Quantification of bell-shaped size selectivity in shrimp trawl fisheries using square mesh panels and a sorting cone after a Nordmøre grid

Abstract

Shrimp trawlers in the Barents Sea use a Nordmøre sorting grid ahead of a small-mesh codend to avoid bycatch while catching shrimps efficiently. However, small fish can still pass through the grid to enter the codend, which increases their risk of being retained. In this study, we quantified the selectivity of a standard Nordmøre grid used together with one of two different codend designs, namely a diamond mesh codend with square mesh panels and a codend with a square mesh sorting cone section, for deep-water shrimp (Pandalus borealis), redfish (Sebastes spp.), and American plaice (Hippoglossoides platessoides). For the first time, the selective properties of these two alternative designs were estimated and compared to those of a Nordmøre grid used together with a 35-mm diamond mesh codend, which is the compulsory gear used in the fishery today. With this traditional codend, the size selectivity of both bycatch species showed the expected characteristic bell-shaped size selection pattern, with low retention probability of very small fish and bigger fish but with high retention probability of certain sizes of juveniles. Using the square mesh sorting cone significantly reduced the maximum retention risk of redfish. The maximum retention with the diamond mesh codend with square mesh panels was estimated to be 14% lower than that of the traditional codend, but the difference was not statistically significant. The two alternative codend designs did not result in any significant reduction in bycatch of American plaice.

Effect of the Nordmøre grid bar spacing on size selectivity, catch efficiency and bycatch of the Barents Sea Northern shrimp fishery

“…Therefore, the results obtained for shrimp grid passage probability in this study should be interpreted keeping in mind that in areas with larger shrimp than those caught in our study, there could potentially be differences in passage probability for the largest individuals. However, the size structure of Northern shrimp and the bycatch composition obtained in this study is similar to those obtained in several earlier experiments that have been carried out in the Barents Sea during the last two decades [ 10 , 16 , 21 – 25 , 45 ]. This shows that there is likely little variation in the shrimp population size structure in the Barents Sea.…”

“…Several different gear modifications have been tested to improve the selectivity of shrimp trawls. These include, for example, additional grids or sieve panels [ 10 , 24 ], modified codend designs regarding codend meshes [ 16 , 22 ], and use of artificial lights [ 21 ]. However, the results obtained during those trials were variable.…”

“…However, it is an important bycatch species to consider because it is caught in great numbers and then discarded by the fleet, which contributes to the negative ecological impact of the fishery. In addition, due to its morphology, it can block large areas of the grid reducing its sorting capacity and can cause practical problems with sorting the catch on board [ 15 , 16 ].…”

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

“…However, juvenile fish such as cod ( Gadus morhua ), redfish ( Sebastes spp . ), haddock ( Melanogrammus aeglefinus ), polar cod ( Boreogadus saida ) or American plaice ( Hippoglossoides platessoides ) are still often caught in large numbers [ 15 – 17 ]. Most of the very small fish entering the gear pass between the vertical bars of the grid but are subsequently released through the codend meshes.…”

“…Several gear modifications have been tested to improve the selectivity of shrimp trawls. These include additional grids or sieve panels [ 15 , 19 ], various codend designs regarding mesh type [ 16 , 17 ], lights [ 20 ], and grid designs [ 21 ]. However, the results from these trials were variable and, therefore, more research is required to determine a design that reduces bycatch while minimising the loss of target animals.…”