Analysis of cod food intake
Baltic Sea cod is under threat, putting a unique and genetically distinct stock at risk of extinction. Since the 1980s, cod catches in the inland sea have fallen sharply, and since the mid-1990s, the number of large cod has been steadily declining. Changes in the cod stock, one of the few top predators in the Baltic Sea, affect the inland sea’s ecosystem.
In recent decades, the cod stock in the Baltic Sea has been gradually depleted, whilst the effects of climate change and eutrophication have made it difficult for the fish to recover. The result is a weakened stock where cod reach sexual maturity at around 20 cm, compared to the normal 40 cm, whilst mortality among juvenile cod and cod larvae has increased dramatically. There is scientific uncertainty regarding the actual causes behind these changes. Experiments have been carried out in which captured cod have been fed, resulting in improved condition and growth, but it remains unclear what is causing the changes in their natural environment. Food shortages can occur in the wild when there are too few prey animals per predatory fish, either because there are too many predatory fish (density-dependent food shortage) or because the abundance of prey animals is too low regardless of the number of predatory fish (density-independent food shortage). Results from a pilot project at the Vega Centre, Swedish Museum of Natural History, suggest a reduced basal metabolic rate in cod and thus that starvation is not the only explanation for the decline in cod productivity.
The project aims to analyse the causes of the decline, poorer growth and health status of Baltic Sea cod. The project will investigate the development of Baltic cod growth conditions using new otolith chemical methods during the period when the stock showed positive population trends (between the 1920s and the 1980s) and compare this with developments in recent decades.
Otoliths are the fish’s ear stones. Just like the annual rings of a tree, otoliths grow radially throughout the fish’s life, providing a timeline of chemical information. In total, the project will carry out chemical analyses on approximately 100 otoliths, which will complement results from previous studies and provide unique comparative data on cod development in recent decades.
The project is being carried out by BalticWaters in close collaboration with Stockholm University and is funded by Thuréus Forskarhem and Naturminne.