Brian MacKenzie is convinced environmental factors will affect cod population more than grey seals. (Photo: Jens Rosenfeldt, DTU/Aqua)

Cod, grey seal stocks could recover together: study

Dual management goals to recover cod and grey seal stocks in the Baltic Sea are realistic even if the grey seals will consume more cod, according to a research paper published recently in PLoS ONE.

Cod and grey seals have hardly encountered each other in the Baltic Sea anymore since a heavy decline in cod and grey seal numbers pushed grey seals to the north while the cod stayed in the southern edges.
 
Management plans to increase the cod and grey seal populations could cause the grey seals to disperse further south, and the cod to disperse further north, and bring the species together again. Researchers looked into whether this interaction would harm cod stocks.
 
”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 as there is a chance that the grey seals can harm the cod stocks as it happened in the 1920-30s. Although, back then the grey seal population was much larger than it is now”, said Professor Brian MacKenzie from the National Institute of Aquatic Resources (DTU Aqua) in Denmark, who worked with Margit Eero from DTU Aqua and Henn Ojaveer from the Estonian Marine Institute.

The scientists believe fisheries and climate change will hit cod harder than the grey seals.

In recent years, cod numbers has increased in the Baltic Sea. (Photo: Jens Rosenfeldt)

Around 10 years ago, overfishing, oxygen depletion and decreased salinity drove the cod stock in the Baltic Sea to record-low numbers. But in recent years, they have increased thanks to some good years of cod reproduction and a fishing management plan containing effective regulation of the fisheries.
 
“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,“ explained MacKenzie.
 
Moreover, climate change could thwart cod stocks’ survival because lower salinity will cause cod to have difficulty reproducing, he clarified.
 
Researchers designed some simulations of future scenarios to gauge the effects of climate change, fisheries and seal behaviour on cod.
 
“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,“ MacKenzie stated.
 
These are the first simulations for the cod population in the Baltic Sea incorporating both the increasing population of grey seals and the possibility that climate change will lower water salinity.

Related articles:

- Baltic cod fishery achieves MSC certification 

- EC increases Baltic cod fishing but cuts herring and sprat

By Natalia Real
[email protected]
www.seafood.media

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