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Report of the Workshop on Nephrops Selection (WKNEPHSEL)

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posted on 2007-01-01, 00:00 authored by ICESICES
The workshop was set up to start to summarize progress in measuring andexplaining Nephrops selectivity and to develop new analytical methods toparameterise Nephrops selection profiles that can be used for managementpurposes.• Data from approximately 56 different data sets from 9 countries and 7 areas werecollated on a haul-by-haul basis. The proportions of Nephrops retained in the testcod-end, relative to those retained in the test and control cod-ends combined,were plotted against carapace length.• Parametric analysis of the data provided was difficult given the mixture ofexperiment types. A non-parametric analysis based on smoothers was thereforeused and the relationship between the standard errors and the gear explanatoryvariables was modelled using a linear mixed model.• The effects of various parameters on L50 and SR derived from a logistic modelwere further investigated by linear regression for the covered codend data.Parametric analysis is more powerful than the smoother-based analysis used inthe meta analysis but realistically we can only apply it to covered cod-end data.• The meta analysis carried out shows at length classes 20, 25, 30, 35, and 40 mm,three variables – mesh size, mesh shape and the presence / absence of a liftingbag – had highly significant effects on Nephrops selection.• The analysis perhaps surprisingly showed twine thickness not to be a significantfactor except in one specific data set. It is felt though that twine thickness remainsa gear parameter that should be considered further given its implicit effect onselectivity.• The analysis showed a significant effect of the strengthening or lifting bag onNephrops selection but this result should be considered with caution. Furtherstudies should be carried out to confirm these models predictions but also toassess whether their use is necessary from a strength and safety perspective.• The data set on Nephrops selection should be explored further, particularly toinvestigate the robustness of the results to different modelling assumptions e.g.Bayesian techniques and to identify if there are fundamental differences betweenmodels for the square and diamond mesh codend data when analysedindependently.• There is considerable amount of catch comparison data for both Nephrops andwhitefish selectivity in Nephrops trawls and this data should be further analysedby SGCOMP.• The potential for improving Nephrops size selection by the use of other selectivedevices such as the French flexi-grid should be considered.• Under WGFTFB a similar workshop be held to consider whitefish selectivity datafrom Nephrops fisheries.• The modelling approach used to examine what population effects might resultwhen implementing gear changes should be repeated with other fisheries to fullyassess its value and accuracy


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