Bycatches of harbour porpoises in Norwegian coastal gillnet
fisheries: implications for management and conservation
Dissertation presented for the degree of Philosophiae Doctor (PhD)
Department of Biosciences
Faculty of Mathematics and Natural Sciences University of Oslo
Marine Mammals Research Group Institute of Marine Research
© André Moan, 2023
Series of dissertations submitted to the
Faculty of Mathematics and Natural Sciences, University of Oslo No. 2393
All rights reserved. No part of this publication may be
reproduced or transmitted, in any form or by any means, without permission.
Print production: Graphics Center, University of Oslo.
This thesis marks the completion of a long and laborious journey that has certainly included its share of ups and downs. Now that this journey is finally coming to an end, I am immensely appreciative of having had this opportunity. In this preface, I want to express my gratitude to some of the most important people who have helped and guided me along the way.
First, I want to thank my two co-supervisors Asbjørn Vøllestad at the University of Oslo and Jon Helge Vølstad at the Institute of Marine Research. You have, without fail, always met me with positivity and enthusiasm, and always been available for chatting about whatever questions and doubts I may have brought to your attention. Thank you!
Next, I want to specially mention Øystein Langangen at the University of Oslo, who became very involved in my work during this last year. Thank you, Øystein, for your support and help.
I have thoroughly enjoyed our co-supervision of master students at the university and our collaboration on individual based models. It has been great to have someone to talk to, and spar and troubleshoot with, that can really get into the dirty and greasy bits of modeling work together with me.
Another person that I owe great thanks is Kjell T. Nilssen at the Institute of Marine Research.
Over the course of my PhD, Kjell took me along on several field trips, trips that not only taught me heaps about our coastal seals, but that were also fun and enjoyable, and a welcome respite for my usual office-centered everyday life.
Finally, there is my main supervisor Arne Bjørge at the Institute of Marine Research. My venture into marine mammal science and fishery research started with Arne many years ago, and ever since then, Arne has always been at my side, guiding me, supporting me, showing me the way, and giving me every opportunity. Arne, thank you for your seeing me and believing in me, and for your unbounded patience and gentle guidance through these years. I consider myself incredibly lucky that I walked into your office all those years ago and asked for a project on marine mammals!
With these thanks out of the way, it is time to dive into the matter at hand.
Table of Contents
Summary in Norwegian ... I Summary in English ... III List of papers ... V Relevant co-authored papers ... V List of abbreviations and acronyms ... VII
1 Introduction: cetaceans and fisheries ... 1
2 The study species – the harbour porpoise (Phocoena phocoena) ... 5
3 Fisheries in Norway, with an emphasis on coastal gillnet fisheries ... 13
4 Monitoring fisheries and harbour porpoise bycatches in Norway ... 15
5 Mitigating harbour porpoise bycatches ... 21
6 Exploring different pinger-based bycatch mitigation plans ... 27
7 Sustainability of bycatches ... 35
8 Limitations and further discussion ... 41
9 Broader international implications ... 43
10 Conclusions and recommendations ... 47
Acknowledgements ... 49
References ... 51
Summary in Norwegian
Nise er en liten tannhval i nisefamilien, som er svært vanlig i Nord-Atlanteren. Det totale antallet niser langs norskekysten er mellom ca. 77.000 og 200.000 individer, avhengig av hvordan man avgrenser området. Niser oppholder seg for det meste i relativt grunne farvann, - farvann som også er i flittig bruk av norske garnfiskere. Når niser støter på fiskegarn skjer det av og til at de setter seg fast og bifanges. Det fører nesten alltid til at nisene må bøte med livet.
I tillegg til at bifangster er dødelige for den enkelte nisen, så kan de også ha store konsekvenser på populasjonsnivå og potensielt føre til store reduksjoner i tallrikheten av niser i norske farvann.
I denne avhandlingen forsøker jeg å tallfeste bifangster av niser i norske garnfiskerier, undersøke effekten av mulige avbøtende tiltak, og vurdere i hvilken grad bifangstene er bærekraftig i det lange løp.
I den første artikkelen (Paper 1) bruker jeg data fra et utvalg av fiskebåter som rapporterer data om sine fangster til Havforskningsinstituttet, den såkalte referanseflåten, til å estimere bifangstraten av nise i norske garnfiskerier. Det er nødvendig å estimere bifangsten siden fiskere nesten aldri rapporterer slik bifangst. Det gjør derimot referansefiskerne.
Bifangstestimater basert på data fra referansefiskerne blir oppskalert ved bruk av statistiske modeller til resten av garnfiskeriene ved hjelp av data fra sluttseddelregisteret, som angir hvor mye av ulike typer fisk som er levert til de ulike fiskemottakene. Det beste anslaget for årlig bifangst er på 2.675 niser per år, der mesteparten tas i torske- og breiflabbfisket. Samtidig viser beregningene at bifangsten var på et mye høyere nivå fra 2006 til 2013 enn fra 2014 til 2018 (3.694 niser per år vs. 1.656 niser per år). Nedgangen i den senere tiden skyldes sannsynligvis en nedgang i fiskeinnsats i breiflabbfisket.
Den andre artikkelen (Paper 2) rapporterer resultatene fra et toårig forsøk der åtte fiskebåter brukte akustiske alarmer (pingere/niseskremmere) på fiskegarnene annenhver uke. Hensikten med forsøket var å undersøke hvor stor effekt slike pingere kunne ha på bifangstraten av nise.
Resultatene etter ca. 750 fisketurer var svært tydelige. Av den totale bifangsten på 20 niser, så ble 19 niser tatt i garn uten pingere, og bare én tatt i garn med pingere. Det viser at pingerne ikke fjernet muligheten for bifangst av niser helt, men reduserte den med om lag 95%.
Forsøkene viste også at mertid/heft pga. pingerbruk var lav.
I den siste artikkelen (Paper 3) bruker jeg en individbasert romlig eksplitt modell til å simulere populasjonsutviklingen for niser i norske farvann i de neste 50 årene i forskjellige scenarioer med ulik grad av fiskeinnsats og bruk av pingere. Resultatene viser at selektiv bruk av pingere i områder og fiskerier med høy bifangstrisiko kan redusere bifangst av nise med opp til ca. 25%, avhengig av fiskeinnsats. Men dersom fiskeinnsatsen i de nærmeste tiårene øker til det samme nivået som den var fra 2006 til 2018 og forblir på dette nivået, så tyder resultatene på at
nisepopulasjonen i norske farvann ila. en 30-årsperiode minker til ca. 45% av nåværende nivå, og at bruk av pingere ikke vil være tilstrekkelig til å hindre nedgang. Pingere kan imidlertid redusere nedgangen. I et slikt scenario kan pingere i stormaska fiskegarn redusere nedgangen til ca. 53% av det nåværende nivået.
Denne avhandlingen utgjør et viktig bidrag til kunnskapsgrunnlaget for tilstanden i den norske nisebestanden og de langsiktige konsekvensene av bifangst av nise i norske garnfiskerier.
Avhandlingen avdekker kunnskapshull og inneholder konkrete forslag til videre forskning, datainnsamling og gjennomføring og oppfølging av avbøtende tiltak for å redusere bifangst av nise i fiskeriene. Funnene i avhandlingen er relevante for beslutningstakere for å kunne ivareta og beskytte våre norske niser, og sørge for god og bærekraftig forvaltning av disse flotte dyrene når vi går inn i framtiden.
Summary in English
The harbour porpoise is a small odontocete that is very common in the North Atlantic. The total abundance along the Norwegian coast is between 77,000 and 200,000 porpoises, depending on how the area is defined. Porpoises spend most of their time in shallow waters, - waters that are also used by Norwegian gillnetters. Porpoises are frequently bycaught in gillnets, and these bycatches are almost always lethal. In addition to the mortal impact on individual porpoises, bycatches can also have severe consequences on the population level, and potentially cause large declines in the abundance of porpoises in Norwegian waters.
The purpose of this thesis is to quantify and assess the population level impact and long-term sustainability of past, current, and possible future levels of bycatches in Norwegian fisheries to harbour porpoises inhabiting Norwegian shelf waters and to assess the mitigative effect of different bycatch mitigation scenarios with acoustic deterrents (pingers) on gillnets.
Paper 1 uses data collected by a sample of contracted fishing vessels that report data on their catches directly to the Institute of Marine Research to estimate harbour porpoise bycatch rates in gillnets. Bycatch estimates are scaled up to the whole fleet using fish landing tickets. The best estimate for bycatch is 2,675 porpoises per year, with most porpoises caught in cod and monkfish fisheries. The results from Paper 1 also show that the bycatch level was at a much higher level between 2006 and 2013 than between 2014 and 2018 (3,694 porpoises per year vs.
1,656 porpoises per year). The recent decrease in bycatches is most likely due to a reduction in fishing effort in monkfish fisheries.
Paper 2 reports results from trials with pingers on gillnets in commercial fisheries. Eight vessels used pingers on gillnets every other week for two years. The results were very clear: a total of 20 harbour porpoises were bycaught, and only one of those was caught in a pingered gillnet.
This shows that pingers did not eliminate bycatches completely but reduced them by 95%.
Results also show that extra time costs associated with pinger use were low.
Paper 3 integrates results from Papers 1 and 2 to set up and run a spatially explicit, individual- based simulation model to model harbour porpoise population dynamics under different possible future fishery trends and bycatch mitigation strategies. Modeling results show that pingers can reduce bycatches of porpoises in high-risk fisheries by up to 25%, assuming fishing effort is on the same level as between 2014 and 2018. However, if the total fishing effort returns to a level corresponding to the average level between 2006 and 2018, then our results indicate that the population will decline dramatically, reaching about 45% of the initial abundance after about 30 years. In this scenario, our results indicate that the use of pingers in selected fisheries can reduce the decrease, but not prevent it entirely. Based on our simulations, using pingers in large mesh fisheries can reduce the decline so that the population reaches 53% of the initial abundance.
The results presented in this thesis contribute to the wider understanding of the current and future status of harbour porpoises in Norwegian waters, and more generally in the Northeast Atlantic, and provide some of the information needed to inform conservation efforts and policymaking to protect harbour porpoises in Norwegian waters and elsewhere.
List of papers
Paper 1 Moan, A., Skern-Mauritzen, M., Vølstad, J. H., & Bjørge, A. (2020).
Assessing the impact of fisheries-related mortality of harbour porpoise (Phocoena phocoena) caused by incidental bycatch in the dynamic Norwegian gillnet fisheries. ICES Journal of Marine Science, 77(7-8), 3039-3049.
Paper 2 Moan, A. & Bjørge, A. (2023). Pingers reduce harbour porpoise bycatch in Norwegian gillnet fisheries, with little impact on day-to-day fishing
operations. Fisheries Research 259, 106564.
Paper 3 Moan, A., Langangen, Ø., Bjørge, A. (2022). Simulating harbour porpoise population dynamics in different scenarios of future fishing effort and bycatch mitigation measures. Ecological Modelling. Manuscript.
Relevant co-authored papers
Bjørge, A., Moan, A, Ryeng, K. & Wiig, J. R. 2022. Low anthropogenic mortality of humpback (Megaptera novaeangliae) and killer (Orcinus orca) whale entrapment in Norwegian purse seine fisheries despite frequent
entrapments. Marine Mammal Science, 1-11.
Elnes, J.O, Moan, A., Nilssen, K.T., Vøllestad, L.A. & Bjørge, A. 2022.
Temporal and spatial distribution of harbor seal (Phoca vitulina) risk of entanglement in gillnets at the Norwegian coast. Fisheries Research.
Moan, A., & Bjørge, A. 2020. Bycatch of coastal seals in Norwegian gillnet fisheries conducted by coastal fishing vessels. SC/28/BYCWG/04. Report to the NAMMCO scientific committee 28, October 21, 2021. Manuscript.
List of abbreviations and acronyms
ADD Acoustic deterrent device (pinger)
ASCOBANS Agreement on the Conservation of Small Cetaceans of the Baltic, Northeast Atlantic, Irish and North Seas
AU Assessment unit
CI Confidence interval (always 95% unless otherwise specified)
CITES Convention on International Trade in Endangered Species of Wild Fauna and Flora
CMS The convention on the Conservation of Migratory Species of wild animals CPUE Catch per unit effort
CRF Coastal reference fleet CV Coefficient of variation
DOF (Norwegian) Directorate of Fisheries GLM Generalized linear model
GAM Generalized additive model
IBM Individual-based (simulation) model
ICES International Council for the Exploration of the Sea IMR (Norwegian) Institute of Marine Research
IWC International Whaling Commission
NAMMCO North Atlantic Marine Mammal Commission NBHF Narrow-band, high-frequency
NOK Norwegian kroner (the currency)
LOA Length overall (meters), used to describe the size of a fishing vessel ORF Offshore reference fleet
OSPAR The Convention for the Protection of the Marine Environment of the Northeast Atlantic
PAH Polycyclic aromatic hydrocarbon
PBR Potential biological removal PCB Polychlorinated biphenyl
PET Protected, endangered, and threatened species POP Persistent organic pollutant
REM Remote Electronic Monitoring SD Standard deviation
SPL Sound pressure level (expressed in dB)
1 Introduction: cetaceans and fisheries
Cetaceans, which are comprised of about 90 different species of whales, dolphins, and porpoises, can be found in almost any marine environment on earth. All cetaceans share a common evolutionary heritage, having adapted to a fully aquatic life from terrestrial ancestors in the early Eocene, about 56 million years ago (Thewissen et al., 2007). Despite their shared ancestry, as a group, Cetacea is quite diverse, with members ranging from the small and very regional (such as the vaquita, Phocoena sinus) to the true behemoths of the seas, that undertake some of the longest migrations in the animal kingdom (e.g., the blue whale, Balaenoptera musculus). It is an interesting fact that the largest cetacean can be about 3,500 times as massive as the smallest, again demonstrating the diversity of this group of animals. As a result of their (often) large body sizes and high energetic demands, cetaceans usually take the role as top predators in their ecosystems and may serve a host of important ecological functions (e.g.
Roman et al., 2014; Williams, 2006). For example, they can structure communities through competition and top-down regulatory effects. In some cases, they themselves can be prey for other species. When they defecate near the water surface, they contribute to primary production, because their feces fertilize the upper water layer. The process of cetaceans feeding in the deep, and then defecating near the surface has aptly been coined “the whale pump”, because nutrients from the deep are “pumped” by way of the animal up to the photic zone, where phytoplankton can make use of them (Roman and McCarthy, 2010). Cetaceans can also bring nutrients to the deep. When they die, and sink to the bottom of the sea, cetacean carcasses provide habitats and energy to other organisms (Smith et al., 2015). The ecological importance of cetaceans to the communities of which they are part cannot be overstated.
Cetaceans in nearly all regions of the world are impacted to some degree by human activities, which are nearly ubiquitous (Halpern et al., 2008). Human activities that can impact cetaceans include underwater noise (Richardson and Würsig, 1997), e.g., from sonar (Parsons, 2017), ships (Erbe et al., 2019), pile driving (Bailey et al., 2010) or wind turbines (Weilgart, 2007).
Ship traffic can disturb (Bedriñana-Romano et al., 2021) and strike/collide with cetaceans (Van Waerebeek and Leaper, 2008). Additionally, cetacean habitats may become degraded and unusable as a result of human activities (Simmonds and Nunny, 2002). Cetaceans are also impacted by pollution (e.g., PCBs, PAHs, POPs, inorganic contaminants, etc.). Other human activities that can impact cetaceans are whale-watching (Parsons, 2012), whaling and fishing.
This list is not exhaustive.
Interaction between humans and cetaceans are not necessarily negative. Some species of cetaceans can seek out humans, like curious dolphins seeking out a small boat or playing in the bow waves of a large ship. But interactions between humans and cetaceans often have negative consequences for the latter. Whaling is an obvious example. Unregulated whaling in the 19th century decimated many populations of baleen whales, such as the bowhead (Balaena
mysticetus), grey (Eschrichtius robustus) and humpback whales (Megaptera novaeangliae).
The 1986 moratorium on whaling, and later protective legislation and international agreements have offered some relief to harvested whale populations, which are now (mostly) well on their way to recovery. Whaling today is limited and well-regulated and does not pose a threat to populations of whales (Friedheim, 2001)
Today, the most important threat to cetaceans in general is fisheries (e.g.; Brownell Jr et al., 2019; Jefferson and Curry, 1994; Read et al., 2006). Cetaceans can get trapped in moving fishing gears, such as trawls or Danish and purse seines (Bjørge et al., 2022), or they can get entangled in hook lines, gillnets, pot traps, or mooring lines for any of the above. These kinds of unintentional and incidental catches of cetaceans in fishing gears intended for other species are called bycatches. Bycatches have serious welfare and conservation implications for the affected species. It has been estimated that globally, more than 300,000 cetaceans are killed in various fishing gears every year (Read et al., 2006). The problem of cetacean bycatches in fishing gears is further compounded by the fact that both cetaceans and fishers are attracted to the same areas, seeking out the same resources. Cetaceans and humans often overlap in nutrient- rich coastal waters with a high degree of mixing and upwelling, and high primary productivity.
Sharing a common food resource is thus a fundamental driver that directly causes interactions between cetaceans and fishing gear. A striking and illustrative example of cetaceans and humans being attracted to the same resource, can be found in Mul et al. (2020) and Vogel (2020), which demonstrate the close association between killer whales preying on herring, and fishing vessels fishing herring in the fjords of northern Norway.
Larger cetaceans, e.g., baleen whales, that get entangled in fishing gear or moorings/ropes (potentially with more gear, buoys and/or anchors still attached) may be strong enough to snap ropes and lines, and free themselves, at least from parts of the gear (Arthur et al., 2015). They may then be able to move by dragging the remains of the entangled gear along. However, such
“fishing gear appendages” are very likely to interfere with the animal’s normal activities (e.g., through reduced foraging success, increased energetic demands and stress), and can lead to permanent damage or death through a potentially slow and protracted process. North Atlantic right whales (Eubalaena glacialis) entangled in fishing gears, for example, have been documented to drag gears around for an average of six months (Moore and van der Hoop, 2012).
The high frequency of large whale entanglements has caused the International Whaling Commission (IWC) to form an expert advisory panel on entanglement response, to develop and share knowledge and expertise, and teach best protocols to personnel that may become engaged in releasing large cetaceans from fishing gear (IWC, 2011). Today there is a Global Whale Entanglement Response Network (GWERN) and active entanglement response teams and communities in many of the regions where large whale entanglements are common.
Unlike large baleen whales, smaller cetaceans, like the odontocetes (the toothed whales) are usually not powerful enough to snap or break entangling ropes and netting. Instead, when they
become entangled, they usually cannot just break free and go about their day. In fact, there is a chance that any movements that they make to free themselves (like thrashing, turning, or writhing), may only cause them to become further entangled. Since getting stuck effectively prevents them from returning to the surface to breathe, the result is that, eventually they perish.
The process of death in fishing gears is poorly understood. Dissections of bycaught small cetaceans have revealed that they often do not have seawater in the lungs, and that they fail the
“diatom test” (a criteria for drowning based on the presence of diatoms in lung tissue). This suggests that rather than drowning, death may be caused by hypoxia and subsequent asphyxiation (Ijsseldijk et al., 2021), but it is unclear what this entails for how long it takes for the animal to lose consciousness, or how the animal generally experiences this “death by fishing gear”. For most species, death most likely occurs within a few minutes.
Small cetaceans often inhabit coastal and shelf waters. This makes them especially susceptible to bycatches in coastal fisheries. In fact, the threat of bycatches in fisheries to small cetaceans has caused great concern in the international scientific community, and increasingly made small cetaceans the focus of conservation efforts all over the world. But this was not always so. Figure 1 shows that the research output of work related to bycatches of small cetaceans (expressed as the number of scientific publications per year, see figure caption) was at a consistent low level from the 1970s to the 1990s. But in the beginning of the 1990s, research output started to increase, and has continued to increase at increasing rates to date. Last year alone (2021), there were a total of 135 articles published on small cetacean bycatch.
Figure 1: Results from an ISI Web of Science query, using the search string (odontocete OR
"small cetacean" OR dolphin OR porpoise) AND (bycatch OR by-catch OR entanglement OR fishery), executed on August 11, 2022.
Today, there are several ongoing international scientific expert groups and committees partly or wholly dedicated to issues dealing with small cetaceans. The Scientific Committee (SC) in
threats to small cetaceans (including bycatches in fisheries). Both the International Council for the Exploration of the Sea (ICES) and the North Atlantic Marine Mammal Commission (NAMMCO) have fixed/recurring expert working groups that deal with small cetaceans (among other protected, endangered, and threatened species, PETS). Additionally, there have been, and continue to be, many initiatives, workshops, and collaborations among researchers both in national and international settings to address small cetacean bycatch issues.
One of the small cetaceans that has received the most attention with the rising awareness of the bycatch issue, is one of the most common ones – the harbour porpoise (Phocoena phocoena).
The harbour porpoise is considered to be especially threatened by gillnet fisheries (e.g. Braulik et al., 2020; Bravington and Bisack, 1996; IMR/NAMMCO, 2019; Jefferson and Curry, 1994;
Orphanides and Palka, 2013; Tregenza et al., 1997; Trippel et al., 1999), and many populations of harbour porpoises seem to be in big trouble, despite repeated warnings from a united scientific community and ongoing (but ineffective) conservation efforts (Carlén et al., 2021;
Rogan et al., 2021). The harbour porpoise and the problem of harbour porpoises dying in gillnets are the foci of this thesis. The next section starts out by giving a brief introduction to the fascinating animal that is the harbour porpoise.
2 The study species – the harbour porpoise (Phocoena phocoena)
The harbour porpoise is a toothed whale in the family Phocoenidae. There are several genera and species in the family, but the harbour porpoise is the most common and widespread.
Porpoises are distinguished from their close relatives among the odontocetes (i.e., narwhals, belugas, and true dolphins) by their spatulated teeth, triangular dorsal fins, ultrasonic sounds, and much less pronounced beaks (Figure 2). Harbour porpoises are physically small compared to most other cetaceans, with adults reaching up to 1.9 meters and 70 kgs, and females growing a little heavier than males. Harbour porpoises become sexually mature when they reach a length of about 1.4 meters, around three to five years of age (Kesselring et al., 2017; Learmonth et al., 2014; Lockyer, 2003). Once mature, females get pregnant almost every year. Gestation lasts from 10 to 11 months. Calves are born in summer (May – August) and weaned about 10 months later. Harbour porpoises usually live for less than 12 years, with a maximum recorded age in the wild of 24 years (Bjørge and Tolley, 2017; Lockyer, 2003).
Figure 2: Adult harbour porpoise with calf. The neutral dark grey back color, triangular shape of the dorsal fin and the absence of a pronounced beak are characteristic of the species.
Illustration: Princeton University Press.
Harbour porpoises are considered flexible generalist predators that feed on a wide range of different fish species in all parts of the water column, with large variations in diet from region to region and from season to season (Gibson et al., 2003), possibly reflecting changing prey availability. Stomach content analyses have shown that although many prey species occur in
the diet, the great majority of the diet at any time is made up of only a few species (Börjesson et al., 2003; Saint-André, 2019). Porpoises in the northwest Atlantic, for example, are known to eat mostly herring (Clupea harengus), silver hake (Merluccius bilinearis) and pearlsides (Maurolicus muelleri), but in different proportions depending on the time of year (Gannon et al., 1998). In the Barents Sea, harbour porpoises eat capelin (Mallotus villosus), herring, saithe (Pollachius virens), haddock (Melanogrammus aeglefinus), blue whiting (Micromesistius poutassou) and greater Argentine (Argentina silus). Further south along the Norwegian coast, the most important prey are herring, saithe, blue whiting, poor cod (Trisopterus minutus), Argentine and pearlsides. In the North Sea and Skagerrak, herring, gobids (Gobiidae), ammotydis (Ammodytidae), sprat (Sprattus sprattus), whiting (Merlangius merlangus) and cod (Gadus morhua) are important prey species (Aarefjord et al., 1995). Börjesson et al. (2003) found that herring and Atlantic hagfish (Myxine glutinosa) were important prey items for harbour porpoises in Kattegat and Skagerrak. A more recent study on the diet of harbour porpoises in Norwegian coastal waters, showed that gadoid fishes and juvenile saithe may have become more important than herring and capelin in this region (Saint-André, 2019). Many of the prey species consumed by harbour porpoises are also commercially important (e.g., herring, saithe, and capelin), but harbour porpoises mostly eat small or juvenile fish (Gibson et al., 2003;
Saint-André, 2019), less than 30 centimeters long. Often, commercial fisheries target larger fish, so even though harbour porpoises and fishers utilize the same resources, they may be utilizing different segments (age and length classes) of those resources.
The harbour porpoise has a circumpolar distribution in the northern hemisphere, with about 18%
distributed in the Pacific, 31% in the western Atlantic, tiny populations in the Baltic and Black seas, and the rest in the Eastern Atlantic (Braulik et al., 2020). There are five putative subspecies of harbour porpoises, in different parts of its range: P. p. phocoena in the North Atlantic, P. p.
meridionalis in Iberia/West Africa, P. p. relicta in the Black and Azov Seas, P. p. vomerina in the northeastern Pacific, and another, unnamed subspecies in the northwestern Pacific (Fontaine et al., 2014; IMR/NAMMCO, 2019; Shirihai and Jarrett, 2019). According to the IUCN red list, the current global abundance of harbour porpoises is well over one million individuals (Braulik et al., 2020). This suggests that the harbour porpoise is one of the most abundant small cetaceans in the world, only rivaled by Dall’s porpoise Phocoenoides dalli (which is only found in the Pacific), and surpassed by a few species of dolphins (e.g. pantropical spotted dolphin Stenella attenuata, bottlenose dolphin Tursiops truncatus, striped dolphin Stenella coeruleoalba and possibly a few others; long-finned pilot whales Globicephela melas, spinner dolphin Stenella longirostris and pacific white-sided dolphin Lagenorhynchus obliquidens).
In its most recent assessment, the IUCN red list gave the global population of harbour porpoises a rating of Least Concern (Braulik et al., 2020). In the Northeast Atlantic, the total population is in the range of 6-700,000 individuals, based on abundance estimates referenced in Braulik et al. (2020). In the Greater North Sea and adjacent regions, the total harbour porpoise abundance estimates did not change significantly between two large-scale surveys conducted in 1994 and
2016 (Hammond et al., 2002; Hammond et al., 2019), indicating that the population in this region is stable. It is unclear to what extent the stability suggested by the SCANS survey results applies to other potential (sub)populations of harbour porpoises in nearby regions in the North Atlantic. The population structure of harbour porpoises in the region is currently unknown, but the leading hypothesis is that harbour porpoises in the North Atlantic represent one continuous distribution, exhibiting isolation by distance (Andersen et al., 2001; IMR/NAMMCO, 2019;
Quintela et al., 2020; Tolley et al., 2001).
Figure 3: Global distribution of harbour porpoises in red, showing their circumpolar distribution, mostly in shallow and coastal waters in the northern hemisphere. Figure created using range data downloaded from the IUCN red list of Threatened Species (Braulik et al., 2020).
Information about the population structure of a species is important for addressing questions concerning the conservation status of that species. In the context of bycatches in fisheries, evaluating bycatch mortality estimates against an incorrect population size and structure could lead to wrong conclusions. One might conclude, for instance, that bycatches in a region are sustainable based on erroneous or inappropriate population estimates, when in fact, they are not, which has implications for what (if any) actions are taken. This is more likely to happen in situations where bycatch impacts a regional population which is not recognized as a distinct population, and the bycatch is instead evaluated against a larger population, thereby erroneously deflating the perceived impact. Thus, when in doubt, the cautionary approach will usually be to err toward using smaller population units rather than large ones in sustainability assessments.
In 2018, the Institute of Marine Research (IMR) and NAMMCO organized a joint workshop focused on the harbour porpoise, with expert collaborators from relevant parts of the North Atlantic. One of the key outcomes of this workshop was the definition of 15 harbour porpoise assessment units in the North Atlantic (Figure 4 and Table 1). The definitions of these 15 assessment units (AUs) were based on an integrated evaluation of relevant aspects of existing
harbour porpoise research and areas used for fisheries management, and were considered appropriate for assessment purposes, but should not be taken to represent the population structure of harbour porpoises in the region.
Figure 4: Map of the North Atlantic showing the 15 harbour porpoise assessment areas used in the 2018 joint IMR and NAMMCO harbour porpoise workshop. Map adapted from Figure 2 in IMR/NAMMCO (2019).
Table 1 gives an overview of the assessments that resulted from the workshop. In summary, the population trends for harbour porpoises were stable or increasing in six AUs, and decreasing or uncertain in the rest. However, the workshop report was reviewed in a subsequent meeting in NAMMCO’s harbour porpoise working group in 2019, in which it was concluded that due to scope of the work and time constraints, results from the workshop should be considered
“preliminary and informal”, pending further work. Norwegian waters (as defined in Papers 1 and 3) overlap with two of the workshop AUs: the Norwegian and Russian coasts (AU 07) and the Greater North Sea (AU 11). The dividing line between AU 07 and AU 11 lies on the 62nd parallel and represents a pragmatic division, not based on the biology of the harbour porpoise.
As stated above, the population trend for harbour porpoises in AU 11 is stable, but the trend in AU 07 is decreasing. However, harbour porpoises can be expected to cross this border and mix to the same extent as any other location along the coast.
Table 1: Status of harbour porpoises in 15 assessment units in the North Atlantic. An assessment is data rich in this context if it was based on at least two abundance estimates and multiple bycatch estimates that cover the same periods.
# Assessment Unit Trend
1 Eastern US (Gulf of Maine, Bay of Fundy,
and Scotian shelf) Increasing (due to declining bycatch) 2 Eastern Canada (Newfoundland, Labrador,
and Gulf of St. Lawrence) Slow decline
3 Western Greenland Increasing
4 Eastern Greenland Not assessed
5 Iceland Increasing
6 Faroe Islands Stable or increasing
7 Norwegian and Russian coasts Declining
8 West Scotland and Ireland Declining
9 Irish Seas Declining
10 Celtic Seas Declining
11 North Sea Stable
12 Belt Sea Increasing
13 Baltic Sea Severely depleted, and declining
14 Iberian Peninsula Uncertain, but probably declining
15 African coast Uncertain, but of concern
Harbour porpoises use sound both passively and actively for foraging (DeRuiter et al., 2009;
Verfuß et al., 2009), navigating/spatial orientation (Verfuß et al., 2005) and communication (Clausen et al., 2011; Sørensen et al., 2018). The acoustic environment is very important for harbour porpoises. Results from bioacoustic studies suggest that harbour porpoises may be nearly continuously vocalizing. Akamatsu et al. (2007) found that one recorded harbour porpoise produced click trains every 12.3 seconds, with 90% of quiet periods lasting less than 20 seconds. Linnenschmidt et al. (2013) concluded, based on results from a bioacoustics study
in Kattegat, that harbour porpoises echolocate almost continuously, only interrupted by infrequent periods of silence (the longest of which was 23 minutes, in their case).
Compared to the delphinids, harbour porpoises have a very limited repertoire of sounds, and produce only narrow-band, high-frequency (NBHF) clicks, centered at 130 kHz. The delphinids, like bottlenose dolphins or killer whales, on the other hand, employ a wide repertoire of sounds of varying frequencies (both low and high) in their sound production. It has been hypothesized that the limited repertoire and high frequency of sounds used by harbour porpoises are indicative of acoustic crypsis, an evolutionary response to reduce predation by killer whales (Orcinus orca), since NBHF clicks are outside the upper effective hearing range of killer whales (e.g., Moriska and Connor, 2007), which is about 100 kHz.
Figure 5: Attenuation of underwater sounds of different frequencies used by odontocetes, at a depth of 25 m in the Norwegian (red line) and the Baltic (blue line) Seas, expressed as sound pressure absorption per km. Peak frequencies of echolocation clicks are highlighted for a few species. Absorption values were calculated based on the simplified method outlined in Ainslie and McColm (1998), and also using the same values for oceanographic variables.
One potentially important consequence of the exclusive use of NBHF clicks by harbour porpoises is that it effectively limits their acoustic range. The acoustic range is limited compared with other echolocating cetaceans because attenuation of sound in seawater increases
with increasing frequency. Figure 5 compares theoretical attenuation at peak frequency of echolocation clicks produced by harbour porpoises, bottlenose dolphins and killer whales in two different oceanic environments; the Norwegian Sea (high salinity) and the Baltic Sea (low salinity).
Figure 5 shows that due to their high acoustic frequency, harbour porpoise echolocation clicks attenuate much faster than clicks produced by bottlenose dolphins or killer whales, and that attenuation is much greater in a full-strength seawater than in brackish water. For example, in the Norwegian Sea, a click produced by a killer whale at a given sound pressure can propagate 2.3 kilometers before attenuating as much as a click produced by a harbour porpoise at the same sound level would in only one kilometer. This has implications for the use of acoustic techniques to prevent harbour porpoise bycatches (more on this in chapter 5, which deals with bycatch mitigation).
Now that we are more familiar with some of the key biological and ecological aspects of the harbour porpoise, it is time to take the proverbial plunge into the world of fisheries.
Understanding how fisheries are conducted and reported is the next vital piece on the way to understanding the issue of harbour porpoise bycatches. The next section will give a brief description of Norwegian fisheries, with an emphasis on fisheries using gillnets. After that, we will consider how the number of harbour porpoise bycatches in Norwegian gillnet fisheries is quantified (Paper 1), how effective acoustic alarms (pingers) can reduce harbour porpoise bycatch risk in gillnets (Paper 2), and what the future might hold for porpoises in Norwegian waters in the coming decades (Paper 3).
3 Fisheries in Norway, with an emphasis on coastal gillnet fisheries
Norway has a long coastline and rich marine waters with many commercially important fish stocks. Norwegian waters intersect several different marine regions, from the Barents Sea in the north, to the Norwegian Sea, the North Sea and Skagerrak in the south. Fisheries in Norway are extensive, and Norwegian fish and fish products constitute Norway’s second largest export commodity. The commercial fishing fleet in Norway is dynamic and heterogeneous, with fishing vessels often participating in many different fisheries in different areas and with different types of fishing gears through the year.
The commercial fishing fleet can be divided into two groups that roughly define the coastal and offshore fleets: vessels below 15 meters overall length (LOA), and vessels at least 15 meters LOA. In the rest of this thesis, these groups will be referred to as small and large vessels, respectively. According to the official registry of fishing vessels (the “Merkeregister”), between 2006 and 2021, there were 5,628 ± 468 small vessels and 562 ± 113 large vessels in the registry every year (means ± SDs). The small vessels mostly operate gillnets, hook lines and fish traps in the coastal zone, often within 12 nautical miles (22.2 km) of the coast. Large vessels on the other hand, primarily fish in offshore waters using pelagic gears, such as trawls and purse seines.
The most important catch species for both groups are cod, herring, saithe, haddock, and mackerel. The average yearly landed catch between 2006 and 2021 was 2,910,716 tonnes, with approximately 10% and 90% of catches contributed by small and large vessels, respectively.
Even though a majority of the total catches are taken by the large vessels, bycatch data collected by the reference fleets (described in the next section) indicate that the fishing gears that they use for most of their activities are not typically associated with harbour porpoise bycatches.
Bycatch data indicate that almost all harbour porpoise bycatches in Norway are taken in gillnets set by vessels fishing in coastal waters (Paper 1). A typical Norwegian gillnetter conducts 77 fishing trips and sets/hauls that number of gillnets per year, but there is a lot of individual variation. Here, a “gillnet” confusingly refers to a string of multiple gillnets, tied together at the ends, that in rare cases can be almost 14 kilometers long. Gillnets usually soak (i.e., fish) for 24 hours, but sometimes longer, especially in fisheries targeting monkfish. For parts of the year, gillnet fishers do not necessarily target one species, but engage in mixed fisheries, where many different species are caught.
Fishers are required to land all their catches, including marine mammal bycatches, at registered fish reception facilities. Even so, some catches go unreported, especially catches of porpoises (Paper 1) , pinnipeds (Elnes et al., 2020) and sea birds (Fangel et al., 2015) that are unintentionally bycaught. After landing the catch, fishers receive a fish landing ticket that details the weighted catch of each species landed, as well as information such as the vessel name, registration number, the date, the general location of the fishing site (specified by cell in an irregular geographical grid, with most cells covering 30 by 30 arc minutes). Fish landing
(e.g., number of nets, number of hooks) or the fishing time. They also do not specify mesh sizes, twine diameters, materials, or heights of gillnets. Discards at sea are generally prohibited, but there are exceptions for viable fish that were caught in violence of regulations on minimum size, etc.). Harbour porpoises that are entangled in a gillnet are almost always dead by the time the gillnets are hauled. They are not part of the exceptions to the requirement to land catches. Even so, harbour porpoises only appeared on 70 out of the 1,331,501 fish landing tickets that were generated from 2006 to 2020 where the gear was specified as gillnet. Those 70 fish landing tickets, with 111 harbour porpoises (count estimated from reported weight), were associated with six different fishing vessels, that all landed their catches between 2016 and 2018. This kind of underreporting of marine mammal bycatches in fisheries is in accordance with the findings of Basran and Sigurðsson (2021), who found large discrepancies between observed and self-reported bycatch numbers in Iceland, New Zealand and the US.
Given 1) the predisposition of harbour porpoises to become entangled in gillnets, 2) the abundances of harbour porpoises in Norwegian coastal waters, 3) the extensive use of gillnets in Norwegian fisheries, and 4) the strong disagreement between fish landing ticket bycatch registrations and reference fleet bycatch registrations (see next section), it is safe to say that voluntary reporting of marine mammal bycatches by fishers does not work and cannot be considered a reliable source of data on bycatch rates. The issue of underreporting implies that an alternative approach is necessary to monitor marine mammal bycatches in Norwegian fisheries, and this is the topic of the next section.
4 Monitoring fisheries and harbour porpoise bycatches in Norway
There are several systems in place to monitor fisheries in Norway. As discussed in the last section, fish landing tickets and logbooks are not suitable sources of bycatch data, since fishers in the general fleet almost always neglect to land bycaught harbour porpoises (or other marine mammals) and/or to report such bycatch in the logbooks. Fish landing tickets and logbooks still give valuable information about catch composition and amounts, fishing effort, gear use, and coarse geographical and seasonal distributions of fishing effort, assuming proper and truthful record-keeping. In Norway, compliance to fishing rules and regulations (including record- keeping) is controlled by the coast guard and the Sea Surveillance Unit (SSU). The SSU is a subdivision of the Norwegian Directorate of Fisheries. For large vessels that undertake multi- day fishing trips, SSU inspectors stay on the vessels for the entire duration of selected trips.
This is not practical for many of the small vessels. But the SSU also has roaming vessels that do random/unplanned and opportunistic inspections, just like the coast guard vessels. In this way, small vessels are also controlled. Collectively, the coast guard and the SSU inspect about 2% of commercial fisheries. Some further details on how SSU inspectors choose which vessels and fisheries to control, are described in Bjørge et al. (2022).
Additionally, coastal (CRF) and offshore (ORF) reference fleets are used to monitor a variety of biological parameters of catches, to assist in stock assessments and setting catch limits/quotas.
Vessels in these reference fleets are contracted by the IMR, financially compensated and trained in sampling and reporting routines. Reference vessels are selected as approximate stratified random samples, with the aim that the CRF and the ORF should be as representative as possible for the entire small and large commercial fishing fleets, respectively (Williams and Clegg, 2020). As of 2019, the CRF was comprised of 22 vessels, and the ORF was comprised of 16 vessels. By vessel numbers, this constitutes about 0.39% and 2.85% of all small and large vessels, respectively, but actual sampling coverage is most likely somewhat higher, since reference vessel tenders include stipulations on minimum fishing effort.
Data reported by the reference fleets contain a lot of the information that is missing from the commercial fish landing tickets. This includes GPS coordinates of fishing locations, bottom depths, fishing depths, highly detailed specifications of gear usage, numbers of fishing gears used, start and end time for each gear (allowing calculation of fishing time, or “soak time”), mesh sizes for gillnets, and more. Most importantly, the reference fleets report bycatches of marine mammals (and other PETS like birds and elasmobranchs). A previous study based on bycatch numbers reported by the CRF between 2006 and 2008 revealed that coastal gillnet fisheries for cod and monkfish in particular bycaught many harbour porpoises (Bjørge et al., 2013). That is why, in Paper 1, and in the following section and throughout the rest of this thesis, these two fisheries receive special attention.
The average monthly total catch of cod and monkfish in the CRF and the corresponding nation-
monthly variation in landed catch through the year is similar among CRF vessels and all small vessels. Based on landed catches, the cod fishery starts in mid-January, peaks in mid-March, and then tapers off towards May. The winter fishery for cod coincides with the arrival of Barents Sea cod that migrate to coastal spawning grounds in the northern parts of Norway in January and February. The monkfish fishery on the other hand peaks in October, but the average monthly catch is relatively high from August to December (Figure 6C), and low from January till June. Figure 6B and 6D show how the yearly total landed catch and fishing effort (expressed as the number of gillnet sets/hauls) in each of these two important fisheries has varied since 2006 to 2018. In the cod fishery, catches have remained consistent throughout this period. The monkfish fishery on the other hand started to decline in 2011, and continued to decline until 2015, with landed catch somewhat higher in the years since then. Figure 6D shows that, overall, the catch per unit effort (CPUE) of gillnet fisheries has shown an increasing trend from 2006 to 2018. Figure 6B and 6D also show that historically, the collective cod and monkfish fisheries have comprised a large proportion (> 50%) of the total yearly gillnet fishing effort among small gillnetting vessels in Norway.
Figure 6: Comparison of average monthly landed catch in gillnets among coastal reference vessels and all small vessels in the cod fishery (A) and the monkfish fishery (C). Panel B shows the yearly total landed catch among all vessels in cod and monkfish fisheries showing a
declining trend in monkfish catches, starting in 2011. Panel D shows the yearly total landed catch vs. yearly total number of gillnet sets/hauls, among small vessels.
The agreement in monthly variation in landed catch of cod and monkfish substantiate the representability of the CRF of the small gillnetter fleet on the national level. Taking for granted that this representability holds on smaller temporal and spatial scales, then harbour porpoise bycatch rates estimated using CRF data in different coastal fishery regions (e.g., number of porpoises bycaught per gillnet set or per kg fish caught) should also apply to similar fisheries conducted by non-reference vessels. Thus, bycatch rates of harbour porpoises estimated using CRF data can be applied to the corresponding data for the whole gillnetter fleet to obtain estimates of total bycatch of harbour porpoises in all commercial gillnets set in Norwegian coastal waters. Bycatch data from the CRF can also be used in conjunction with other explanatory covariates in a more sophisticated modeling approach, where a generalized linear or additive (mixed) model (GLM or GAMM) can be used in an analogous way to predict bycatches of harbour porpoises in gillnets in Norwegian coastal waters.
In Paper 1, we used both design-based and model-based approaches to estimate average yearly total bycatch of harbour porpoises in Norwegian coastal gillnet fisheries. Since monkfish fisheries are very different from cod fisheries, and from other fisheries (in terms of the mesh sizes of the nets used, the number of nets, and the soak time; chapter 3), it is reasonable to expect that bycatch rates vary between these three fishery groups. To account for this, we classified fishery data from the CRF and from fish tickets into three groups: “cod”, “monkfish”
and “other”. The “other” group was a heterogeneous mixed fisheries group that contained all fisheries that could not be classified as either cod or monkfish. To also account for variations in bycatch rates across spatial and temporal scales, fishery data were annotated by adding variables for region, season, and year. Data were post-stratified according to these variables, resulting in a total of 104 strata, or métiers, where strata/métiers represented unique combinations of stratification variables. Fishery regions were defined based on an appropriate combination of different coastal fishery blocks (Figure 7), while season was a binary variable, representing the first (January to June) or second half of the year (July to December).
In the design-based approach, harbour porpoise bycatch rates were calculated for each métier using a separate stratified ratio estimator (Cochran, 1977). We separately estimated bycatch using landed catch and number of sets/hauls as proxies for fishing effort. In the model-based approach, we used negative binomial GLMs and GAMMs to fit CRF data using stepwise forward regression, and applied the best fitted models, as chosen by Akaike’s information criterion (AIC) (Burnham, 1998), to fish ticket data to obtain total estimates. The GLM was included to facilitate comparisons with results from Bjørge et al. (2013). In the GAMM, reference fishing vessels were considered primary sampling units, and fitted as a random effect.
This had the added advantage that individual vessel effects could be taken into account when estimating bycatch rates.
Figure 7: Map showing administrative fishery blocks along the Norwegian coast. Colors indicate how blocks were grouped into fishery regions in Paper 1. The 62nd parallel, separating AU 07 and AU 11 (chapter 2) is also used as the dividing line between fishery blocks 07 and 28 and regions 3 and 4.
Figure 8 shows the estimated yearly total bycatch of harbour porpoises in gillnet fisheries from 2006 to 2018 based on the design-based approach (panels A and B) and the model-based approach (panels C and D). Overall, the bycatch estimates agreed well across methods, but year to year variation was less pronounced in the GLMM. Estimates for the same year calculated
with different methods were not significantly different. While the GAMM bycatch model most closely represent the sampling design of the coastal reference fleet and can be preferred over other estimates for that reason, the most “heuristic” bycatch estimates (taken as the average of yearly estimates produced with the four different estimation methods) from 2006 to 2018 was 2,675 porpoises per year (CV 0.217), with most porpoises bycaught in the northern parts of Norway, and especially in the cod and monkfish fisheries. Bycatch estimates from 2006 to 2013 were substantially higher than estimates from 2014 to 2018 (3,694 [CV 0.236] vs. 1,656 [CV 0.189] porpoises per year, respectively). This decrease can most likely be explained by a reduction in fishing effort in monkfish fisheries, as evidenced by the observed decline in landed monkfish catch that started in 2011 (Figure 6B).
Figure 8: Estimated yearly total harbour porpoise bycatch in gillnets from 2006 to 2018 using a stratified ratio estimator based on landed catch (A) and number of sets/hauls (B) and using a generalized linear model (C) and a generalized additive mixed model (D). Error bars indicate 95% confidence intervals, based on bootstrapping.
Bycatch of harbour porpoises in Norwegian coastal gillnet fisheries for cod and monkfish has previously been estimated to 6,900 animals per year (Bjørge et al., 2013), also using CRF data.
However, it was later discovered that the data used for extrapolating bycatch rates in that study included all fishing gears (not only gillnets), thereby inflating the total bycatch estimates. The GLM estimates of bycatches in cod and monkfish fisheries from Paper 1 represent a
reapplication of their bycatch model on corrected data (and a longer timer series) and can be considered an update of those estimates. One important distinction with the new estimates, however, is the inclusion of other fisheries besides cod and monkfish. Thus, the total estimates presented here, and in Paper 3, represent the most comprehensive and up-to-date estimates of bycatches of harbour porpoise in Norwegian commercial gillnet fisheries.
5 Mitigating harbour porpoise bycatches
The harbour porpoise is protected in most of its range, through national and international legislation and agreements. Many countries and regions have management plans specifically for the harbour porpoise, and/or umbrella legislation that generally covers many groups of species, including the harbour porpoise. In US waters, the harbour porpoise is protected under the Marine Mammal Protection Act (MMPA). In Canadian waters, the harbour porpoise is protected under the Marine Mammal Regulations (MMR) of the Fisheries Act and is also considered a species of “special concern” and additionally protected under the Species at Risk Act (SARA). In the EU and many other European countries, the harbour porpoise is protected under several conventions (OSPAR, CITES, CMS/Bonn, Bern, and others) and annex II and IV of the Habitat Directive. Harbour porpoises are also protected under the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS). In Norway, the harbour porpoise is also protected through national legislation via the Act of June 6, 2008, no. 37 on the management of wild marine resources (the Marine Resources Act).
International and national policy and legislation that include the harbour porpoise strongly imply that bycatches of harbour porpoises in fisheries should be avoided. This general protection is not necessarily contingent on specific bycatch levels or sustainability ratings, although ASCOBANS has a defined target of reducing bycatches of species of marine mammals to less than 1.0% of the best abundance estimate. For harbour porpoises in Norwegian waters, which over the last couple of decades have been subject to an average known bycatch mortality of 2,675 porpoises per year (Paper 1), bycatches may well be unsustainable in the long run, as indicated by the declining trend in the 2018 IMR/ASCOBANS status assessment for the Norwegian coast (AU07) (IMR/NAMMCO, 2019) (more on sustainability in the next chapter). This is a clear case where the precautionary principle should be applied in implementing measures to reduce bycatches as soon as possible.
There are many measures that can mitigate harbour porpoise bycatches in gillnets, and a general overview of methods for preventing cetacean bycatches can be found in a review article by Leaper and Calderan (2018). The most effective method is widely considered to be to ban fishing, although time and area closures come with their own set of problems. For example, they can shift fishing effort to nearby regions (Beare et al., 2013; Poos and Rijnsdorp, 2007) or have other unintended consequences (Abbott and Haynie, 2012; Novak Colwell et al., 2019).
For socioeconomic reasons, a less invasive approach is often preferred. One alternative is to modify or change the gillnets to reduce the risk of entanglements to harbour porpoises while at the same time allowing fishing to continue with normal levels of effort. There have been many attempts to make gillnets more easily detectable for harbour porpoises by taking advantage of the fact that harbour porpoises have well-adapted hearing and use their echolocation nearly continuously.
These measures include increased net stiffening (Bordino et al., 2013), LEDs (Bielli et al., 2020), nylon barium sulphate nets (Koschinski et al., 2006; Mooney et al., 2004; Trippel et al., 2003), ion-oxide gillnets (Larsen et al., 2007) and passive acoustic reflectors (Kratzer et al., 2020). The idea behind these measures is to increase the acoustic reflectivity of the nets, so that echolocating harbour porpoises can detect them. However, active acoustic deterrent devices (ADDs, or pingers) have seen the greatest success and widest application (FAO, 2021).
Conceptually, the simplicity of pingers is very enticing: pingers can be easily fitted to existing gillnets and once submerged, send out short bursts of sounds (“pings”) in the frequency range where harbour porpoise hearing is the most sensitive. Pingers have been demonstrated in many studies to effectively deter harbour porpoise from approaching gillnets (e.g., Barlow and Cameron, 2003; Dawson et al., 2013; Kraus et al., 1997; Omeyer et al., 2020; Palka et al., 2008).
Today, pingers are mandatory in many gillnet fisheries in both the US and the EU.
Paper 2 reports results from field trials conducted in Norway to determine the effect of pingers on harbour porpoise bycatch rates in commercial coastal gillnet fisheries. While it may seem unnecessary to test pingers when their effects are already so well documented in the scientific literature, the fact is that there are many different pinger designs, and there are regional differences in how fisheries are conducted. Additionally, the marine acoustic background environment can vary a lot from one region to the next. As pointed out in chapter 2, this has implications for pinger use, for example because of sound attenuation (Figure 4) and different levels of signal-to-background noise ratios for emitted pings. These circumstances may conceivably be expected to affect the deterring effects of pingers on harbour porpoises, so that results from pinger trials in other areas may not necessarily be applicable in Norwegian gillnet fisheries. There are also sociopolitical reasons for why a local test is necessary – it may be the only way that local stakeholders can be persuaded or convinced of the appropriateness of using pingers in their fisheries, which is critical for compliance to regulations on pinger usage, and ultimately, for the success of bycatch mitigation using pingers. The pinger trials reported in Paper 2 also present a novelty: a description of time-costs associated with pinger use, from the perspective of the fishers. The time cost of using pingers is an important aspect of their use, and data on time costs can help in finding optimal and well-balanced management solutions that includes the fisher’s perspective.
Our pinger trials were conducted using eight commercial fishing vessels fishing with bottom- set gillnets, targeting mostly either cod, saithe, or monkfish in areas associated with high levels of harbour porpoise bycatches. Fishers used pingers on their gillnets every other week. In this way, each vessel served as its own control. Haul-level catch data from the pinger trials were fit to a bycatch GAMM in a similar fashion to the GAMM used in Paper 1, but using a Poisson distribution and a different model definition, since the purpose of this model was to explain the effects of the different covariates (in particular the effects of the pingers) on modeled harbour porpoise bycatch rates, rather than predicting total harbour porpoise bycatch for unobserved segments of the small vessel fleet. From 2018 to 2020, eight fishers conducted a total of 735
fishing trips and bycaught 20 harbour porpoises and nine seals, assumed to be harbour seals (Phoca vitulina), which are also known to be frequently bycaught in coastal gillnet fisheries (Moan and Bjørge, 2020). Total fishing effort and catches for each season and fishery are summarized in Table 2. There was no significant difference in fishing effort between nets with and without pingers (F(1,733) = 2.4, p = 0.13), but almost all harbour porpoises (19 out of 20) and six out of nine harbour seals were bycaught in control nets (i.e. gillnets without pingers).
Table 2: Fishing effort, catch and bycatch for nets with and without pingers, for pinger trials conducted in cod, saithe, and monkfish fisheries from 2018 to 2020. Bycatch is given as counts, with HP = harbour porpoise and HS = harbour seal.
Year Fishery Group Trips Net KM days Catch (kg) HP HS
2018 Monkfish Control 55 525.8 5,519 5 0
2018 Pinger 34 503.8 2,708 0 2
2018 Saithe Control 6 12.7 6,481 1 0
2018 Pinger 2 3.3 1,287 0 0
2019 Cod Control 70 177.0 320,737 6 4
2019 Pinger 57 156.0 246,689 0 0
2019 Monkfish Control 67 880.9 29,236 6 2
2019 Pinger 81 699.5 30,822 0 1
2019 Saithe Control 31 78.1 148,087 1 0
2019 Pinger 97 219.3 96,514 1 0
2020 Cod Control 46 35.5 84,577 0 0
2020 Pinger 48 38.9 111,085 0 0
2020 Saithe Control 73 84.9 13,544 0 0
2020 Pinger 68 84.9 14,329 0 0
Total All Control 343 1794.8 608,181 19 6
Total All Pinger 392 1705.7 503,434 1 3
Total All Both 735 3,500.5 871,302 20 9
Modelling results indicated that estimated bycatch rates in control nets increased with increasing fishing effort and decreased with increasing depths, which agrees with results from a previous analysis done of harbour porpoise bycatch rates based on CRF data (Bjørge et al., 2013). The difference in estimated harbour porpoise bycatch rates between pingered and control nets in Paper 2 was highly significant in all three fishery groups, with estimated relative rates of 0.06 in saithe nets and 0.02 in cod and monkfish nets. This corresponds to a reduction in harbour porpoise bycatch rates in pingered nets, when controlling for fishing effort and other
covariates, of 94% to 98%, depending on the fishery. These relative bycatch rate estimates are similar to relative rates reported in Kraus et al. (1997), Gönener and Bilgin (2009), Larsen and Eigaard (2014), and others (Dawson et al., 2013).
Figure 9 shows the time costs associated with different activities on board vessels using pingers that could conceivably be slowed down because of the pingers. Setting gillnets with pingers attached had an average time cost of 1.7 ± 4.2 minutes per set (mean ± SD). Hauling gillnets with pingers attached had an average time cost of 3.9 ± 3.8 minutes per haul. Maintenance (changing batteries, redoing knots, replacing ejected pingers into their casings, etc.) had an average time cost of 7.3 ± 7.4 minutes per week. The weekly on/off cycle had an average time cost of 8.7 ± 12 minutes, but since this was a necessity imposed on the fishers due to the experimental setup, it will not be further considered (cycling would not be necessary in normal fisheries). Time costs for setting and hauling nets were incurred because both activities had to be slowed down slightly to ensure that there were no issues, or to handle any issues that did occur. The main issues encountered were that pingers could pop out of their casings or get entangled in the gillnets. Pinger pop-outs could occur when pingers came on board, either because they were bent over the side of the vessel, or when going through the gillnet hauler.
The issue of pinger pop-outs has been reported before for the banana pinger (Crosby et al., 2013), and instructions from the manufacturer include a note to make sure pingers are not subjected to tension during hauling. In our trials, pop-outs could be partially mitigated by slowing down the hauling machinery as pingers came through, and by attaching pingers to the floatline with some slack. However, if the pingers were given too much slack, they could become entangled in the nets. Pinger entanglements could in turn cause tears and rifts in the gillnets as the nets were handled. This indicates that optimal pinger usage depends on an appropriate attachment.
Figure 9: Boxplot showing extra time costs because of pingers, for different activities. Black and red vertical lines represent medians and averages, respectively.
According to fishery data (chapter 3), a typical Norwegian gillnet fisher conducts 77 hauls per year, while the most prolific gillnet fishers might conduct up to 300 hauls in a year. The