The cod capelin interaction in the Barents Sea: spatial dynamics in predator prey overlap and functional response

What are the factors determining cod's capelin consumption in the Barents Sea? In the CODFUN project, we have found some of the answers to this question - answers that we need to be able to fish capelin sustainably - also in the future. The Barents Sea is huge - four times the size of mainland Norway. This is the home of some of the largest fish stocks in the world, including Northeast Arctic cod. In the Barents Sea both bottom depth, sea temperatures and the abundance of species composition of cod prey varies geographically. Capelin - a small shoal fish is easy to digest and has a high fat content. Capelin is therefore great food for cod. However, cod prefer staying close to the seabed, whereas capelin mostly live higher up in the water column, and therefore may not be so easy find and eat. In the Barents Sea, there are large differences in light conditions, temperature and ice cover throughout the year. Both capelin and cod migrate north in the summer to find food and south to spawn and escape the ice in winter. Seasonal variation, geographical variation and variation between years - where, when and how much capelin consumes is not constant. In CODFUN, we have linked theoretical models of feeding behavior with large amounts of data collected on cruises over the last 15 years to gain insight into this variation. Large populations need large areas to sustain themselves. Therefore it is mainly the stock sizes of cod and capelin that determine how they are distributed geographically and how much they overlap in space. In recent years there has been a lot of cod in the Barents Sea. During this period, cod and capelin have overlapped in two areas during the winter months - in the north, where immature capelin over-winter, and along the coast of Finnmark and Russia, where mature capelin are spawning. In the autumn, cod has been distributed throughout the Barents Sea, while capelin has been concentrated in a smaller area east of Svalbard. In the autumn, no capelin is found in cod-free zones, which means that all capelin may be eaten by cod. However, within the overlap area in autumn, capelin consumption vary between individual cod, and somewhat surprising the total amount of capelin around each cod is not so important. Rather, the physical conditions play a crucial role in determining how easily cod may capture capelin. On banks and bank slopes there is enough light at the bottom during the day allowing cod to see capelin. On the banks cod stomach has a high capelin content and here the density of cod is high. At dawn, when light start penetrating the water column and before capelin forms shoals, capelin migrates towards the seafloor. At this time of day cod can easily feed on capelin without having to move more than a few meters up from the bottom. The distribution area of capelin in autumn is far north, and the water is cold, only around 1 ° C. Since cod has the same body temperature as the environment, digestion is very slow. Most likely, cod do not feed every day, but spend most of the time digesting. The cold water, bottom depth and capelin behavior these factors create variation between the cods and limit how much capelin each cod eats. CODFUN has given new insight into the distribution patterns of capelin and cod and cod's feeding behaviour relevant for fisheries management. Today, the amount of large capelin is measured on a cruise in the autumn. Thereafter, the proportion of capelin surviving until spawning in the spring is estimated, and from this a capelin fishing quota advice is given. CODFUN has generated knowledge can be used to improve and update the mathematical model used to calculate how much capelin the cod stock consumes. It is a goal

Resultater fra CODFUN vil utgjøre en del av kunnskapsgrunnlaget for oppdatering av modell som brukes til kvoterådgiving for lodde i Barentshavet. Kompetanse og metodikk utviklet i CODFUN åpner opp nye muligheter til å bruke bredbåndsakustikk for å studere predator-byttedyr interaksjoner i marine økosystemer.

Cod and capelin are abundant species with significant influence on the Barents Sea ecosystem dynamics and fisheries, as they form a tight trophic link and sustain large fisheries. In short-lived species like capelin, natural mortality caused by predation is high and variable, influencing optimal harvesting. However, in natural systems, predation mortality is difficult to quantify. The number of prey eaten by each predator, together with the abundance of predators and prey, determine natural mortality due to predation. In natural heterogenous environments, abundances of prey and predators vary locally. Therefore spatial overlap between prey and predators becomes a very important factor determining the probability of being eaten. Ecological processes are typically scale dependent and predator-prey overlap may change depending on the scale of observation. By ignoring the local variation in overlap, or extrapolating across scales without accounting for spatial heterogeneity, the estimates of natural mortality at the population level may be biased. In this project we propose to use a combination of state-of-the-art ecological statistical modelling, multi-scale retrospective analysis, and a small-scale field study to investigate the capelin-cod interaction at different spatial scales. In addition to an increased understanding of a fundamental Barents Sea ecosystem process, we expect to be able to use the results to improve the formulation of natural mortality included in the existing capelin assessment model for the Barents Sea.