Can vertical separation of species in trawls be utilized to reduce bycatch in shrimp fisheries?

Abstract

Several shrimp trawl fisheries use a Nordmöre sorting grid to avoid bycatch of fish. However, small fish can pass through the grid. Therefore, the retention of juvenile fish often remains an issue during shrimp trawling. We investigated the vertical distribution of deepwater shrimp (Pandalus borealis) and dominant bycatch species at the point where the Nordmöre grid section is installed. This was achieved using a separator frame which split the net vertically into three compartments of equal entry size. Our results showed that shrimp predominately follow the lower part of the trawl belly, whereas species such as redfish (Sebastes spp.), cod (Gadus morhua), polar cod (Boreogadus saida) and American plaice (Hippoglossoides platessoides) preferred the mid-section in the aft of the trawl. Haddock (Melanogrammus aeglefinus) primarily entered through the upper section of the trawl belly. Using these results, we predict that a vertical separation device installed forward of a 19 mm Nordmöre grid combined with a 35 mm codend would result in a significant reduction in bycatch with only minor loss of shrimp.

Increasing catch efficiency for Nephrops in deep-water shrimp (Pandalus borealis) trawl fisheries

“…Previous studies reported that the vertical distribution of different flatfish species in the extension piece of the trawl can vary, but that in relation to the moving trawl, these fish drift backwards close to the lower panel (Melli et al, 2019;Larsen et al, 2021). We found that the retention of American plaice for length classes < 30 cm was greater when the lower gap was present in the grid, which suggests that substantial numbers of this species remained close to the lower panel in the extension piece.…”

Bycatch estimation of narrow‐clawed crayfish (Pontastacus leptodactylus) in Turkish fyke net fisheries

“…We found that undersized crayfish had the highest rate of bycatch (43.6% of commercial catches), likely because of small size selectivity of fishing gears (Bolat et al, 2010; Cilbiz et al, 2019). The FAO and the European Commission encourage more selective fishing practices to reduce or eliminate bycatch (Pérez Roda et al, 2019; Suárez et al, 2021), so studies aimed at improving selectivity of fishing gears increased in recent years (Aydın et al, 2011; Brinkhof et al, 2022; Eigaard et al, 2021; Larsen et al, 2021; Sistiaga et al, 2022; Veiga‐Malta et al, 2020), one of which was to improve size selectivity of crayfish fishing in Turkey (Cilbiz et al, 2022).…”