Predicting optimal combinations of by‐catch reduction devices in trawl gears: A meta‐analytical approach

Abstract

Global efforts to reduce unwanted catches have led to the development of a vast array of by‐catch reduction devices (BRDs), in particular for mixed trawl fisheries. Some of these BRDs could likely benefit from being combined. However, the number of possible combinations would be prohibitive to be tested experimentally. Therefore, in this study we propose a meta‐analytical approach that combines the data available on BRDs tested independently in a fishery and predict the theoretical selectivity of all possible combinations of those devices. This allows to identify promising BRD combinations, worth experimental investigation and flexible trawl configurations, where the selectivity can be substantially modified by adding or removing one BRD, thus aiding fishermen in adapting to high variability in catch composition and quota availability. To illustrate the approach, we used BRDs developed for the well‐studied Nephrops (Nephrops norvegicus, Nephropidae) directed mixed trawl fishery in the Skagerrak and Kattegat seas. We predicted the selectivity of 100 BRD combinations for Nephrops, cod (Gadus morhua, Gadidae) and haddock (Melanogrammus aeglefinus, Gadidae), compared them in terms of absolute selectivity and performance under realistic catch scenarios, from both single‐ and multispecies perspectives, and identified 15 BRD combinations that could be worth future experimental investigation. The meta‐analytical approach makes best use of existing knowledge and leads to new insights about the potential for improvement and flexibility in trawl selectivity. This could benefit a variety of mixed trawl fisheries and help developing a new generation of more flexible gears, with multiple BRDs integrated in their structure.

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

“…The larger values obtained in this study can potentially be explained by the higher complexity of the gear tested in this study, different fishing grounds, and/or seasons (e.g. O'Neill et al, 2006;Fryer et al, 2016;Melli et al, 2020). Furthermore, the level of variability observed in this study for the selectivity parameters at the haul level is similar to those reported by Polet (2002).…”

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

“…Utilizing vertical separation in terms of only directing the shrimp and fish entering the lower compartment EPL ( l ) or the lower and middle compartments EPL ( l ) + EPM ( l ) in front of the Nordmöre grid and codend, the modified overall size selection rm L ( l ) and rm L + M ( l ) are expected to be: Where we have used Eq 4 and assumed that the vertical separation in the frame and the size selection processes in the aft of the trawl are independent and sequential. This is similar to the prediction approach applied by Melli et al [ 39 ]. r ( l , ϑ ) represents the combined size selection of the 19 mm Nordmöre grid and the 35 mm diamond mesh codend ( Fig 1 ) that is compulsory for the Barents Sea shrimp fishery [ 18 ].…”

“…The 95% confidence intervals for Δ r L ( l , vL , ϑ ) and Δ r L+M ( l , vU , ϑ ) were obtained based on the bootstrap population results for r ( l , ϑ ), rm L ( l , vL , ϑ ) and rm L + M ( l , vU , ϑ ) in a similar way as described above for rm L ( l , vL , ϑ ) and rm L + M ( l , vU , ϑ ) [ 9 , 19 , 39 ].…”

Mitigating Bycatch: Novel Insights to Multidisciplinary Approaches

“…A technology standard specifies bycatch reduction technologies or production processes that producers must implement for avoidance or minimization. Examples include the prohibition of sundown sets to reduce dolphin mortality when setting on dolphins to capture large yellowfin tunas in the Eastern Pacific Ocean (Gjertsen et al, 2010;Hall et al, 2017), discarding offal on the opposite side of the vessel from which gear is released and required use of Tori lines on longline vessels (seabirds) (Gilman et al, 2014(Gilman et al, , 2016, selectivity requirements for gear such as mesh size, use of circle hooks with mackerel-type bait rather than J-hooks with squid bait (sea turtles) (Kerstetter and Graves, 2006;Reinhardt et al, 2017), bycatch reduction devices on trawl nets (Melli et al, 2020), and the use of pingers on drift gillnets (marine mammals, sea turtles) . Technology standards may change as technology changes.…”