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

Abstract

It has been assumed that Westland Petrels, Procellaria westlandica, feed extensively on fisheries waste and this habit has been considered at least partly responsible for an increase in the Westland Petrel population. However, no studies have tested this assumption. In this study, 12 Westland Petrels were tracked by satellite for a total of 22 foraging trips. The birds’ tracks were compared with the distribution of Hoki, Macruronus novaezelandiae, fishing vessels, predominantly deep-sea trawlers, and the proportion of time that they spent in the vicinity of fishing vessels was assessed. This satellite tracking study was complemented by diet sampling and a survey of Westland Petrels at sea in a wider study of the importance of fisheries waste in the diet of Westland Petrels.

There was considerable variation in the amount of time that Westland Petrels spent in the vicinity (i.e. within 5 km) of Hoki fishing vessels. The mean length of foraging trips of satellite-tracked birds was 4.1 days. On average, tracked birds spent one third of a foraging trip in the vicinity of vessels. Some birds spent most of a foraging trip near the fishing fleet, while others spent very little time near vessels. While near fishing vessels, flight speeds considerably lower than those attained away from fishing vessels suggested that the birds were 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 tracking overestimated the importance of fisheries waste.

The amount of time that birds spent outside the Hoki fishing areas shows that, despite the large-scale fishery, Westland Petrels continue to forage over much wider areas than those occupied by the Hoki fishing fleets.