Catchability of Pelagic Trawls for Sampling Deep-living Nekton in the Mid North Atlantic

Heino, M. ORCID: https://orcid.org/0000-0003-2928-3940, Porteiro, F.M., Sutton, T.T., Falkenhaug, T., Godoe, O.R., & Piatkowski, U. (2011). Catchability of Pelagic Trawls for Sampling Deep-living Nekton in the Mid North Atlantic. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-11-012

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Abstract

We use the material collected in summer 2004 from the Mid-Atlantic Ridge between Iceland the Azores with three pelagic trawls to estimate relative catchabilities for the common fish, cephalopod, decapod and jellyfish species. Catchability is defined as the ratio of numbers or weight caught between two trawls, standardised for towed distance. We estimate taxon-specific catchability coefficients for two large pelagic trawls with graded meshes, using a smaller pelagic trawl with uniform mesh size as the reference trawl. Two of the trawls were equipped with multiple opening-closing codends that allowed for sampling in different depth layers. Generalized linear and mixed models suggest that most of the taxa have catchabilities much less than expected from the area of opening alone, indicating that only a few species are herded by the large meshes in the mouth of larger trawls. Catchability coefficients across taxa show a very large spread, indicating that the sampled volume for the larger trawls with graded meshes were highly taxon-specific. Part of this variability can be explained with body size and taxonomic group, the latter probably reflecting differences in body form and behaviour. The catchability estimates presented here form the basis for combining data for quantitative analyses of community structure.

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