4.1 Feeding and growth of capelin and polar cod

Interactions, drivers and pressures 2016
Typography
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Capelin

Ten years (2006-2015) of capelin diet was examined from the Barents Sea where capelin is a key forage species, especially of cod (Gadus morhua). The PINRO/IMR mesozooplankton distribution shows low plankton biomass in the central Barents Sea, most likely due to predation pressure from capelin. In the Barents Sea, a pronounced shift in the diet from smaller (<14 cm) to larger capelin (≥14 cm) is observed.

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. The migration of capelin into northerly areas (>80°N) was observed until 2013 due to larger ice free area, which may give capelin more access to the arctic zooplankton. However, since 2014 the northern limit of the capelin distribution is moved southwards, this is probably related to the decrease in the capelin stock.

Figure 4.2.2 shows that the growth of capelin has decreased from 2009 onwards in a similar way as in previous periods of relatively high capelin abundance (1990–1992, 1998–2002). There was a corresponding decrease in stomach fullness of capelin from 2009 onwards. These trends were reversed after the capelin stock started to decrease in 2014, and weight at age and stomach fullness is now both at relatively high levels.

Slow growth is generally associated with slow maturation (since capelin matures according to size rather than age), which is indicated by relatively large proportion of age 3 fish and also some age 4 fish (Figure 3.5.6). The slow growth during the period from 2009–2014 may have been associated also with some individuals maturing at smaller size. During the Norwegian capelin fishery during winter-early spring the proportion of capelin of body length below 14 cm increased from <5% in 2011–2014 to 15% in 2015.

Figure 4.1.1. Stomach fullness of capelin during survey in August-September 2006–2015. Number of fish sampled each year in brackets.Figure 4.1.1. Stomach fullness of capelin during survey in August-September 2006–2015. Number of fish sampled each year in brackets.

The decrease in individual growth rate and condition of capelin observed until 2014 for the large capelin stock in that period may have been caused by reduced food availability due to strong grazing on the largest plankton organisms. This is suggested by reduction of the largest size fraction (>2 mm) in the Norwegian part of the autumn survey (see section 3.3). The plankton species composition in the northeastern area has changed; abundance and biomass of large copepod species (Calanus finmarchicus, C. glacialis), which are important prey items for capelin, decreased in the last years with increasing abundance of small copepods (Pseudocalanus minutus) which are practically not eaten by capelin. The change in the composition of the plankton community is most likely caused by warming in the Barents Sea and high grazing pressure from capelin and other species.

Figure 4.1.2 Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of capelin in 1973–2016.Figure 4.1.2 Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of capelin in 1973–2016.

Capelin growth depends on the state of the plankton community (Skjoldal et al., 1992; Dalpadado et al., 2002; Orlova et al., 2010). Capelin is able to produce a strong feedback on zooplankton stock levels through predation (Figure 4.1.3, Dalpadado et al., 2003; Stige et al., 2014), which has been found previously 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 1984–2016.Figure 4.1.3. Fluctuation of capelin stock and zooplankton biomass in the Barents Sea in 1984–2016.

There is some evidence of a density-dependent effect on capelin growth. This is reflected in decreasing individual fish length of 2- and 3-years old capelin with increasing capelin numbers (Figure 4.1.4).

Figure 4.1.4. Average length as function of abundance for capelin at age 2 and 3. The datapoint from 2016 is marked in red.Figure 4.1.4. Average length as function of abundance for capelin at age 2 and 3. The datapoint from 2016 is marked in red.

Capelin length significantly decreases with increasing numbers, which indicates density-dependent growth (Figure 4.1.4). The effect is most pronounced in 2 year old capelin, and not seen in 1-year-olds. Density-dependent growth in capelin has been investigated previously, but should be investigated in more depth in light of more recent data.

Polar cod

The diet data from 2007 to 2015 indicate that polar cod mainly feed on copepods, amphipods (mainly hyperiids Themisto libellula, occasionally gammarids) and euphausiids, and to a lesser degree on other invertebrates (Figure 4.1.5). The consumption of amphipods has decreased somewhat since 2010. Large polar cod also prey on fish. The total stomach fullness index decreased after 2011 but now seems to have stabilized at a lower level than in 2007–2011, while growth of polar cod has been relatively stable (Figure 4.1.6).

Figure 4.1.5. Stomach fullness of polar cod during survey in August-September 2007–2015. Number of fish sampled each year in brackets.Figure 4.1.5. Stomach fullness of polar cod during survey in August-September 2007–2015. 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 1986-2016.Figure 4.1.6. Growth (weight at age from ecosystem survey) and stomach fullness (TFI) of polar cod in 1986-2016.

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