Species interactions

The seal population declined in the first part of the century and since then cod has been the only major fish predator in the system. Predation by sea birds and also salmon have been assessed to have only limited influence on the utilized fish stocks. Cod in the Baltic mainly feed on clupeid and invertebrates. Other fish species including the cod itself constitute other but minor parts of the cod diet.

Predation mainly affects herring below 20 cm. Larger herring i.e. age groups 2 and 3 are also preyed upon but to a lesser extent. Because of its smaller size all age groups of sprat are equally preyed upon. Cannibalism by cod on cod mainly affect age group 0 and 1 as prey, while the effect on older age groups is negligible. Biomass losses due to predation by cod vary with cod abundance.

Thus, the impact of a relatively small cod stock on the overall biomass of herring and sprat has been limited since the middle of the 1980s (i.e. less than the assumed mortality from other natural causes). However, an increasing cod stock has the capability of reducing the sprat stock to considerably lower levels. For example, the yearly consumption in the beginning of the 1980s was estimated by the Baltic Multi Species Model (MSVPA) to 33-86% of the standing stock.

A reduction of the sprat stock, independently of whether it is caused by high predation or recruitment failure, will probably result in an increased predation pressure on young herring or perhaps also on cod. Even if the biomass consumed is not substantial, this predation on juveniles in terms of numbers will be extensive.

Cannibalism has been suggested as a strong self regulatory mechanism in the eastern Baltic cod and to a lesser degree in the western Baltic cod. In addition to cannibalism, predation on early life stages of cod by clupeoids has been identified as a process influencing the recruitment success of cod.

The increase of the seals populations are also leading to increased predation on mainly salmon, sea trout, herring and whitefish. This creates problems for coastal fishery communities because seals empty and destroy gears.