Multivariate effects on seabird bycatch in the legal Patagonian toothfish longline fishery around Crozet and Kerguelen Islands

The effects of temporal, spatial, environmental and operational effects on seabird incidental mortality in the legal Patagonian toothfish longline fishery operating, between 2003 and 2006, in French exclusive economic zones of Crozet and Kerguelen Islands were analysed. During the study period, the mean bycatch rate varied from 0.05 to 0.12 birds per 1,000 hooks. Two species were concerned by incidental mortality: white-chinned petrels (88%) and grey petrels (11.5%). Males of white-chinned petrel seemed more at a risk than females. Logbooks data tended to underreport mortality when compared with dedicated fishery observers. The results indicate that temporal (season or phenology) and spatial (area) factors reflecting mortality risk for seabirds played the most significant role in the incidental mortality of the two species. Operational (integrated weight mainline, number of scaring lines and number of hooks hauled) and environmental factors (wind/vessel angle, moon brightness) were also influential, although less significantly, in increasing this mortality risk. Our two steps analyses by separately modelling the probability of presence and the abundance given presence suggest that the decrease in seabird bycatch over the period was mainly due to an important decrease in probability (occurrence) of mortality.