Modeling Harvest Intensity of Sooty Shearwater Chicks by Rakiura Maori in New Zealand

Cultural evidence suggests that sooty shearwater (Puffinus griseus) chicks have been harvested by Rakiura Maori on islands in southern New Zealand since prehistoric times. Concerns exist that modern harvests may be impacting sooty shearwater abundance. We modeled human-related and ecological determinants of harvest (total no. of individuals harvested) of sooty shearwater chicks on 11 islands and examined the relationship between shearwater abundance and harvesting rates (chicks/hr) and harvester behavior throughout the harvesting season. Models best explaining variation in harvest between harvesting areas (manu), for both the early and late parts of the harvesting season, included harvester-days (included in all models with change in deviance information criteria [Delta DIC], Delta DIC < 8.36 and Delta DIC < 11.5, for the early and late periods, respectively). Other harvest determinants included shearwater density, size of the manu, and number of people helping harvesters (all included in the top 5 models within Delta DIC = 2.25 for the late period). Areas harvested by several families under a common-property harvesting system had higher harvest intensity for their size (24% points higher, 95% credible interval 11-36%) than those managed as an exclusive resource for one family. The slowest harvesters spent more time harvesting but on average only harvested 36% (95% credible interval 15-65%) and 34% (95% credible interval 12-63%) of the harvest taken by the fastest harvesters during the early and late periods, respectively. Our results highlight the possibility of elevated harvest intensity as the population of harvesters increases. However, our models suggested that a corresponding reduction in harvesting rate at low prey densities during the most productive period could potentially regulate harvest intensity. Future research will integrate these results into prospective shearwater demographic models to assess the utility of a range of harvesting strategies in ensuring harvest sustainability.