Feeding, growth, and maturation of cod

Photo: Frederik Broms, NPI.

Interactions, drivers and pressures 2020
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Cod is the major predator on capelin; although other fish species, seabirds and marine mammals are also important predators. The cod stock abundance in the Barents Sea peaked around 2013 and have declined since, although it is still above the long-term average. The cod spawning stock and thus the abundance of old, large fish is still high.

Feeding, growth, and maturation of cod

Figures 4.2.1 and 4.2.2 show the consumption and diet composition of cod.

Figure 4.2.1. Cod consumption 1984–2020. Consumption by mature cod outside the Barents Sea (3 months during first half of year) not included. Norwegian calculations, from AFWG 2021. Figure 4.2.1. Cod consumption 1984–2020. Consumption by mature cod outside the Barents Sea (3 months during first half of year) not included. Norwegian calculations, from AFWG 2021.

Figure 4.2.2. Cod stomach content composition in the Barents Sea in 1984–2019, by weight (aggregated over all size groups). NOT UPDATED with 2020 data. Figure 4.2.2. Cod stomach content composition in the Barents Sea in 1984–2019, by weight (aggregated over all size groups). NOT UPDATED with 2020 data.

Estimated biomass of capelin consumed by cod in recent years has been close and in some years above the biomass of the entire capelin stock (Figure 4.2.3). Abundance levels of predators other than cod are also high and, to our knowledge, stable.

Figure 4.2.3 Size of the capelin stock and estimated consumption of capelin by cod. Note that the capelin biomass is estimated in September and may not be representative for the biomass available for cod during the year when year-to-year variability is high. Figure 4.2.3 Size of the capelin stock and estimated consumption of capelin by cod. Note that the capelin biomass is estimated in September and may not be representative for the biomass available for cod during the year when year-to-year variability is high.

Estimated consumption of capelin by cod during first and second parts of the year has indicated different temporal patterns. Consumption during the 1st and 2nd quarters has been high during earlier periods and includes consumption during the spawning period, and during spring and early summer prior to seasonal capelin feeding migrations. During the last decade, however, a major difference has been the pronounced increase to a much higher level of consumption in the 3rd and 4th quarters (Figure 4.2.3). This reflects the northward movement of cod stock, and a larger spatial overlap between cod and capelin under the recent warm conditions.

Figure 4.2.4 Acoustic estimates of polar cod compared to consumption of polar cod by cod and % of polar cod in cod diet, 1986-2020. Figure 4.2.4 Acoustic estimates of polar cod compared to consumption of polar cod by cod and % of polar cod in cod diet, 1986-2020.

Figure 4.2.4 shows that there generally is a reasonable correspondence between the proportion of polar cod in the cod diet and acoustic estimates of polar cod.

Figure 4.2.5. Cod consumption by shrimp and shrimp catch vs. biomass estimates of shrimp (ICES NIPAG 2020). Figure 4.2.5. Cod consumption by shrimp and shrimp catch vs. biomass estimates of shrimp (ICES NIPAG 2020).

The estimated biomass of shrimp in the Barents Sea peaked in 2018 and then decreased. The trends in shrimp biomass are to some extent reflected in the cod diet (Fig. 4.2.5). Note that the proportion of shrimp in the cod diet in 2019 was the highest since 2007.

Figure 4.2.6. Cod diet composition in the northern part of the Barents Sea during the ecosystem survey in August–September 2011–2019. Red dots indicate capelin, and blue dots polar cod. Figure 4.2.6. Cod diet composition in the northern part of the Barents Sea during the ecosystem survey in August–September 2011–2019. Red dots indicate capelin, and blue dots polar cod.

Capelin is the main prey item for cod. High or low stock biomass of capelin affect the biological state of cod.

During the first capelin collapse (1985–1989) the importance of capelin in cod diet decreased from 53% in 1985 to 20–22% (maximum) 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 capelin diet and redfish which constituted 3–18%.

During the second capelin collapse (1993–1997), the proportion (by weight) of capelin in the cod diet was high during the first 2 years (47% and 30%), followed by a decreased to 6–16%. During 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 (1–12%), but the proportion of hyperiids consumed was still much lower than during the first collapse.

During the third capelin collapse (2003–2006), consumption of capelin by cod was rather high (10–26%). Several alternative prey groups were present in the cod diet in similar quantities: juvenile haddock (6–11%) and cod (5–10%); herring (3–11%); blue whiting (1–5%); and hyperiids (1–12%). Consumption of capelin by cod during the most recent years has remained somewhat stable (17–31%) but has been much lower than during earlier periods of high capelin abundance (average 36–51%). In recent years, a relatively diverse cod diet has been recorded: with stable high consumption of juvenile cod and haddock (6–11 and 5–11%, respectively); other fish species (11–15%); and other food types (21–33%) (mainly ctenophores and crabs).

Investigations of cod diet in the area north of 76°N showed different types of feeding intensity in three different local areas (Dolgov and Benzik, 2014). Cod feeding intensity was low (149–1690/000) in areas near western and southern Spitsbergen — where cod feed on non-commercial fish. Other local areas were characterized by high feeding intensity (MFI 214-251-169 0/000) with capelin as dominant; non-target species (snailfish and sculpins), polar cod, and hyperiids were also consumed. These two are traditional areas of cod distribution during summer. The third area (Franz Josef Land, northern Novaya Zemlya, and adjacent areas) has become available habitat for cod only since 2008; in this area, cod (MFI 284-340 0/000) feed intensively on polar cod and capelin. Northward expansion of cod distribution, and their movement into northeastern Barents Sea results in better feeding conditions for cod under their high stock biomass and decreasing of main prey (capelin and polar cod). However, cod intensively fed on capelin and polar cod in 2015-2018 despite their low stocks (Figure 4.2.5). In 2019, consumption of these important prey was much lower and occurred in rather restricted areas in the norther Barents Sea compared to previous years.

In addition, some new prey items have recently appeared in the cod diet. The non-indigenous snow crab (Chionoecetes opilio) has become a rather important prey item for cod, especially in for large cod in the eastern Barents Sea alongside Novaya Zemlya (Dolgov and Benzik, 2016, Holt et al., 2021). The percentage (by weight) of snow crab in the cod diet sharply increased from 2014 onwards (Figure 4.2.6). In contrast, two other non-indigenous crab species (red king crab and deep-water crab Geryon trispinosus) have not become more important in the cod diet. The difference is probably related to higher overlap between cod and snow crab, and more appropriate body shape and size of snow crab than the other crab species as prey for cod.

Figure 4.2.7. Importance of snow crab in cod diet (% weight of total consumption) in 1984–2020. Based on Norwegian consumption calculations. Figure 4.2.7. Importance of snow crab in cod diet (% weight of total consumption) in 1984–2020. Based on Norwegian consumption calculations.

Weight at age for young cod as measured from the winter survey has decreased in recent years (Figure 4.2.8) and was in 2021 the lowest observed or very close to that for age groups 3-7. The decreasing trend in growth seems to have been halted now, however. Concerning consumption, the biggest decrease in per capita consumption was observed for age 2 and this corresponds well to the decrease in growth from age 2 to 3 as measured from the winter survey (Figure 4.2.9). Maturity-at-age for cod decreased considerably in 2015-2016 but then increased again. The 2021 values are however somewhat below the level observed in 2000-2010 (Figure 4.2.10).

Figure 4.2.8. Cod weight at age 3-7 as calculated from the winter survey, for 1994-2020. Figure 4.2.8. Cod weight at age 3-7 as calculated from the winter survey, for 1994-2020.

Figure 4.2.9. Cod consumption at age 2 vs growth from age 2 to 3.  Figure 4.2.9. Cod consumption at age 2 vs growth from age 2 to 3.

Figure 4.2.10 Maturity-at-age for cod ages 6-9 (ICES 2021). Figure 4.2.10 Maturity-at-age for cod ages 6-9 (ICES 2021).

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