Projected impacts of climate change, bycatch, harvesting and predation on the New Zealand tītī (Ardenna grisea) population

Abstract

Many factors have contributed to a decline in the tītī (sooty shearwater) Ardenna grisea population in Aotearoa New Zealand since at least the 1960s. The relative impacts of Southern Oscillation Index (SOI) variation, bycatch, predation, and traditional harvesting by Rakiura Māori were recently estimated in a study fitting population models to data from the period 1976-2005. Annual mean SOI was related to both adult survival and fecundity. We used the results from that study to project abundance of tītī under a range of management strategies and future SOI scenarios, based on 41 climate models. Projections over the period 2019-2070 showed marked variation across the climate models. When the proportion of chicks harvested and the level of depredation by weka Gallirallus australis were set at their historical means and the proportion of birds killed in bycatch was set at an upper bound based on current estimates, the probability of a decline ranged from 0.30 to 1.00, across all climate models. When both bycatch and depredation by weka were set to zero, the probability of a decline ranged from 0.11 to 1.00, across all climate models. Our results suggest that future abundance of tītī in Aotearoa New Zealand will depend to a large extent on SOI conditions over the coming decades. As climate-model uncertainty makes reliable prediction of future SOI conditions difficult, adaptive management is likely to be the best option for Rakiura Māori, the kaitiaki (environmental guardians) of the Rakiura Tītī Islands, to maintain sustainable tītī harvests.