Seabird beachcast events associated with bycatch in the Norwegian purse seine fishery
Beachcast events, where a large number of seabird carcases drift ashore, occur with irregular intervals. These events are due to specific situations where mass mortality of seabirds have occurred. Disentangling the cause of these events can provide valuable information on stressors that can impact seabird populations. Following several mass mortality events involving large gulls in northern Norway, an investigation on the probable cause of death was initiated. In total 75 dead birds were collected at two occasions and necropsies carried out. The findings from the necropsy of the gulls were consistent with drowning as the primary cause of death. Mortality caused by bycatch in purse seine fishery was considered a potential cause of the mortality and monitoring of seabird bycatch in this fishery was thus initiated. In this monitoring of the fishing operations, 10% of 91 fishing events observed led to bycatch, with a total of 32 bycaught seabirds. These bycatch events resulted in a fairly high total estimated bycatch rate of 0.356 (95% CI = 0.133–0.949) birds per haul. These findings are consistent with the hypothesis that the registered mortality events were caused by bycatch in the purse seine fishery. The highly episodic and unpredictable nature of these events makes it demanding to achieve solid estimates of the occurrence and extent of bycatch without a very high monitoring effort. Our study shows that systematic investigation following beachcast events can shed light on the occurrence of such extreme events.