Size selection of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in the Northeast Atlantic bottom trawl fishery with a newly developed double steel grid system

Abstract

11In recent years, Norwegian fishermen have reported problems with fish accumulation in front 12 of the mandatory sorting grids (Sort-X, Sort-V, and Flexigrid). These problems are associated with 13 high fish entry rates and low water flow through the grid sections. In this study, we replaced the lifting 14 panel in the original design of a sorting grid section (Sort-V) by another steel grid ("lower grid") in 15 order to improve water flow and increase sorting area. Two different inclination angles of this new 16 additional "lower grid" were tested. The results demonstrated that both the lower grid and the main 17 grid contributed to the release of cod and haddock. However, the release efficiency of the lower grid 18 was low compared to that of the main grid. A larger proportion of fish contacted at least one of the 19 grids with the lower grid set at 40 o compared to at 35 o . The new double grid was found to release 20 significantly more haddock between 38 and 50 cm long than the mandatory Flexigrid. For cod, the 21 sorting system was at least as good as the Flexigrid at releasing undersized fish. Thus, the new double 22 grid system represents a potential alternative to the Flexigrid. Although the Sort-V single grid releases 23 significantly more undersized cod and haddock than the new double grid system, it also releases a 24 significantly higher proportion of the targeted commercial sizes. 25

Size selectivity and length-dependent escape behaviour of haddock in a sorting device combining a grid and a square mesh panel

“…The findings that the smallest haddock are more likely to utilize an escape opportunity more efficiently than bigger fish that would still manage to escape through the sorting device if they attempted to do so, is in line with the findings of Grimaldo et at. (2018).…”

“…Earlier studies have all assumed/approximated the contact factor to a fixed value and have not allowed it to vary with fish size (Zuur et. al, 2001;O'Neill et al, 2006;Sistiaga et al, 2011Sistiaga et al, , 2017Larsen et al, 2018).…”

“…However, several studies in the literature have documented that haddock of different sizes may exhibit different behaviour in a trawl (Grimaldo et al, 2018;Melli et al 2017Melli et al , 2018. Therefore, it is relevant to investigate if a modelling approach that avoids the assumption of length independency on the behavioural aspect can improve the modelling of size selectivity processes for haddock in relation to grids and square mesh panels in trawls.…”

“…It was based on the Sort-V single grid system already used in the Northeast Atlantic gadoid fishery (Jørgensen et al, 2006). A detailed description of the Sort-V sorting system is provided in Herrmann et al 2013 andin Grimaldo et al 2018. In the new sorting section, the grid was installed upside down compared to the original Sort-V section.…”

“…In model (1) l represents the length of the fish. Examples for using 1 2016and Larsen et al (2016Larsen et al ( , 2017Larsen et al ( , 2018. A shared limitation for all these selectivity studies is that all sizes within the species investigated were assumed to be equally likely to seek escape/contact with the selection device investigated.…”