Cod is the main predator on capelin, although other fish species as well as seabirds and marine mammals are also important predators. In the last 6-7 years there has been an extremely high cod stock level in the Barents Sea. Estimated biomass of preyed capelin by cod in recent years has been equivalent to the biomass of the entire capelin stock (Figure 4.2.3). Under good conditions the capelin stock tolerated a high grazing pressure; the biomass produced during the year was equivalent to the standing stock biomass measured in autumn. The abundance of predators other than cod is also at high and, to our knowledge, stable levels.
Figures 4.2.1 and 4.2.2 shows the consumption and diet composition of cod.
Figure 4.2.1 Cod consumption 1984–2016. Consumption by mature cod outside the Barents Sea (3 months during first half of year) not included. Norwegian calculations, preliminary Figures, final numbers to be found in AFWG 2017.
Figure 4.2.2. Cod diet in the Barents Sea in 1984–2016, by weight
Figure 4.2.3 Size of the capelin stock and estimated consumption of capelin by cod.
The estimated consumption of capelin by cod for the first and second parts of the year has shown different temporal patterns. The consumption during the first and second quarters has been high also in previous capelin periods and includes consumption during the spawning period and also the spring and early summer situation before the seasonal feeding migration of capelin. A major difference, however, is the pronounced increase to a much higher level of consumption in the third and fourth quarters during the last capelin period (Figure 4.2.3). This reflects the northward movement of cod and a larger spatial overlap between cod and capelin under the recent warm conditions compared to the situation earlier, e.g. during the capelin period in the 1990s.
Figure 4.2.4. Cod consumption during the ecosystem survey in August-September 2010–2015. Red dots indicate capelin, and blue dots polar cod.
During the first capelin collapse (1985–1989) the importance of capelin in cod diet decreased from 53% in 1985 down to maximum 20–22 % for the remainder of the collapse period. During that period, an increase of other prey was observed, in particular hyperiids which constituted 7–23% of the diet and redfish with 3–18%.
During the second collapse (1993–1997) weight proportion of capelin in the cod diet was high in the first 2 years (47 and 30%), and then decreased to 6–16%. In this period cannibalism in cod increased sharply from 4–11% to 18–26% of the diet. In addition, more intensive consumption of hyperiids was observed again (1–12%), but the proportion of hyperiids was still much lower than during the first collapse.
During the third collapse period (2003–2006), consumption of capelin by cod was rather high (10–26%). During this collapse, several alternative prey groups for cod were present in the diet in similar quantities: juvenile haddock (6–11%) and cod (5–10%), herring (3–11%), blue whiting (1–5%) as well as hyperiids (1–12%). Consumption of capelin by cod during the most recent years has remained more or less stable (17–31%), but has been much lower compared to earlier periods of high capelin abundance (average 36–51%). In recent years, a relatively diverse diet has been recorded with stable high consumption of juvenile cod and haddock (6–11 and 5–11% respectively) as well as other fish (11–15%) and other food (21–33%) (mainly ctenophores and crabs). It should be noted that there has been an increasing amount of snow crab in cod diet – from 0.1–0.3% in 2009–2010 to 1.2–1.5% in 2011–2012 to 6.1 % in 2014.
Investigations of cod diet in the area northwards 76°N showed that three groups of local areas can be separated (Dolgov and Benzik, 2014). Feeding intensity of cod was low (149–169 0/000) in areas near western and southern Spitsbergen, where cod feed on non-commercial fish. Other group of local areas is characterized by high feeding intensity (MFI 214-251-169 0/000) with dominance of capelin and also non-target fish (snailfish and sculpins), polar cod and hyperiids. These two groups of areas are more or less traditional for cod distribution in summer period. The third group (Franz Josef Land, northern Novaya Zemlya and adjacent areas) became available for cod only since 2008. Cod extremely actively (MFI 284–340 0/000) feed here on polar cod and capelin. Thus such northward extension of cod and their intrusion in northeastern Barents Sea results to better feeding conditions for cod under their high stock biomass and decreasing of main prey (capelin and polar cod).
In addition, some new prey items appeared in cod diet. Invasive snow crab Chionoecetes opilio became a rather important prey items for cod, especially in eastern Barents Sea alongside Novaya Zemlya (Dolgov and Benzik, 2016). Weight percent in cod diet sharply increased from 0.1–0.3%m in 2009–2010 to 5.6–6.1%m in 2014–2015 and start to decrease in 2016 (5.2%m) (Figure 4.2.5). In contrast, two other new crab species has no high importance in cod diet until now, their weight percent did not exceed 1.0%m for red king crab and 0.2%m for Geryon spp. probably it related more to overlapping between cod and snow crab and more appropriate body shape and size.
Figure 4.2.5. Importance of three species of invasive crabs in cod diet in 1984–2016.
Growth and maturation
Consumption and growth for younger cod has been fairly stable in recent years (Figure 4.2.6), with a slight decrease for older cod (Figure 4.2.7). However, the proportion mature-at-age 6–9 decreased considerably from 2014 to 2016 (Figure 4.2.8), and the decrease in maturation is much stronger than the change in weight at age should indicate (Figure 4.2.9).
The biomass of the main prey species relative to the cod stock size has decreased somewhat in recent years (Figure 4.2.10). However, the consequences of the fourth collapse of capelin stock in the Barents Sea (2015 and onwards) on cod conditions were so far minor compared to previous collapses. It can be related to northward expansion of cod to the northern Barents Sea with available food resources not used by cod earlier.
So, a combination of using new areas in the northern Barents Sea available for cod feeding as well as switch on new prey item allowed to cod compensate for decrease of traditional prey like capelin and polar cod under recent warming period.
Figure 4.2.6 Cod growth and consumption at age 3 (ICES 2016c).
Figure 4.2.7. Cod growth and consumption at age 6 (ICES 2016c).
Figure 4.2.8. Maturity-at-age for cod ages 6-9 (ICES 2016c).
Figure 4.2.9. Cod maturity and weight at age 7 (ICES 2016c).
Figure 4.2.10. Abundance of major prey stocks and shrimp compared to cod abundance.