What limits the harvest of sooty shearwaters (Puffinus griseus) on Poutama Island?

Abstract

The traditional harvest of sooty shearwaters by Rakiura Maori was studied in the 1994 and 1995 muttonbirding season on Poutama Island, New Zealand. Chicks were captured much faster during the rama (the second phase of the harvest when chicks are caught at night on the surface) than during the nanao (the first phase of the harvest when chicks are extracted from breeding burrows during the day). Harvest rates (mins/chick) decreased, and strike rates (chicks/burrow) increased in areas and years with higher chick density. The relationship between strike rates and chick density was curvilinear, so observed changes in harvest rate will not be directly proportional to the actual change in density on Poutama. Burrow occupancy was highest in areas with intermediate burrow entrance densities, perhaps because crowding at higher density reduces breeding success, or because harvest is most intense in high density areas. A 20 year record of captures on Poutama showed that the harvest rates almost doubled between 1989 and 1998. The muttonbirders were able to compensate slightly for decreased catches by working an extra 31 minutes per day during the nanao, but there is little scope for further compensation on Poutama because the working day is taken up almost entirely by catching and processing chicks. Density is the main indirect determinant of the number of chicks that can be gathered on Poutama. The number harvested is determined directly by how many chicks a muttonbirder can catch and process in a day on Poutama. Replicate studies are now needed on other islands to test whether similar limits operate elsewhere.