Ulf Lindstrøm2,4, Mette Skern-Mauritzen1, Margaret McBride2, Kit M. Kovacs3, Nils Øien1, Christian Lydersen3, Gisli Vikingson5
1Institute of Marine Research, Bergen, Norway
2Institute of Marine Research, Tromsø Department, Tromsø, Norway 3Norwegian Polar Institute, Tromsø, Norway
4Department of Arctic and Marine Biology, Tromsø, Norway
5Institute of Marine Research, Reykjavík, Island
In terms of biomass and consumption, marine mammals are significant players in the northeast Atlantic. Sigurjónsson and Víkingsson (1997) estimated the total annual consumption by 12 cetacean species occurring around Iceland to 6 million tons, corresponding to around four times the total Icelandic fishery landings at the time. Further, Bogstad et al. (2000) estimated that the two most abundant marine mammal species in the Barents Sea, the harp seals Pagophilus groenlandicus and the minke whales Balaenoptera acutorostrata, consumed around 1.8 and 3.4 million tons per year, and were the second and third most important predators in this system in terms of biomass consumed, only surpassed by the Northeast Atlantic cod (Gadus morhua). With diverse diets, also including commercial species such as herring, capelin and cod, marine mammals may interact both directly and indirectly with fisheries. For example, Lindstrøm et al. (2009) found that more minke whales in the Barents Sea resulted in more capelin available for fisheries, due to strong indirect effects on minke whale predation on cod reducing cod predation on capelin. Also, while marine mammals may not deplete prey stocks to critical levels they may impede recovery of fish stocks collapsed due to e.g. overfishing (e.g. Bundy et al., 2009; O’Boyle, Sinclair, 2012). Furthermore, marine mammals can be sensitive to fluctuations in the marine ecosystems, including climate change related changes in prey or habitat availability, potentially increasing their vulnerability to impact from fisheries (e.g. Nilssen et al., 1998; Trites et al., 2007; Bogstad et al., 2015). However, these interactions are system specific, and the potential for direct competitive interactions between marine mammals and fisheries is both related to harvesting intensity, the trophic levels targeted by the fisheries (e.g., large predatory fish, forage fish, zooplankton), the diets and dietary breadth of the mammals, the functional form of marine mammal – prey interactions, and simplicity of ecosystem in terms of number of species and trophic links (e.g. Mackinson et al., 2003; Kaschner, Pauly, 2005, Morissette et al., 2012).
In this study, which is based on a ms (Skern-Mauritzen et al. in prep.), we estimate the prey consumption by 21 marine mammal species (7 seal species, 8 toothed- and 6 baleen whale species) in three areas in the northeast Atlantic: 1. the Iceland shelf, Denmark Strait and Iceland Sea (ICE), 2. the Greenland and Norwegian Seas (GN) and 3. the Barents Sea (BS). A well-established modelling framework (Leaper, Lavigne, 2007; Smith et al., 2015) is used to estimate the prey consumption. Further, the potential competition between marine mammals and fisheries in these areas is assessed in this study using three different properties: 1. Trophic level overlap, 2. Morisita’s overlap index (Krebs, 1999) and 3. Overlap in the Cumulative biomass - TL relationship (Link et al., 2015).
The preliminary results from this study suggest that marine mammals consume ca. 17 mill. ton food annually, which is ca. twice as much food as the average fishery catches in the northeast Atlantic in 2013/2014. The total prey consumption by marine mammals in the ICE, GN and BS areas sums up to ca. 7.5, 4.5 and 5 million tons, respectively. The prey consumption by the marine mammals
varies between areas and groups due to differences in prey preferences and prey availability; baleen whales are by far the most conspicuous predator group in the ICE (~5.2 mill. tons) and GN (~2.2 mill. tons) areas whereas seals dominate the prey consumption in the BS (~3 mill. tons) area. Krill, cephalopods and amphipods are the overall most important prey for baleen whales, toothed whales and seals, respectively. Krill, herring and polar cod are the single most import prey species in the ICE, GN and BS areas, respectively.
Except for herring in the ICE and GN areas and capelin in the BS area the fishery catches exceed the prey consumption by marine mammals. The marine mammal-fishery competition analysis suggests weak and variable competition; there is a significant competition between marine mammals and the fishery in the GN area but not in the other areas. The degree of marine mammal-fishery competition varies between marine mammal groups; the competition between toothed whales and the fishery is strongest. The Cumulative Biomass-TL relationship indicate that the exploitation pattern of marine mammals and the fishery differ.
The preliminary conclusions from this study are: 1. the total prey consumption by the marine mammals varies much between both areas and groups due to differences in prey preferences and prey availability; baleen whales are by far the most conspicuous predator group in the ICE and GN areas whereas seals dominate the prey consumption in the BS area, 2. Krill, cephalopods and amphipods are the overall most important prey species for baleen whales, toothed whales and seals, however, herring and polar cod are the overall most import prey in GN and BS, respectively, 3.
Marine mammals in the northeast Atlantic consume twice as much food as the fishery removes, however, except for herring in the ICE and GN areas and capelin in the BS area the fishery catches exceed the prey consumption by marine mammals, 4. the overall marine mammal-fishery competition analysis, i.e. all marine mammal included, suggest there is a significant competition between marine mammals and the fishery in the GN area, driven by the predation on herring, but not in the other areas, 5. the degree of marine mammal-fishery competition varies between marine mammals; the competition between toothed whale and the fishery appears to be significant in all areas except, perhaps in the Barents Sea, depending on the property used to assess the competition, 6. The three properties display similar competitive picture which is that the exploitation pattern of the marine mammals and fishery catches differ, particularly in the ICE and BS areas.
Finally, to explore the dynamical consequences of various harvesting scenarios on the dynamics of marine mammals and vice versa, we need to include these marine mammal species in ecosystem or multispecies models and run scenarios. The Norwegian-Barents Sea Atlantis model (Hansen et al., 2016), which presently contains nine marine mammal species, will be used to run such scenarios.
Keywords: Nordic and Barents Seas, marine mammals, consumption, fishery interactions
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Supplementary
Table 1. Marine mammal species regularly occurring in the Nordic and Barents Seas, categorized as all year residents (Residents) or summer migrants (Migrants).
Species Resident/
Migrant Body mass, kg Habitat
Pinnipeds
Harbour seals Resident 90 Coastal
Grey seals Resident 200 Coastal
Ringed seals Resident 75 High Arctic
Bearded seals Resident 250 High Arctic
Harp seals Resident 120 Arctoboreal
Hooded seals Resident 250 Arctoboreal
Atlantic walrus Resident 1 200 High Arctic
Odontocetes
White whales Resident 1 350 High Arctic
Narwhal Resident 1 300 High Arctic
Killer whale Resident 4 400 Arctoboreal
Sperm whale Migrant 40 000 Arctoboreal
Lagenorhyncus dolph. Resident 210 Arctoboreal Pilot whales Migrant 1 700 Arctoboreal//Temperate
Harbour porpoise Resident 55 Coastal
Bottlenose whales Migrant 6 000 Arctoboreal
Mysicetes
Minke whales Migrant 6 600 Arctoboreal
Fin whales Migrant 55 500 Arctoboreal
Humpback whales Migrant 30 400 Arctoboreal
Blue whales Migrant 100 000 Arctoboreal
Sei whales Migrant 17 000 Arctoboreal/Temperate
Bowhead whales Resident 80 000 High Arctic