Evaluating and optimizing stock enhancement of natural flatfish stock

The overall objective of this thesis is to evaluate and optimize a stock enhancement program of natural population of flatfish in which artificially reared fish are released in coastal areas using turbot (Psetta maxima) as model. Biological and ecological criteria are developed to evaluate the likelihood for success, i.e. increasing the stock size, based on ecological knowledge. Special reference is given to processes involved in the regulation of the natural population, such as density-dependent and density-independent mortality and growth. Field data on the temporal and spatial variation in the abundance of age-0 turbot is analyzed and implications for stock enhancement are discussed. It is concluded that there is a solid theoretical background indicating that stock enhancement through releases of artificially reared juvenile individuals is a viable option for the turbot stock. Support for the hypothesis that stock enhancement is a viable option is provided by the results of a series of filed experiments conducted under a stock enhancement program in the southern part of Denmark. The focus of these experiments was on the growth and mortality of released fish in comparison to wild fish, and the importance of the characteristics of the release habitat. The growth of the released fish was similar to, or higher than, that of their wild counterparts, and the mortality, even though highly variable, was on a comparable level. No indication of replacement of the wild individuals with released ones was found. The importance of the release habitat was investigated by releasing turbot into three areas that differed in wind exposure. The area with the highest degree of wind-exposure was, as predicted, the area where the highest abundance of wild juvenile turbot was found. The highest growth of released individuals was not found in this area but was found in the one characterized by the lowest degree of wind-exposure, an atypical nursery ground for turbot. The effect of the release habitat on the growth could be related to the quality of the prey items observed in the stomachs of wild turbot sampled in the three areas. Besides the choice of a suitable release habitat, the release size of the fish and the conditioning to the release habitat was shown to be critical for survival of released fish. In order to study the mortality of the released fish, a methodology applying diffusion theory was developed and tested. Using this method the post-release mortality was found to be as high as 14%•day–1 for the 9 day period examined in 7.5 cm sized fish. Mortality decreased with increasing release size and became negligible at a release size of 17cm. After conditioning reared fish to the release environment allowing fish to develop their natural burrowing behavior prior to release, the post-release mortality was found to be insignificant. Since it was possible to enhance the turbot stock through releases of artificially breed individuals, stock enhancement may be considered as a supplement to traditional stock management. It can be applied in species with a turbot like life cycle where density-dependent regulation occurs early in life, and where the cost of production young fish is less than the gain in fisheries yield. As long as the artificially reared juveniles are added to the population after the stage where density-dependent regulation occurs, it can be expected that the released fish will contribute to the natural stock. Further, in order to optimize the outcome when stock enhancing, two aspects should be given special attention, 1) finding a proper release habitat where not only the food quantity but also its quality provides optimal growth conditions and 2) the risk of a server loss of fish during the post release period which can be avoided either by manipulating the size of the individuals released or by applying a conditioning period prior to the release