Seine nets and beam trawls compared by day and night for sampling fish and crustaceans in shallow seagrass habitat
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Densities of nekton were estimated by comparing catch rates of two previously uncompared gear types, a beam trawl and a seine net, by day and night in a shallow seagrass (Zostera capricorni) habitat in Moreton Bay, Queensland, Australia. A total of 39,676 fish and crustaceans representing 42 species was caught. The catch rates of nekton were 1.4-68.1 times higher at night than in the day for 8 of the 17 common species, and were 1.4-9.2 times higher in seines than trawls for 11 of the common species. None of the common species had higher catch rates in the day than the night, or in the trawls than the seines. For some species there was no significant difference in catch rates amongst the sampling combinations. Night-time seine collections had a greater proportion of larger individuals than day and trawl samples. The differences in catch rates and size of nekton are probably a consequence of both gear avoidance and the movement of nekton out of seagrass during the day. Catch rates were estimated more accurately and precisely with the seine than the trawl, with higher catch rates at night. An analysis of the overall composition of the catch (based on presence/absence data) by multi-dimensional scaling separated the samples into four main groups: day-trawl, night-trawl, day-seine and night-seine. The results suggest that seine nets are a better choice for determining the relative proportion of species in a seagrass habitat, and estimating the density of most species. Such sampling should also be done by day and night, or by night alone.
Fisheries Research: an international journal on fishing technology, fisheries science and fisheries management
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