The sustainability of a harvest, i.e. whether harvest permits non-negative population growth, depends on the characteristics of the harvest, the organism's life history and additional sources of mortality. Important characteristics of the harvest include the number, proportions and type of individuals harvested, and the timing of the harvest within the year and within the life cycle (e.g. Olmsted & Alvarez-Buylla 1995; Jensen 2000; Freckleton et al. 2003). The life history of the organism determines how harvest affects the potential for population growth. Finally, harvest is rarely the sole source of human-imposed mortality, and the effects of harvest must be considered in the context of other such impacts (e.g. bycatch, pollution or habitat destruction).
Selective harvest, in which individuals of different types are taken in proportions other than those in which they occur in the population, affects both the number and type of individuals harvested. Selection may result from harvester preferences, harvesting technology, behaviour of the harvested species or management requirements, and may be intentional or unintentional. Selective harvest affects sustainability because different types of individuals contribute differently to population growth.
Selection on a gross scale, e.g. juvenile vs. adult, is often imposed on managed harvests and accounted for in evaluating harvest impacts. Selection on less obvious characteristics, e.g. foraging ability or physiological condition, may be equally important. It is well-documented that apparently similar individuals can differ greatly in their reproductive contributions to future generations (e.g. Clutton-Brock 1988; Newton 1989). For example, less than one-third of breeders and less than 10% of fledglings produce future recruits in some avian species (Coulson 1988; Wooller et al. 1988; Mills 1989; Newton 1989; Dann & Cullen 1990). In such cases, selectivity can have profound implications for sustainability. Yet such differences are less likely to be identified or measured, so are likely to be overlooked in evaluating harvest impacts.
Issues of sustainability and selectivity arise in the harvest of the sooty shearwater Puffinus griseus in New Zealand. The sooty shearwater is a medium-sized (650–1000 g) procellariiforme. It is long-lived, has delayed maturity (first breeding at about 7 years) and low fecundity (lays at most one egg each breeding season). It is a colonial breeder, nesting in burrows, primarily on offshore islands. Sooty shearwaters are thought to number in the millions (Warham & Wilson 1982; Miskelly et al. 2001) but the species is listed as Near Threatened by the IUCN (BirdLife International 2004) because of population declines. Population declines have been indicated on both the wintering grounds (Veit, Pyle & McGowan 1996; Veit et al. 1997) and on the breeding grounds (Lyver, Moller & Thompson 1999; Gaze 2000; Jones 2000; Lyver 2000a; Scofield 2001; Scofield & Christie 2002).
Bycatch of sooty shearwaters in ocean fisheries, particularly the central North Pacific driftnet fisheries, has also been a significant problem (Uhlmann 2003). More than 350 000 were taken annually from 1978 to 1990 (Ogi et al. 1993) and DeGange et al. (1993) suggest a worst-case scenario of 1·2 million caught per year in the 1980s. High-seas driftnet fisheries were closed in 1993, but driftnet fishing persists in Russian and Mediterranean waters (Uhlman 2003) and current mortality levels are unknown.
The Rakiura Maori people of New Zealand conduct a traditional harvest of sooty shearwaters (tl̄tl̄ in the Maori language) on 36 islands around Stewart Island, the southernmost of New Zealand's three main islands (Department of Lands & Survey 1978; Wilson 1979; Robertson & Bell 1984; Anderson 1997). This harvest, generally referred to as muttonbirding (Wilson 1979; Waitangi Tribunal 1991) is governed by the Tl̄tl̄ Regulations, which prohibit the harvest of adults, restrict access to the islands and the timing of the harvest and include provisions to protect the island habitat (e.g. measures to prevent predator introductions). The extent of the harvest is unknown but may be as high as 250 000 birds annually (Warham 1996). The capacity for harvest has increased with new technology such as charter boat and helicopter transport, wax cleaning methods and changes in the attendance of harvesters (Lyver 2000b). Maori culture places great importance on this harvest, and recent perceived changes in population status have led to concerns about its sustainability (e.g. Wilson 1979).
The harvest season (1 April−31 May) is divided into two periods. The first period, the ‘nanao’ in the Maori language, starts on 1 April (Wilson 1979). During this period chicks are extracted from their burrows using a wire probe. The second period, the ‘rama’, begins when chicks start emerging from their burrows at night in preparation for fledging (usually about 20 April) allowing harvesters to pick them up off the ground. The harvest ends when chicks become scarce, usually around mid-May.
The harvest is selective: harvesters prefer heavier, more developed chicks (Hunter, Moller & Kitson 2000b; Lyver 2000a). We investigate the impacts of this selectivity on population growth, and how those impacts interact with fisheries bycatch of adults. We apply our model to patterns of selectivity estimated on a harvested island (Putauhinu), and discuss the problems involved in determining levels of harvest compatible with sustainability.