Comparison of the selectivity of three pelagic sampling trawls in a hydroacoustic survey

For the estimation of the stock biomass measured during hydroacoustic surveys it is necessary to take fish samples. These samples should represent the length distribution and species composition of the fish estimated by the sounder. Therefore the trawl has to catch every object in the volume of water sampled. To approximate this, the selectivity of the used trawl should be low. However, specially designed trawls are rarely available. Usually modified commercial trawls are used. Two research vessels from Sweden and Germany carried out a co-ordinated survey, to investigate trawl catch composition of high fish densities scattered layers of herring and sprat in the Baltic Sea. The analysis indicated that the selectivity for young fish was different between trawls because of the different mesh sizes in the codend. Thus, there is a need to estimate the real size distribution. A correction function was designed by applying selectivity data obtained from commercial trawls. However, these corrections are only possible for data obtained with mesh sizes not much larger than the optimum size. Results of this study also indicated that the larger the trawl, the larger the proportion of larger fishes in the length distribution. This effect could not be explained by selectivity. Thus further research is necessary. In every case it is advisable to check the selectivity of the sampling trawl in the described way. (C) 1999 Elsevier Science B.V. All rights reserved.