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Lack of oxygen and starvation behind slim cod in the Baltic Sea

Published: 26 October 2016
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Under the latest twenty years the condition of cod has deteriorated in the Baltic Sea. A new study shows that the bad condition has two main causes: lower availability of sprat and an increased extent of low-oxygen bottoms.

The deterioration of cod condition, i.e. body weight in relation to length, has been observed since the mid-1990s and it has been heavily debated due to the important consequences for both the fishery and the ecosystem. An increasingly high number of thin and bony cod has been found during this period.

With the use of biological data from standardized sampling and hydrological observations, a research group led by SLU Department of Aquatic Resources (SLU Aqua) has now shown that the lack of oxygen and lower availability of food can explain the bad condition of the Baltic cod.

– The population of sprat, the main fish prey species for cod, has decreased in the areas where cod is currently concentrated. We also see a fivefold increase of low-oxygen bottoms, which caused a reduction of benthic organisms, say Michele Casini, professor at the Department of Aquatic Resources.

With the increase in the hypoxic bottoms, the areas where cod can live is reduced, causing habitat changes as well as crowding and higher competition.

From a management perspective, these new results are important since they provide new insight on the factors affecting the Baltic Sea cod. According to the researchers, to assure the profitability of the future cod fishery it is very important to reduce the nutrient inputs in the Baltic Sea to dampen eutrophication and combat hypoxia.

– Our study supports also the fisheries advice from the International Council for the Exploration of the Sea. We should limit the fishery on pelagic species such as sprat in the main cod distribution area to prevent cod starvation, say Michele Casini.

More information
Prof. Michele Casini, Department of Aquatic Resources, SLU, michele.casini@slu.se , +46-10-478 40 16

Scientific article

Casini M. et.al. "Hypoxic areas, density-dependence and food limitation drive the body condition of a heavily exploited marine fish predator", Royal Society Open Science

http://rsos.royalsocietypublishing.org/content/3/10/160416

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