Food overfishing - the cod fishery in the Baltic Sea through the eyes of aquaculture
A common goal of fisheries management and conservationists is to conserve natural resources productively or to restore the resources to this state. Fisheries management must furthermore ensure that fishing enterprises can operate profitably. In this respect, a lack of recruitment and discarding of small fish is a problem, but also an obvious contradiction and it is assumed that significantly higher yields would be possible. In order to maintain or achieve productivity, feed production of the ecosystem must be or be brought in line with feed consumption of the target stock. This was not the case in the eastern Baltic Sea in particular and there could be observed a density-dependent reduction in the growth rates. An improvement of the food supply in the sea can be achieved by reducing the number of recruits, an established method which is applied by analogy also in agriculture, gardening and forestry. Recruitment management allows the remaining part of the recruits to grow at the maximum rate, ensuring that comparatively little waste is generated. The reduction of recruits by discarding small fish is currently carried out in conjunction with the catch of the target species. In order to achieve the maximum sustainable yield (MSY) the fishing mortality was reduced and the mesh size was increased. This, however, increased also the food requirements of the fish remaining in the sea. Curiously, the food requirement is not taken into account in the model usually used to optimise the fishing pattern. As a consequence the model predicts "ghost yields" which cannot be realized in practice due to the lack of food. Here, two "MSY fishing patterns" are compared with two fishing patterns which, according to conventional definition cause significant growth overfishing. The comparison shows that about three times the amount of feed is required per kg of fish produced if the stock is managed according MSY-criteria. If one compares the fishing pattern currently used with a proposed fishing pattern, minor disadvantages in terms of yield, but advantages in terms of revenue can be expected. The large mesh sizes used in this pattern make the so-called high-grading obsolete, but a reduction in recruits to around 50% is imperative. Therefore, it is proposed in addition to separate the reduction of recruitment from the actual fishing process in order to avoid that each individual fisherman has to process the unwanted small fish on his cutter.