4.1 Feeding and growth of capelin and polar cod

Polar cod. Photo: Fredrik Broms, Norwegian Polar Institute

Interactions, drivers and pressures 2018
Typography
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Not updated as 2017 data for diet of capelin and polar cod were not yet available

Capelin

Eleven years (2006–2016) of capelin diet were examined from the Barents Sea where capelin is a key forage species, especially of cod. The PINRO/IMR mesozooplankton distribution shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin and other pelagic fish. This pattern was also observed in 2017. In the Barents Sea, a pronounced shift in the diet from smaller (<14 cm) to larger capelin (≥14 cm) is observed. With increasing size, capelin shift their diet from predominantly copepods to euphausiids, (mostly Thysanoessa inermis - not shown), with euphausiids being the largest contributor to the diet weight in most years (Figure 4.1.1). Hyperiid amphipods contributed a small amount to the diet of capelin.

Figure 4.1.1. Stomach fullness of capelin during survey in August-September 2006-2016. Number of fish sampled each year in brackets.Figure 4.1.1. Stomach fullness of capelin during survey in August-September 2006-2016. Number of fish sampled each year in brackets.


Capelin growth decreased from 2009 onwards in a way similar to earlier periods of relatively high capelin abundance (1990–1992, 1998–2002) (Figure 4.1.2). 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; abundance and biomass of large copepod species (Calanus finmarchicus and C. glacialis) — which are important prey items for capelin — decreased in recent years. While the abundance of small copepods (Pseudocalanus minutus) — which are not important to the capelin diet — has increased. This change species composition of the plankton community is most likely caused by warming in the Barents Sea, and high grazing pressure from capelin and other species. Compared to 2015, the 2016 abundance of large copepods (C. finmarchicus and C. glacialis) increased, while that of the smaller copepod (P. minutus) decreased slightly. In 2016, increase biomass of C. finmarchicus was observed relative to 2015, while biomass of C. glacialis remained at the same level.

 Figure 4.1.2 Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of capelin in 1973–2017.Figure 4.1.2 Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of capelin in 1973–2017.

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 4.1.3, Dalpadado et al., 2003; Stige et al., 2014); has been found to be particularly pronounced for krill in the central Barents Sea (Dalpadado and Skjoldal, 1996). 

Figure 4.1.3. Fluctuation of capelin stock and zooplankton biomass in the Barents Sea in 1984–2017.Figure 4.1.3. Fluctuation of capelin stock and zooplankton biomass in the Barents Sea in 1984–2017..


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 4.1.4).

Figure 4.1.4. Average length as function of abundance for capelin at age 2 and 3. The data point from 2017 is marked in red.Figure 4.1.4. Average length as function of abundance for capelin at age 2 and 3. The data point from 2017 is marked in red.

Polar cod

Diet data from 2007–2016 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 4.1.5). 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 (Figure 4.1.6). The growth rate of polar cod was low in 2016 and, thus, did thus not reflect the increased stomach fullness observed that year. 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.

Figure 4.1.5. Stomach fullness of polar cod during survey in August–September 2007–2016. Number of fish sampled each year in brackets.Figure 4.1.5. Stomach fullness of polar cod during survey in August–September 2007–2016. Number of fish sampled each year in brackets.

Figure 4.1.6 Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of polar cod in 1986-2017.Figure 4.1.6 Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of polar cod in 1986-2017.

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