# Food intake of early juvenile western Baltic cod (Gadus morhua) during settlement transition

## TL;DR

This study investigates the diet of young cod in the Western Baltic Sea and finds that their transition from a pelagic to a benthic lifestyle is linked to specific prey like Diastylis rathkei, which may be affected by low oxygen levels.

## Contribution

The study identifies Diastylis rathkei as a key prey species during the settlement transition of cod, linking its availability to cod recruitment success.

## Key findings

- Settlement transition in cod occurs at 46–87 mm total length, marked by a shift in prey preference.
- Diastylis rathkei is a crucial prey species during the early benthic phase of cod.
- Low oxygen levels in the Western Baltic Sea may threaten cod recruitment by reducing D. rathkei availability.

## Abstract

This study examines the gut contents of 203 early juvenile Atlantic cod [17–101 mm ± 18.48 mm standard deviation (SD)] from the Western Baltic Sea (ICES Subdivision 22) collected between 2020 and 2022. According to the observed prey (proportion of pelagic, intermediate and benthic items) in the cod guts, settlement transition from a pelagic to a benthic lifestyle is estimated to take place at 46–87 mm cod total length (TL). Copepod species were the preferred prey item of pelagic feeding juvenile cod, dominated by the genus Acartia, which is also the most abundant copepod genus in the area. With increasing cod size, Centropages spp. and Cladocera species were favoured. Intermediate prey consisted mostly of late bivalve veliger larvae. Although a switch from planktonic to intermediate prey was not observable in every cod individual (probably due to differences in prey availability between years and stations), our results showed that especially at the beginning of the demersal life, all examined cod relied almost exclusively on the Cumacean species Diastylis rathkei. Its importance to cod during the settlement transition is in accordance with earlier findings from the same and adjacent areas highlighting it as potential key, but also bottleneck, species for cod recruitment success. Because D. rathkei is highly sensitive to low oxygen conditions, and oxygen minimum zones are spreading in the Western Baltic Sea, the decreasing access to D. rathkei as prey might be a contributing factor to the low recruitment success of cod in recent years.