Westland Petrels and the Hoki fishery: determining co-occurrence using satellite telemetry

It has been assumed that Westland Petrels, Procellaria westlandica, feed ex tensively on fisheries waste and this habit has been considered at least pa ltry responsible for an increase in the Westland Petrol population. However , no studies have tested this assumption. In this study, 12 Westland Petrel s were tracked by satellite fur a total of 12 foraging trips. The birds' tr acks were compared with the distribution of Hoki, Macruronus novaezelandiae , fishing vessels, predominantly deep-sea trawlers, and the proportion of t ime that they spent in the vicinity of fishing vessels was assessed. This s atellite tracking study was complemented by diet sampling and a survey of W estland Petrels at sea in a wider study of the importance of fisheries wast e in the diet of Westland Petrels. There was considerable variation in the amount of time that Westland Petrel s spent in the vicinity (i.e. within 5 km) of Hoki fishing vessels. The mea n length of foraging trips of satellite-tracked birds was 4.1 days. On aver age, tracked birds spent one third of a foraging trip in the vicinity of ve ssels. Some birds spent most of a foraging trip near the fishing fleet, whi le others spent very little time near vessels. While near fishing vessels, flight speeds considerably lower than those attained away from fishing vess els suggested that the birds mere scavenging there. In 68% of tracked flights, birds apparently stopped at or near the fishing fleet during the 12 h before their return to the colony Because of this, it is likely that diet studies undertaken in conjunction with satellite track ing overestimated the importance of fisheries waste. The amount of time that birds spent outside the Hoki fishing areas shows th at, despite the large-scale fishery, Westland Petrols continue to forage ov er much wider areas than those occupied by the Hoki fishing fleets.