At-sea distribution of breeding male and female grey petrels (Procellaria cinerea) determined from New Zealand fisheries bycatch

Seabird bycatch data collected between 1996 and 2016 in commercial fisheries within the New Zealand Exclusive Economic Zone (EEZ) were analysed to determine if male and female grey petrels (Procellaria cinerea) have different atsea foraging distributions during the breeding season. Data collection includes the return of bycaught (killed) seabirds from commercial fishing vessels by government fisheries observers. A total of 408 bycaught breeding grey petrels with known sex (214 males, 194 females) were analysed for a locational and seasonal sex bias. Data were also examined to determine whether where carcasses were returned from sea, there were different proportions of males and females captured by different fishing methods: offshore bottom longlining, surface longlining, and offshore trawling. There was no significant difference in the totals of male and female grey petrels returned from fishing operations, but capture locations for the sexes varied widely. More males than females were caught in April, May, August, and September. July showed a reverse trend, while June was the only month with no difference in captures between sexes. More males than females were caught in offshore bottom longliners and trawlers, with the opposite for surface longliners. This study emphasises the importance of a large-scale approach to capture locations and season when analysing impacts of fisheries on seabird populations, and highlights different foraging areas according to sex during the breeding season. Spatial segregation has important management implications as changes in fisheries practice in foraging areas may affect the sex ratio of the grey petrel population.