Selective harvest of sooty shearwater chicks: effects on population dynamics and sustainability

Abstract

Summary Selectivity of harvest influences harvest sustainability because individuals with different characteristics contribute differently to population growth. We investigate the effects of selection based on chick weight on a traditional harvest of the sooty shearwater Puffinus griseus by Rakiura Maori in New Zealand. We develop a periodic stage‐structured matrix population model and incorporate seasonal harvest of three weight classes of chicks. Intensity and selectivity of harvest are defined in terms of weight‐specific hazard functions. We investigate the effect of harvest intensity and selectivity on population growth rate, λ, and the chick exploitation rate, E. We also consider the interaction of chick harvest and adult mortality. λ decreases and E increases as harvest intensity increases. At low harvest intensities, selection has little effect on λ. At high harvest intensities, λ increases as selectivity increases because of the non‐linear relationship between harvest intensity and the probability of being harvested. λ is determined almost completely by E, irrespective of the combination of harvest selectivity and intensity producing E. This is true for both general patterns of selectivity and specific patterns estimated from empirical data. The elasticities of λ, the net reproductive rate and the generation time are unaffected by selectivity and show only small responses to harvest intensity. Adult sooty shearwaters are killed as bycatch in long‐line and driftnet fisheries. Such mortality of adults has an effect on λ about 10‐fold greater than an equivalent level of chick harvest. The sustainability of any combination of chick harvest and adult mortality depends on the resulting reduction in λ. We explore these results in relation to indices of sustainability, particularly the United States Marine Mammal Protection Act (MMPA) standards.