Fourteen years (2006–2019) of capelin diet were examined from the Barents Sea where capelin is a key species both as a prey and predator. The PINRO/IMR mesozooplankton distribution usually shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin and other pelagic fish.
Feeding and growth of capelin and polar cod
This pattern was also observed during 2017-2020. In the Barents Sea, a pronounced shift in the diet from smaller (
Capelin growth decreased from 2009 onwards in a way similar to earlier periods of relatively high capelin abundance (1990–1992, 1998–2002) (Figure 4.1.3). There was a corresponding decrease in stomach fullness of capelin from 2009 onwards. These trends were reversed in 2014; both weight-at-age and stomach fullness are now at relatively high levels.
The decrease in individual growth rate and condition of capelin observed before 2014 for the large capelin stock may have been caused by reduced food availability linked to strong grazing on the largest planktonic organisms; as suggested by reduction of the largest size fraction (>2 mm) in the Norwegian zone during the autumn survey (see section 3.3). Plankton species composition in the northeastern area has changed. Large calanoid copepods, Calanus finmarchicus and C. glacialis are important prey items for capelin. Abundance and biomass of boreal C. finmarchicus and arctic C. glacialis have increased since 2015 providing favourable conditions for capelin feeding. Small copepods Pseudocalanus spp. are one of the most numerous copepods species in the Barents Sea. However, this copepod is not consumed by capelin, likely due to its small size (approximately 2.5 times smaller than C. finmarchicus), though its abundance had been increased up to 2018 and stayed at high level in 2019.
Capelin growth depends on the state of the plankton community (Skjoldal et al., 1992; Dalpadado et al., 2002; Orlova et al., 2010). Capelin produces a strong feedback mechanism on zooplankton stock levels through predation (Figure 4.1.4, Dalpadado et al., 2003; Stige et al., 2014); it has been found to be particularly pronounced for krill in the central Barents Sea (Dalpadado and Skjoldal, 1996).
There is evidence of a density-dependent effect on capelin growth. This is reflected in decreasing length of individual (2- and 3-year old) capelin with increasing capelin abundance (Figure 4.1.5).
Figure 4.1.5. Average length as function of abundance for capelin at age 2 and 3. There are highly significant negative relationships between the variables; adjusted R2 is 0.46 for the two-year-olds and 0.33 for the three-year-olds.
Diet data from 2007–2019 indicate that polar cod mainly feed on copepods, amphipods (mainly hyperiids Themisto libellula and occasionally gammarids), euphausiids, and other invertebrates (to a lesser degree) (Figure 4.1.6). Large polar cod also prey on fish. The total stomach fullness index decreased after 2011, and was at a fairly low level in 2012–2015; the index increased again in 2016 to the highest level measured in this 10-year time-series and remained relatively high in 2017-2019 (Figure 4.1.7). In 2019, the portion of copepods and euphausiids slightly increased, compared to 2018. In spite of increased number of Themisto libellula in the Barents Sea in 2019, the portion of amphipods in polar cod diet slightly decreased compared to 2018.
The growth rate of polar cod was low for age 3 and intermediate for ages 1-2 in 2016-2018 (Figure 4.1.8) and, thus, does not reflect the increase in stomach fullness from the 2012-2015 to the 2016-2018 period. It should be noted that spatial coverage for polar cod is incomplete during most years of the BESS; thus, growth and stomach fullness data may not reflect the status of the entire population. The stomach fullness was the highest north of Spitsbergen and in the northern Barents Sea. The higher proportion stomach filling (high TFI) in the diet specially in 2018 but also in 2019, compared to 2017 may reflect the high abundances of Themisto libellula in the east Svalbard region.