In many fisheries, fish, invertebrates and other animals are caught as bycatch and returned to the sea, a practice known as discarding. Most animals do not survive this procedure. Reasons for discarding are various and usually have economic drivers:
· Fish smaller than the minimum landing size
· Quota for this specific species has already taken
· Fish of undesired quality (high-grading)
· By-caught species of no commercial value
Theoretically, the use of modern fish finding technology used to find schools of fish should result in low by-catch. However, if species mixing occurs in pelagic schools (most notable of herring and mackerel), non-target species might be discarded. Releasing unwanted catch from the net (slipping) or pumping unsorted catch overboard also results in discarding.
Discarding of herring in the pelagic fisheries was considered not to be a large problem, with discards below 5%, estimated by onboard observer programmes. In the area considered by HAWG, only two nations reported discards from their fleets in 2004. For those nations, dis-card figures were raised to national landings (based on the spatial and temporal distribution of the fleet), and used in the assessment of North Sea autumn spawning herring (UK/Scotland and Germany, see Section 2.3) and VIaN (UK/Scotland, see Section 5.1.3). All other nations did not report notable amounts of discards of herring in the pelagic fisheries, either because they did not occur, catches were not sampled for discards or difficulties with raising proce-dures. No discard estimates for the total international catch were calculated.
The inclusion of discarded catch is considered to reduce bias of the assessment and thus give more realistic values of fishing mortality and biomass. However, they might also increase the noise in the assessment because the sampling level for discards is usually lower than that for landings (Table 1.7.1, 1.7.2). This is, as for sampling of landings, caused by the large number of different metiers in the pelagic fishery and the difficult to predict behaviour of the fisheries (in terms of target species and spatial and temporal distribution). Raising discard estimates to the national landings might result in a higher bias than an area based estimate of discards from
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the total international fleet, if sampling is insufficient. HAWG therefore recommends that the development of methods for estimating discards be based on a fleet based method, rather than on a national basis.
1.8 Ecosystem considerations, sprat and herring- response to WGRED and SGRESP.
HAWG welcomes the moves within ICES to attempt to reconcile the single species advice within an ecosystem context. It is important when considering ecosystem advice that the qual-ity and robustness of the science is supportable and the advice is able to withstand the rigors of scientific and stakeholder scrutiny. Measures and objectives must be testable and based on high quality science that is defensible to the non-scientific community.
1.8.1 Ecosystem Areas
HAWG considers it productive to break up problems into tractable components and acknowl-edges that ecosystems from different regions vary from each other. However, the setting of rigid boundaries between neighbouring seas worries HAWG.
With regard to the WGRED descriptions of eco-regions, it is obvious that the demarcation of boundaries between regions is problematic and must be based on certain criteria that may be contentious, e.g. boundaries of the North Sea. Some limits of the WGRED eco-regions contra-dict well established definitions, e.g. the separation between North Sea and Baltic. The criteria used for the selection of these regions are not transparent to HAWG.
Even if the suggested eco-regions may fit for some demersal stocks, many pelagic ones mi-grate among areas. In addition currents have an impact on fish distribution, especially for early life stages, and supporting exchange between areas. Thus the classified eco-regions should not necessarily be considered as suitable areas for pelagic management purposes.
1.8.2 North Sea
HAWG notes the comments from WGRED about the decline in sandeel, Norway pout and the copepod Calanus finmarchicus abundance in the North Sea. It also acknowledges that the plankton community in the North Sea has shifted to a dominance of more “southerly” species, as shown by CPR data (Beaugrand et al., 2002, Reid et al., 2003). Both Calanus and juvenile sand eels are common prey of herring and recent evidence from the Baltic has shown that ju-venile herring positively select Pseudocalanus and Temora and avoid eating Acartia (Casini et al., 2004). Acartia is associated with summer blooms and warmer temperatures as shown by Gowen et al (1998).
The individual fish from the strong 2000 year class of herring have been smaller in size and are less mature at age. This suggests that either more slower-growing fish have survived in that year class or that the ecosystem has failed to provide enough food to allow the full poten-tial growth for that cohort i.e. that food has been limiting for that cohort. This cohort grew well up to 1 wr of age.
In terms of the impact of a high biomass of herring on the North Sea ecosystem, some studies are ongoing, but more resources are required to obtain new estimates of stomach contents and feeding by sprat and herring. With low sandeel and Calanus abundances, the herring may well be having a stronger impact than in the previous last 2 decades. However a high biomass of herring may also be providing an alternative prey source to piscivores such as horse mack-erel and Minke whales (Olsen & Holst, 2001) reducing the pressure on sandeel. These last three sentences are very speculative and if the quantitative trophic-complexities of the system are to considered a priority by ICES, more resources need to be spent on understanding the
trophic interactions in the North Sea and developing spatial and temporal models of trophic dynamics in the system.
The production of herring has increased since the collapse caused by overfishing in the 1970s (Figure 1.8.1) and is dominated by the growth in 1wr fish. The methods used to determine these productions are described in WD 21, and based on that of Dutil & Brander (2003). Sur-plus production has been of the order of 700 k tonnes for the last 25 years and the recent posi-tive net production has lead to an increase in available herring biomass in the system.
Little analysis is currently taking place into the relative roles of sprat and herring as ‘sinks of biomass”, predators and prey within the southern North Sea. The interactions of the two spe-cies have been shown to be very dynamic in the neighbouring Baltic Sea (Mollmann & Kos-ter, 2002). With the decline in sandeel and other planktivorous fish, HAWG would support further studies into the interaction and associations (or not) of herring, sprat, anchovy and pil-chard (sardine).
Kattegat and Skagerrak is also considered an important area for herring by HAWG, it supports both local spawning populations and is the major nursery ground for North Sea herring. The impact of the higher saline inflows through this area into the Baltic Sea in recent years on the resident herring populations is at present unknown. Studies presented to HAWG in 2005 about the HERGEN project suggest that salinity may play a role in the genetic integrity of local spawning components.
Most herring fisheries deploy gear that is deployed clear of the seabed. The impact of gravel extraction on the conservation and productivity of herring is still unclear, and there are virtu-ally no studies to provide evidence at present (CM2003/ACFM:17). The limited evidence available at present records no incidences of cetacean mortality due to pelagic trawling (0 catches observed out of 218 pelagic hauls by commercial trawlers from 1999-2004). There are also very few other by-catches of fish, beyond the targeted fisheries of herring, mackerel, horse mackerel and blue whiting.
1.8.3 Celtic Seas
WGRED did not look at the Celtic Seas in great detail, although SGRESP has considered the region. Across the region information on the comparative dynamics of sprat and herring, par-ticularly in the areas used by juveniles, may prove useful to HAWG. Information on the vari-ability in hydrography, and its influence on larval drift may also be of benefit. In the region, there is no evidence to support the likelihood of wide scale catching of cetaceans by vessels targeting herring. As in the North Sea, there is a severe paucity of data on herring feeding and stomach contents.
Within the Celtic Sea itself, HAWG would like information on the trends in planktonic pro-ductivity and recent changes in temperature and related hydrography that may help explain the changes seen in Celtic Sea herring. It should be noted that Celtic Sea herring is the second most southerly population of herring exploited in Europe and thus it may be more effected by sea warming.
Similar requests are made for the continental shelf west of Scotland and the Irish Sea. HAWG would like information on planktonic productivity of the region and any evidence for shifts that coincide with the years of higher herring productivity in the 1970s, particularly in the con-text of increased yield of recruits per spawner.
Factors that may interest SGRESP and WGRED, include the recent change in the maturity at age ogive in Irish Sea herring. In certain years, the proportion mature 1wr fish (almost 2 years old) can be higher than 30%, and in 2004 100% of 2wr fish were mature.
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Despite recent evidence from WESTHER and HERGEN that there is little genetic differentia-tion between the stocks, their phenotypic characteristics and populadifferentia-tion dynamics are differ-ent. A comparison of the relative trends in surplus production indicates that after the collapses due to overfishing in the 1970s, the Celtic Sea shows a very different pattern compared to both the west of Scotland and the Irish Sea stock (Figure 1.8.2, methods in WD21). The Celtic Sea stock appears to have been more dynamic in terms of surplus production (biomass available to fish) than the stocks further to the north.