The size selectivity of the main body of a sampling pelagic pair trawl in freshwater reservoirs during the night

摘要

The aim of this study was to test the selectivity of a trawl main body, especially for small fish, during night sampling. The tested trawl had a mesh size of 80/40/20 mm in the main body, a mouth opening of approximately 100 m(2), and a length of 48 m and was originally designed for sampling vendace, Coregonus albula L Trawl selectivity was determined by comparing the catch using this gear with that from a purse seine net (mesh size 6-10 mm, length 120m, height 12m) at the same sites and time. The size distributions of the total catch and densities of three dominant species (common bream. Abramis brama (L.), roach, Rutilus rutilus (L) and bleak, Alburnus alburnus (L)) were compared between the two types of gear. Densities of these species were divided into size groups before comparison: (i) small fish (60-120 mm, standard length SL); (ii) middle-sized fish (125-180 mm SL); and (iii) large fish (>180 mm SL), which included only roach and bream. Significant differences in the fish density between the trawl and purse seine nets were found. Generally, the densities in all size categories of bleak and small roach were underestimated in trawl catches, whereas the densities of middle- and large-sized bream were underestimated in the purse seine catch. We conclude that both types of gear are size selective - small fish escaped through the mesh of the trawl main body, whereas larger bream were able to escape from the purse seine before closing. The trawl with the given mesh sizes is recommended for quantitative sampling only of fish larger than 180 mm (SL) under low light-intensity conditions