Diets of the Barents Sea cod (<i>Gadus morhua</i>) from the 1930s to 2018

Abstract

Abstract. A new dataset on the diet of Atlantic cod in the Barents Sea from the 1930s to the present day has been compiled to produce one of the largest fish diet datasets available globally. Atlantic cod is one of the most ecologically and commercially important fish species in the North Atlantic. The stock in the Barents Sea is by far the largest, as a result of both successful management and favourable environmental conditions since the early 2000s. As a top predator, cod plays a key role in the Barents Sea ecosystem. The species has a broad diet consisting mainly of crustaceans and teleost fish, and both the amount and type of prey vary in space and time. The data – from Russia, Norway and the United Kingdom – represent quantitative stomach content records from more than 400 000 fish and qualitative data from 2.5 million fish. Many of the data are from joint collaborative surveys between Norway and Russia. The sampling was conducted throughout each year, allowing for seasonal, annual and decadal comparisons to be made. Visual analysis shows cod diets have changed considerably from the start of the dataset in the 1930s to the present day. There was a large proportion of herring in the diets in the 1930s, whereas in more recent decades capelin, invertebrates and other fish dominate. There are also significant interannual asynchronous fluctuations in prey, particularly capelin and euphausiids. Combining these datasets can help us understand how the environment and ecosystems are responding to climatic changes, and what influences the diet and prey switching of cod. Trends in temperature and variability indices can be tested against the occurrence of different prey items, and the effects of fishing pressure on cod and prey stocks on diet composition could be investigated. The dataset will also enable us to improve parametrization of food web models and to forecast how Barents Sea fisheries may respond in the future to management and to climate change. The Russian data are available through joint projects with the Polar Branch of the Russian Federal Research Institute of Fisheries and Oceanography (VNIRO). The UK and Norwegian data (Townhill et al., 2020) are being released with this paper at https://doi.org/10.21335/NMDC-2139169383.

Empirical evidence of nonlinearity in bottom up effect in a marine predator–prey system

“…The NEA cod is a predator of the BS capelin as shown by diet studies [19,20] and we indeed found a negative effect of cod on capelin stock, similar to previous findings [35]. Conversely, capelin abundance is expected to have a positive effect on cod stock [36,37] and our results also support the claim.…”

“…This high plasticity in its diet may explain our result that cod populations are not affected by capelin abundance when the latter is under a relatively high threshold of 209 billion individuals (note that the median capelin abundance during the studied period is 227 billion individuals, data ranging from 14 billion to 1016 billion individuals). In addition, a low capelin abundance has been associated with high herring Clupea harengus abundance, another major predator of capelin larvae in the BS [35,41] that is also part of the cod diet [19,20].…”

Stock collapse and its effect on species interactions: Cod and herring in the Norwegian‐Barents Seas system as an example

“…Conversely, recent works on historical stomach's contents of NEA cod in the Barents Sea indicate that before the collapse, the herring was much more abundant than capelin in the cod diet (Townhill et al, 2015(Townhill et al, , 2021. Such diet difference is another indication of the change of the link herring-capelin-cod after the collapses.…”

“…Since 1984, the NEA cod diet (fish sized 20–90 cm) is composed on average of only about 3% of herring and about 33% of capelin (Holt et al, 2019). Conversely, recent works on historical stomach's contents of NEA cod in the Barents Sea indicate that before the collapse, the herring was much more abundant than capelin in the cod diet (Townhill et al, 2015, 2021). Such diet difference is another indication of the change of the link herring‐capelin‐cod after the collapses.…”

“…In addition, a recent work shows a relatively little predation of cod on herring in the Barents Sea (Holt et al, 2019) which fit with an expected predation pressure relief after the cod stock collapses (e.g., for Gulf of Maine–Georges Bank Atlantic herring; Overholtz & Link, 2007). Furthermore, the change of cod diet toward a higher capelin diet after the 70s (Townhill et al, 2015, 2021) may also have led to a predation pressure relief.…”