Researchers from the Swedish University of Agricultural Science have developed a new method to get answers to why over-exploited fish populations, such as cod in the Baltic Sea, does not recover even though the fishery has declined. They do so by analysing the pattern of growth during various life stages as a mean to see if low recovery rates may be due to lack of particular type of food.
The Baltic Sea has undergone major changes in species composition. In the late 1980s the Baltic Sea underwent a regime shift where it, in an oversimplified explanation, went from being dominated by large stocks of cod to be dominated by large sprat stocks. On Canada’s Atlantic cod stocks collapsed in a similar way a few years later. Both these ecosystems have undergone regime shifts which have been linked to an excessive fishing pressure.
The new study set out to find answer on what it is that prevents the cod to recover and why the Baltic Sea ecosystem is retained in a “sprat state” despite environmental conditions being comparable to those in the 1980s and the reduced fishing pressure.
Researchers at SLU together with Canadian scientist have developed a new method to highlight the issue. It involves analyzing environmental monitoring data showing cod growth patterns and the availability of prey during different life stages.
Anna Gårdmark, associate professor at the Department of Aquatic Resources SLU and one of the lead authors said “We show how to analyze cod somatic growth during different life stages to distinguish between different mechanisms that may underlie why overfished stocks of predatory fish such as the Baltic cod have not been able to recover. It is about different kinds species interactions between cod and its prey”.
The new method is now being applied on the Baltic cod and cod off Canada’s east coast. If the fish-eating cod individuals grow poorly rather than the smaller cod which feed on zooplankton and benthic invertebrates, it indicates a lack of fish prey of the right size for cod. This in turn shows how fishing for predatory fish and fishing on their prey are linked, and why they must be managed together.
“To understand why fishing predatory fish does not increase in number despite low fishing pressure is crucial to create a fisheries management that enables recovery of cod. Our new method is an important tool for achieving such knowledge”, says Anna Gårdmark.
The article: Regime Shifts in exploited marine foodwebs: detecting mechanisms underlying alternative stable states using size-structured community dynamics theory is published in the latest issue of Philosophical Transactions of the Royal Society B with theme Marine regime shifts around the globe : theory, drivers and impacts.