Factors associated with bycatch of marine mammals in United Kingdom static net fisheries

The accidental capture (‘bycatch’) of marine animals can pose a conservation threat to populations of endangered, threatened, and protected species. We evaluated the factors influencing or associated with estimated bycatch-per-haul (BpH) using a unique long-term data set with almost 20 000 monitored static net fishing operations collected between 1996 and 2023 as part of the UK Bycatch Monitoring Programme. We developed statistical models to explore relationships between BpH and potential explanatory variables for two small cetacean species, harbour porpoise (Phocoena phocoena) and common dolphin (Delphinus delphis), and a pinniped category [which included grey seal (Halichoerus grypus) and harbour seal (Phoca vitulina)]. Explanatory variables were spatio-temporal [ICES Division, season (day of year), year], environmental (depth), and operational [metier, effort, presence of acoustic deterrent devices (ADDs)]. Harbour porpoise and common dolphin BpH were relatively stable up to 2014, but since then have generally shown opposing trends with harbour porpoise BpH decreasing and common dolphin BpH increasing. Seal BpH has been gradually increasing across the time-period. Seasonal patterns in BpH were also evident, with harbour porpoise rates highest in spring and autumn, and common dolphin and seal rates highest in winter. Harbour porpoise and seal BpH generally decreased with increasing depth; no clear association with depth was found for common dolphin BpH. BpH exhibited an increasing trend as haul effort increased (effort being estimated per haul based on soak time and net length), and there was a tendency for bycatch rates to increase more slowly at high effort levels. The use of ADDs was associated with lower BpH of harbour porpoise (approximately a 75% reduction) but no clear effect was seen for common dolphin. BpH of seals was positively associated with use of ADDs. Model predictions for ICES Divisions, metiers and seasons with relatively higher BpH for the different species can be used to refine current bycatch estimation procedures and inform the development of targeted bycatch mitigation.