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Report on meta-analyses of gear selectivity data in terms of gear design parameters, and of the vertical distribution of fish as they enter trawls; sensitivity analysis of predictive methods to estimate selectivity for data poor species, and economic model to evaluate impact of selective gears at vessel level

O' Neill, Barry; Noble, Struan

Ulrich, Clara

Selection by the codend has been the subject of much research over the past thirty years, with trials in artisanal and industrial fisheries around the world. These studies typically test only a few gears, partly for logistic and/or economic reasons and partly to ensure there are sufficient hauls to estimate the selection of each gear with reasonable precision.  To explore a broad range of selective gear options for use in a fishery, and to understand better the relative influence of the important variables related to gear design, it is necessary to develop models that predict selection across all of these variables.  Such empirical models are best constructed in meta-analyses that combine the data from many trials.  There are, however, few meta-analyses in the size-selection literature and these usually only consider the effect of codend mesh size.  In order to make best use of existing selectivity data we review the meta-analyses that have taken place and extend these studies to investigate the vertical distribution of fish at the mouth of trawl to estimate selectivity for data poor species. These analyses advance our understanding of the potential of using selective gears to reduce discards and will be of particular importance in relation to potential ‘choke species’ which may not have had much attention in the past.

We also describe the development of an Excel toolkit that will provide best practice guidance on collecting the required data and methods to evaluate economic implications of trial fishing gear. This is the result of a technical workshop with 25 contributors including vessel operators, policy makers, scientists, fishing gear manufacturers and gear technologists the purpose of which was to prepare a Best Practice Guidance document on how to undertake such gear trials and to develop an excel workbook with embedded formulae intended to be a practical aide for vessel operators or scientists trialling new fishing gear.

Report Highlights: • codend selection depends on codend mesh size, the number of open meshes around the circumference and twine diameter; • panel selection depends on panel mesh size; • For gadoids, panel contact probability depends on where the panel is positioned and the time of year when fishing takes place; • the relationship of L50 with number of meshes in circumference and twine thickness can be opposite between roundfish and flatfish; • it should be possible to separate the three categories of (i) haddock, whiting and saithe, (ii) cod, plaice and lemon sole and (iii) monkfish and Nephrops using vertical separation; • A Best Practice Guideline and an Excel toolkit assessing the economic implications of selective gears are being developed.
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