Grey seals do not prevent cod recovery in the Baltic Sea

Years ago, this scenario could be experienced by yachting enthusiasts and others, who were lucky to be out on the Baltic Sea at the right time and place. But today, it is rare that cod and grey seals encounter each other in the Baltic Sea since a heavy decline in cod and grey seal numbers have caused the remaining grey seals to mainly inhabit the northern parts of the Baltic while the cod is to be found in the southern parts.

Management plans have been made in order to increase the cod and grey seal populations in the Baltic Sea. The potential resulting rise in grey seal and cod numbers could cause the grey seals to disperse further south, and the cod to disperse further north. Thus, predator and prey will once again inhabit the same areas.

"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 who with Margit Eero from DTU Aqua and Henn Ojaveer from the Estonian Marine Institute have published their results in the scientific journal, PLoS ONE. Their conclusion is that fisheries and climate change will affect the cod much more than the grey seals.

The grey seal in focus

Around ten years ago, the cod stock in the Baltic Sea hit record-low numbers due to over-exploitation, oxygen depletion and decreased salinity. But in recent years, cod numbers have increased due to some good years of cod reproduction, and a fishing management plan with effective regulation of the fisheries.

"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," says Brian MacKenzie.

Today, cod fishing in the Baltic is at a sustainable level, but it is important to address other ecosystem issues that might affect the cod population in the future. This puts the grey seal in the spotlight:

"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," explains Brian MacKenzie.

Adjust fishing levels

But the grey seal is not the only factor that might affect future cod stocks. Climate change can cause the salinity in the Baltic Sea to decline which will affect the cod's ability to reproduce because their eggs and larvae will suffer from increased mortality. Fishing will also affect cod numbers, but fishing can be regulated according to how many cod there are. It is much more difficult to regulate climate changes and the behaviour of the grey seal. Thus, it is important to adjust fishing levels to other factors affecting the cod stocks. And that can only be done if the other factors are identified.

"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 Brian MacKenzie.

In order to investigate how the cod in the Baltic Sea can be affected by grey seals, climate change and exploitation in the future, the researchers made a number of simulations of future scenarios.

"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," says Brian MacKenzie, who after many hours in front of the computer was able to conclude that the grey seals will not work against the management plans to recover the cod populations. This means that it is possible to increase numbers of cod and grey seals at the same time.

These simulations are the first simulations for the cod population in the Baltic Sea that includes both the increasing population of grey seals as well as the possibility that climate change will decrease the salinity of the Baltic Sea. They indicate that grey seals in the future can get more cod on the menu. Without harming the cod populations.