Were macrobenthic changes induced by thinning out of flatfish stocks in the Baltic proper?

Abstract

Possible causes of longterm increase in macrobenthic biomass in the Baltic proper are discussed.

Eutrophication by means of nutrient enrichment is shown to be a process of the last three decades. It is assumed that primary production has doubled during this time. Macrobenthic data from the southern Baltic indicate highly increased biomass values above the halocline between the 1920s and 1950s. During the last three decades no biomass increase was found. Because of these facts, eutrophication is rejected as a major cause of increased macrobenthic biomass.

Flatfish populations in the southern Baltic were very dense before the 1920s. This resulted in stunted growth due to strong competition for food. After heavy exploitation in the 1920s and the early 1930s, these stocks were thinned out and showed significantly higher growth rates due to the reduced competition. In the late 1930s the catches of cod increased markedly.

Based on evidence from studies of predator-prey relationships it is proposed that the increased macrobenthic biomass values are due to the decreased grazing pressure following the exploitation of the flatfishes. Furthermore, it is assumed that the increase of the stock of cod in the 1930s was due to reduced food competition between young cod and flatfishes, especially the dab.