How much of the Western Baltic cod stock is eaten by cormorants?

Birds are an integral part of marine ecosystems. In Danish waters, cormorants (Phalacrocorax carbo carbo) are among the most important, where they shape the food web as top predators. Cormorants feed almost exclusively on fish, which has made them subject to one of the most pronounced human-wildlife conflicts in Denmark, because they pressure a coastal fishery that is already struggling with depleting fish resources.
The coastal fishers have essentially two complaints: 1) depredation on active fishing gears reduces their revenue, and 2) predation is a driver for the poor state of some commercial fish stocks. A key challenge in this conflict is the polarized debate, where fishers and conservationists argue in opposite directions, without much willingness to find common ground. Therefore, there is a strong need for evidence-based management advice, with quantitative measure of the impact cormorants have on the coastal fishery. In this project, we investigate the impacts of cormorants on commercial fish stocks in inner Danish waters. We use the Western Baltic cod (Gadus morhua) as a case study, and quantify the annual cormorant consumption of this stock. Our analysis is based on a time-series of the population size of cormorants feeding in the Western Baltic, and a large dataset of fish otoliths found in pellets over the past five decades. We use the otolith data to estimate a continuous time-series of cormorant diets, predicted from the relative availability of key prey species. We estimate cormorant consumption of Western Baltic cod back in time and compare it with other major fish removers, i.e., seals and the fishing industry.
This study is the first to look at stock level impacts of cormorant predation on fish over a wide span of years in Danish waters. It thus contributes to developing ecosystem based management advice, that acknowledges the current concerns of the coastal fishers.