Natural mortality of capelin is currently very high. The main predator for capelin is cod. The size of the cod stock is probably a main factor in the decline in capelin stock size. However, the relationship between changes in stock size of cod and capelin is not very strong. Historical data show that the probability of increase of capelin stock to a high level is low when the cod stock is large (Figure 5.2.1).
The 2016 capelin year class was strong at the 0-group stage and preliminary reports from the 2017 winter survey state that 1-group capelin was abundant and widely distributed.
Whether the 2016 year class will be subject to the same high mortality from age 0 to 1 as the 2015 year class is uncertain. However, the low abundance of immature capelin in 2016 indicates that the abundance of mature capelin in 2017 and 2018 will also be low.
Figure 5.2.1. Capelin total-stock biomass vs. cod spawning-stock biomass in previous year. Circle size is proportional to capelin stock biomass the previous year. Capelin data are from the acoustic survey 1973–2015, cod data are from the report of the AFWG 2015 (ICES 2015c).
Warm conditions are expected in the Barents Sea in the coming years, and with that cod will still have a large area available for feeding. However, given that capelin, as their main prey, is now at a low level and not likely to recover to high levels in 2017 or 2018, cod must compensate by feeding more on alternative prey.
During the first capelin collapse, cod were not fully able to compensate the low amounts of capelin with alternative prey and suffered severe growth decline. During the second collapse, cannibalism was high, whereas during the third collapse no negative impact on cod was detected. However, during the third collapse, the abundance of main fish prey and shrimp (Figure 4.2.10) was higher than it is at present, and the cod abundance was lower.
Other fish and other food including benthos have made up about 40% of the recent diet of cod in the Barents Sea. The abundance and productivity of these food sources is not known and the predation impact by cod is therefore difficult to quantify. Long rough dab, Norway pout and small demersal Arctic fish such as eelpouts, blennies and sculpins are potential prey as are various crabs, shrimps and other benthos. The large cod stock, which will likely continued to feed in the northern Barents Sea, may have a large impact on other parts of the ecosystem. It is desirable to document the feeding and predatory impact of cod on these other prey species in order to distinguish that predatory impact from other direct impact, in particular climate change. This is important also for understanding the effect on the cod stock itself.
Hyperiids and juvenile cod and haddock have been important alternative prey for cod in previous capelin collapses (Figures 4.2.1 and 4.2.2). Currently there is very low amounts of hyperiids in the Barents Sea (see section 3.3). Krill is also an alternative prey for cod, and krill biomass appears to stay high. Snow crab and cod have overlapping distribution on the banks of the Barents Sea, and cod have increased their consumption of snow crab as the latter has become more abundant. The cod could have a top down effect and regulate the snow crab expansion.
There are now more large cod in the stock than previously. Large cod could feed on a wider range of prey and swim faster than small cod so they can exploit a larger area for feeding. This might partly counteract some of the potential negative impact of the capelin decline on cod feeding and growth. Compared to the last capelin collapse, the availability of alternative prey at present appears somewhat lower although accurate quantitative estimates are not available.
If cod switch to feed on a larger proportion of juvenile cod and haddock now when capelin abundance is low, recruitment to the stocks of cod and haddock may suffer. In addition, the cod stock is larger than during the previous collapses, potentially increasing the impact. The likely implication is a moderate decrease in individual cod growth eventually affecting the stock biomass.
Polar cod is a key species in the food chain in the Barents Sea and important prey for cod, Greenland halibut, seals, whales and seabirds. Estimation of harp seal diet shows that harp seals alone can consume up to 100–400 thousand tonnes of polar cod during migration to the whelping area in the White Sea during November -December and up to 350 thousand tonnes during northwards migration in July. The significance of polar cod prey increases in summer when parts of the northern and eastern Barents Sea are free of ice and polar cod becomes available for migratory species. Due to predation pressure from a large cod stock that are distributed to the north in the Barents Sea, and the expected continued warm conditions in the Barents Sea in the coming years, the strong 2015 year class of polar cod may be strongly reduced by predation from cod.