Predictive framework for codend size selection of brown shrimp (Crangon crangon) in the North Sea beam-trawl fishery

Abstract

The brown shrimp (Crangon crangon) fishery is of great socio-economic importance to coastal communities on the North Sea. The fishery is exploited by beam trawlers often using codends with very small mesh sizes, leading to concerns about catch rates of undersized shrimp. However, little information is available on codend size selection, making it difficult to provide scientifically based advice on alternative codend designs. Therefore, this study establishes a predictive framework for codend size selection of brown shrimp, based on a large selectivity dataset from 33 different codend designs tested during four experimental fishing cruises, during which more than 350,000 brown shrimp were length measured. Predictions by the framework confirm concerns about the exploitation pattern in the fishery, because the retention probability of undersized shrimp reaches 95% with the currently applied designs. The framework predictions allow the exploration of obtainable exploitation patterns depending on codend design. For example, increasing codend mesh size to 25–29 mm would reduce the retention rate of undersized shrimp to a maximum of 50%, depending on codend mesh type.

Testing a size sorting grid in the brown shrimp (Crangon Crangon Linnaeus, 1758) beam trawl fishery

“…Stepputtis et al, 2016). Therefore, this portion of the catch will only be subjected to the size selection of the M22 codend, which has a lower 50 and than the M26 (Santos et al, 2018). In contrast, a part of the marketable sized shrimp that contact the grid is selected out.…”

“…The 11 mm mesh size codend has been considered to be adequate when estimating the selectivity of test gears in the brown shrimp fishery considering the range of lengths that are usually encountered in the brown shrimp fishery (e.g. Polet, 2000;2002;Santos et al, 2018). It was not possible to accurately measure the mesh sizes of M11, since the meshes size range was within the lower limit of measurable sizes by the Omega gauge (10 mm ± 1 mm precision).…”

“…All brown shrimp was sorted and weighed, and a sub-sample of approximately 1000 individuals was weighed and length measured, with the remaining weight of the unmeasured shrimps added to the total catch of each gear. Total length measurements were obtained by digital image analysis by use of ridge detection in ImageJ, as described in Santos et al (2018). The total lengths obtained were rounded down to the nearest millimetre for the subsequent statistical analyses.…”

“…The MSC evaluation revealed that the selectivity of a 26 mm diamond mesh codend would reduce the catches of non-marketable sized brown shrimp considerably, with all the associated ecological effects of such reduction (Addison et al, 2017;Santos et al, 2018). Consequently, the MSC management plan stipulates that the minimum codend mesh size is to be progressively increased from 22 mm to 26 mm by 2021 (Addison et al, 2017).…”

“…Consequently, the MSC management plan stipulates that the minimum codend mesh size is to be progressively increased from 22 mm to 26 mm by 2021 (Addison et al, 2017). However, Santos et al (2018) estimated that increasing the mesh size to 26 mm will result in considerable loss of brown shrimp above the marketable size. Therefore, concerned with this loss of marketable sized brown shrimp, the Danish fishermen proposed the use of a size sorting grid with a bar spacing of 6 mm in conjunction with a codend of 22 mm diamond mesh as a potential alternative to the 26 mm diamond mesh codend.…”