Will grey seals counteract cod recovery in the Baltic Sea?

Grey seals and cod used to be found in great abundance throughout the Baltic Sea, but today the seals are chiefly present in the northern parts of the sea while the cod is found in the south.

Management programs are working on increasing the number of both species and this will likely cause predator and prey to once again inhabit the same region.

Since the grey seal and cod populations could overlap in the future, we investigated whether the management plans to re-establish the populations of cod and grey seals are contradictory since there is a chance that the grey seals can harm the cod stocks as happened in the 1920-30s. Although, back then the grey seal population was much larger than it is now,” says Professor Brian MacKenzie from the National Institute of Aquatic Resources (DTU* Aqua) in Denmark (www.aqua.dtu.dk/English.aspx). DTU is the Technical University of Denmark.

Together with Margit Eero (also from DTU Aqua) and Henn Ojaveer from the Estonian Marine Institute (http://www.sea.ee/en/), MacKenzie has conducted a study where they conclude that fisheries and climate change will affect the cod much more than the grey seals. The study has been published in the scientific journal PloS ONE.

About a decade ago, over fishing, oxygen scarcity and decreased salinity cause the cod populations in the Baltic Sea to plummet to record low levels. The amount of cod is now gradually increasing again, chiefly thanks to a strict fishing management plan and a few good years of cod reproduction.

The environmental conditions of the Baltic Sea are still not perfect, but fishing levels are low at the moment, giving the cod a chance to rebuild the population. This has given rise to an increase in cod numbers during the last four to five years,” MacKenzie explains.

Since grey seals have effected the cod populations in the past, the team of Danish-Estonian scientists wanted to take a closer look at how they may affect a slowly recuperating cod population.

Historically, seals have affected cod stocks, and in many areas they are suspected of being the reason why the recovery of cod stocks has been delayed. Therefore, it was important to determine whether the grey seals could pose a threat to the cod stock in the Baltic Sea,” says MacKenzie.

In order to investigate how grey seals in the Baltic Sea could effect the cod, the researchers made a number of simulations of future scenarios – scenarios that also factored in commercial fishing and climate change. If climate change makes the climate warmer, the salinity of the Baltic Sea would decrease and this would affect the reproductive capacity of the cod. The cod would still reproduce, but the mortality rate for eggs and larvae would be higher.

If the Baltic Sea experiences lower salinity due to climate change, the cod stocks will most likely suffer because the cod will have difficulty reproducing. Furthermore, there will be predation by grey seals. These two ecosystem issues can greatly affect the cod stock so this is why we wanted to find out how large these impacts might be in order to regulate fishing levels accordingly,” says MacKenzie. “Our results show, that fishing and environmental factors like oxygen depletion and decreasing salinity will affect the cod population more than the grey seals in the years to come.”

According to the study, it would be possible to simultaneously increase numbers of cod and grey seals in the Baltic Sea.

For more information, the paper can be found at PloS ONE (http://www.plosone.org)

Brian R. MacKenzie, Margit Eero, Henn Ojaveer. Could Seals Prevent Cod Recovery in the Baltic Sea? PLoS ONE, 2011; 6 (5): e18998 DOI: 10.1371/journal.pone.0018998