Modelling the effect of mesh size and opening angle on size selection and capture pattern in a snow crab (Chionoecetes opilio) pot fishery

Bent Herrmann,Eduardo Grimaldo,Jure Brčić,Kristine Cerbule

doi:10.1016/j.ocecoaman.2020.105495

Abstract

Size selection in commercial snow crab (Chionoecetes opilio) pot fisheries is important for reducing on-board sorting and unintended mortality of undersized individuals. In this paper, we tested whether snow crab of various sizes geometrically could pass through diamond meshes of different sizes and opening angles, to estimate a model for predicting the effect of mesh size and mesh opening angle on snow crab size selectivity. The model was able to explain the size selection results from earlier sea trials using commercial snow crab pots. Size selection was strongly dependent on mesh opening angle, making it less well-defined for the conical pots often used in snow crab fisheries where mesh opening angle varies. We predicted the optimal mesh size for the Norwegian snow crab fishery is 140 mm with a 65° opening angle, resulting in high capture efficiency for target size snow crab (larger than 100 mm carapace width) and low sorting effort, with approximately 10% of the catch consisting of undersized snow crab. Our model can potentially be used for other snow crab fisheries.

Highlights

Snow crab (Chionoecetes opilio) is distributed in the polar regions of the Northern Hemisphere and has become an important commercial species in the USA, Canada, Russia and Norway (Alvsvåg et al, 2009; Winger and Walsh 2011; Mathis et al, 2015; Olsen et al, 2019a, 2019b)
We were able to obtain estimates for selection parameters CW50 and SR for 50 of the 56 mesh templates (Fig. 5), making 50 sets of corre sponding values for mesh sizes (MS), opening angles (OA), CW50 and SR. Those 50 sets of data were subsequently used to establish the predictive model for mesh size se lection of snow crab for diamond meshes dependent on mesh size and mesh opening angle following the procedure described for Step 3
Instead of being built based on expensive and time-demanding sea trials using different pot designs, our model was constructed based on fall through ex periments in the laboratory, using dead snow crabs collected during a research cruise

Summary

Introduction

Snow crab (Chionoecetes opilio) is distributed in the polar regions of the Northern Hemisphere and has become an important commercial species in the USA, Canada, Russia and Norway (Alvsvåg et al, 2009; Winger and Walsh 2011; Mathis et al, 2015; Olsen et al, 2019a, 2019b). Snow crabs are attracted to the pot using bait that has been placed inside. Once the crabs enter the pot, they stay there until one or more mechanisms trigger their willingness to escape (Olsen et al, 2019b). These triggers vary from total or partial consumption of the bait, to behavioural patterns such as competition with other snow crabs or other species (Chiasson et al, 1993; Vienneau et al, 1993). A snow crab captured in a pot will not be able to escape unless it is able to pass through the meshes of the netting covering the pot. This means that apart from the size distribution of snow crabs in the fishing area, the size selective properties of the pot’s netting affect the size distribution of the snow crabs captured by the pot

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Results

Conclusion