The fish in the Baltic Sea influence the whole marine ecosystem and the fisheries management is therefore key, not only for the fishing industry but also for the status of the marine ecosystem in general. Algal blooms and the filamentous algae known to most as the “slime” growing in shallow areas are symptoms of too much nutrients in the water but the amounts of algae are also influenced by fish. By increasing the number of large cod and other big predatory fish species, the amounts of algal blooms and filamentous algae can be reduced. Even if the Baltic states are working intensely today to decrease the leakage of nutrients to the Baltic sea this will take over 50 years to rectify and it is therefore important the fish is managed in a way that algal blooms are minimized.

The key role of fish in the Baltic
Excess of nutrients in the Baltic Sea increases the growth of phytoplankton (algal blooms), phytoplankton are eaten by zooplankton that are the food source for herring and sprat. Herring and sprat are eaten by larger predatory fish such as cod. This means that if there is a lot of herring and sprat who eat zooplankton, there will be no one to eat the phytoplankton and phytoplankton thereby increase, creating algal blooms (Cassini et al., 2008. Multi-level trophic cascades in a heavily exploited open marine ecosystem. Proc. R. Soc. B 275: 1793-1801). It is therefore important for the Baltic Sea ecosystem that large predatory fish such as cod and pike are increased at the expense of sprat and herring rather than the other way around. Algal blooms are a recurring natural phenomenon in the Baltic Sea but the extent of these blooms has increased due to leakage of nutrients from human activities. These blooms also lead to the depletion of oxygen at the sea bottom and thereby dead bottoms which also means increased leaked of nutrients from the bottom. Dead bottoms are also thought to affect feed availability for cod since they feed on bottom animals.

Sustainable fish stocks benefits everybody
Fisheries management in the Baltic are nowadays considered one of the success stories in EU fisheries management. In 2005 several Baltic fish stocks were on the verge to collapse but since then, Baltic countries has agreed on management plans and harvest strategies, they have been restrictive on quota increases and minimized illegal fisheries. This has led to a tripling of some of the fish stocks and fishing quotas have doubled since. The quotas have also become much more stable from year-to-year, allowing the fishing industry to plan and maximize profits. This shows that fish stocks are well as the fishing industry benefit from a good management and that improving stock status does not need to take a long time to accomplish.

Baltic issues
The cod in the Baltic has still a small average individual size and there are comparably few old individuals. The targets for the fisheries management are still not taking the effects on the Baltic environment in account and fisheries management is to a large degree disconnected from the management of the Baltic environment in large. There is also a need for a Baltic discussion on what the target relation between cod, sprat and herring should be. BalticSea2020 has in different ways engaged in these issues and you can read more about our projects here.