Harbour porpoise (Phocoena phocoena) predation on cod (Gadus morhua) in the Kattegat : a review of current knowledge and implications for stock assessments

• Main Author: Wallin, Adam
• Format: Other/Unknown Material
• Language: English; Swedish
• Published: SLU/Dept. Of Aquatic Resources 2017
• Subjects: Atlantic cod; diet; Gadus morhua; harbour porpoise; Kattegat; Phocoena phocoena; prey; atlantic cod; Harbour porpoise
• Online Access: https://stud.epsilon.slu.se/10643/

Kattegat is an area between Denmark and Sweden that connects the Atlantic Ocean to the Baltic Sea. It has long been an important source for fishery of cod (Gadus morhua). In a time where the future of the Kattegat cod stock is uncertain, the relationship between the predator harbour porpoise (Phocoena phocoena) and the Kattegat cod remains relatively unknown. In this study, available data that can be used to assess this prey-predator relationship are compiled and evaluated, and the information gaps that need to be filled are identified and discussed. The most recent harbour porpoise abundance assessment from SCANS-III can be approximately remodelled for the Kattegat area to 23 190 (15 550-30 830) animals, eating an estimated average of 3.7 (1.9-5.5) kg of prey each day. Of this, an estimated average of 22 % (5.5-29.3) in weight is comprised of cod, a number which varies seasonally, annually and depending on age and sex of the predator. These results give a mean total annual consumption of 7 020 (593-18 150) tonnes of cod from harbour porpoises in the Kattegat. When applying harbour porpoise predation to the stock assessment model of Kattegat cod, it is important to have accurate representations of harbour porpoise abundance and diet composition of harbour porpoise from the Kattegat region specifically. As a stock assessment model is to be used to assess potential threats in the future of the Kattegat cod stock, it is important to be able to connect these values to the variations of prey compositions available in the Kattegat, so that the harbour predation can be assessed for the future. Sufficient data is currently unavailable, and the timeframes in which the samples have been collected are too long and too old to be able to take variations over time into account. More samples from the Kattegat and would be required to make an accurate harbour porpoise predation assessment, ideally in timeframes that are comparable to the ongoing assessments of the cod stock in the Kattegat. As such data is not currently being ...