The summer overlap between cod and capelin has increased, especially in the northern area, mainly due to the increased size of suitable habitat for cod, and the size of the cod stock. There is, however, a low correspondence between changes in horizontal overlap and changes in capelin consumption. The cod-capelin feeding interaction mainly takes place on the banks of the northern Barents Sea, where a vertical overlap with capelin is much more important for explaining variation in capelin consumption than capelin density. The northward expansion of cod has probably also affected the polar cod negatively, since polar cod has become more available to cod.
Cod-capelin-polar cod interaction
The interaction among cod, capelin, and polar cod is one of the key factors regulating the state of these stocks. However, this interplay is far from fully understood. Cod prey on capelin and polar cod and can strongly influence the numbers of these species, while the availability of these species for cod varies. In addition, 0-group cod may also feed on 0-group capelin. A prerequisite for feeding interactions is geographical overlap and both the overlap magnitude and the size of the overlap area will affect the consumption of forage fishes. In a recent analysis, it was found that increasing cod population size and water temperatures have influenced a northward shift of the late summer overlap area (Fall et al. 2018). The overlap between immature cod and capelin showed a rising trend, while the mature cod-capelin overlap had a less clear trend over time (Fig 4.5.1 B). The overlap area also increased in size, particularly for immature cod, which reflects the northeastwards expansion of the cod stock. The overlap was nevertheless relatively low, which was interpreted as a weak aggregative response of cod to capelin. The observed increased overlap over time could be interpreted as facilitating increased consumption of capelin per unit of cod stock in recent years. However, when comparing the amount of capelin consumed by cod in the summer-autumn period with the overlap, there is no obvious relationship (Figs 4.5.1 A-B). Consumption by immatures peaked during a period of average overlap, and for mature cod, the overlap trend is a near inverse of the trend in consumption. Generally, correlations between spatial overlap and consumption have been weak in summer, at several spatial scales (WD 1 by Johanna Fall). Overlap is three-dimensional; two stocks may be overlapped in the horizontal dimension but still be segregated by depth, always or during parts of the day. The key to a better understanding of the feeding process may be the study of vertical overlap. One of the more important predictors of variation in cod feeding in summer was capelin depth distribution (Fall, 2019); more capelin was found in cod stomachs when capelin was distributed closer to the seafloor. Cod consumed more capelin at the Great and Central banks (100-250 m depth), where capelin was distributed closer to the seafloor throughout the diel cycle, than in deeper areas (Fig 4.5.2). An important factor that is likely to influence the distribution pattern of capelin is the near-bottom distribution of zooplankton on the banks (Aarflot et al. 2018). However, even when accounting for variation in capelin depth distribution, changes in capelin density did not have a strong effect on cod consumption. A potential explanation for this is that once capelin is present, it is present in densities that allow cod to reach satiation (Fall and Fiksen 2019).
Figure 4.5.1: Deviations from mean A) capelin biomass, cod biomass, and estimated capelin consumption by cod, B) mean overlap and size of the overlap area, and C) density of cod on the banks across the period 2004-2015. Panel A shows the same data as Figure 4.2.3 divided by cod maturity (age 3-6 were considered immature) with the addition of cod stock biomass. Panel B shows mean overlap and size of the overlap area (overlap extent) from Fall et al. (2018) using predictions from the area north of 74 N. Bank density in panel C was calculated as the average predicted density of cod at 100-250 m depth using the distribution models in Fall et al. (2018).
Given the apparent importance of the banks for the cod-capelin interaction, it is of interest to examine trends in average cod density on these banks over time (Fig 4.5.1 C). Here, we see that the peak in immature cod consumption did not coincide with an increase in cod density on the banks, suggesting that the increased consumption resulted from behavioral changes in individual cod. For mature cod, on the other hand, consumption appears to follow the trend in bank density. In conclusion, there seems to be a weak relationship between horizontal overlap and consumption of capelin by cod in summer, and capelin density is not an important predictor of variation in cod’s feeding on capelin beyond a very low-density threshold. On the other hand, behavioral responses to varying bottom depths by capelin may affect the feeding intensity of cod on capelin. How these findings reflect the feeding dynamics in other seasons should be further explored.
Cod can prey intensively on polar cod. When polar cod and capelin occur in mixed concentrations, which they often do in northern and eastern areas, polar cod may be easier to catch because they may have a lower swimming speed (confirmed by trawl catch analyses) and are distributed closer to the bottom. However, capelin is a fatter and energetically a more valuable prey item. According to the estimated consumption of various prey species by cod (Fig 4.2.1-2) the consumption of polar cod is much smaller than for capelin. Possible under-sampling of cod stomachs where and when the most intense feeding on polar cod takes place may have biased the consumption estimates somewhat, but no doubt the capelin is a much more important prey for cod than polar cod is. The polar cod biomass in the Barents Sea is currently substantially less than the capelin biomass, thus, the effect of cod consumption on the stocks of these two species are different. Besides, the length of the period with cod and polar cod overlap is much shorter (September-December) compared to the overlapping time of cod and capelin. Interspecific interactions in this trophic system are very complex and require more detailed study.