Short-term survival of discarded target fish and non-target invertebrate species in the “eurocutter” beam trawl fishery of the southern North Sea

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Few studies have examined discard survival in beam trawl fisheries, especially in 4 m beam trawl fisheries using chain mats and limited haul durations. This so-called “eurocutter” fishery is carried out by beam trawlers with an engine power ≤221 kW and is allowed in the 6 to 12nm zone in contrast to larger beam trawlers which operate solely outside of the 12 nm limit. Chain mat beam trawling was developed to prevent large boulders from entering the net, and is typically conducted at lower fishing speed than tickler chain beam trawling. This study obtained short-term survival estimates for this “eurocutter” fishery by monitoring post-capture mortality in tank-held organisms. Survival was high to very high (>75%) for benthic invertebrates, but not for fish. All examined whiting (Merlangius merlangus) and pouting (Trisopterus sp.) died. Only 14% of sole (Solea solea) survived to 91 h of observation, and 48% of plaice (Pleuronectes platessa) to 77h. The survival probability was higher for cod (Gadus morhua) (66% to 88h) and skates (Rajidae) (72% to 80h). However, the mortality rate had not stabilized within the period of observation. Survival models were used to estimate the minimum duration of captivity required to properly evaluate short-term survival, and to investigate the role of physical injuries and other pertinent covariates (catch weight, fish length, fishing depth, salinity, sea surface temperature, air temperature and fishing trip) in determining fish discard survival. The results of this study indicate a high variability in discard survival amongst taxa and highlight that physical injuries when taken alone are a limited proxy for survival of 4m beam trawl discards and that small fish specimens have a limited chance of surviving discarding.