Cod diet as an indicator of Ctenophora abundance dynamics in the Barents Sea

There is growing scientific interest in gelatinous zooplankton, including Ctenophora, as prey and their role in the ecosystem. Ctenophora are difficult to sample using traditional methods. Fish stomach content data is one possible indicator of variations in the abundance of Ctenophora, although predation by fish on gelatinous zooplankton such as Ctenophora is generally low. We examined Atlantic cod Gadus morhua as epibenthic feeders that are distributed widely in the Barents Sea. More than 340 000 cod stomachs analysed for the period 1984-2014 were used to investigate the occurrence of Ctenophora in the cod diet. Ctenophora was found in about 5% of stomachs analysed, and generally low mass and low frequency of occurrence indicated that Ctenophora is not an important food item for cod. Cod mainly preyed on Ctenophora in the eastern and southern part of the Barents Sea in the autumn to early winter period. The proportion of Ctenophora in the cod diet increased with increasing cod size, especially in recent years, when the number of large cod in the stock increased considerably. An increased amount of Ctenophora in cod diet was observed during more recent years, with higher water temperature. The ongoing warming seems to be favourable for Ctenophora in the Barents Sea. This study showed that lack of data on the amount and distribution of Ctenophora could be partially solved by using predator diets.