The population decline of the New Zealand sea lion Phocarctos hookeri: a review of possible causes

Abstract

1. The New Zealand (NZ) sea lion Phocarctos hookeri is NZ's only endemic pinniped and is listed as 'nationally critical'. The species breeds in the NZ sub-Antarctic: 71% of the population at the Auckland Islands (2010 pup production: 1814 Ϯ 39) and the remaining 29% on Campbell Island (726 pups in 2010). 2.Pup production at the Auckland Islands has declined by 40% since 1998 (1998: 3021 pups produced): only 1501 pups were born in 2009. This decline is directly linked to philopatric females not returning to breeding areas. While the Auckland Island population has declined, the Campbell Island population appears to be increasing slowly. 3. Potential reasons for the decline in the Auckland Island population, but not in the Campbell Island population, include non-anthropogenic factors: (i) disease epizootics, (ii) predation, (iii) permanent dispersal or migration, (iv) environmental change; and anthropogenic impacts: (v) population 'overshoot', (vi) genetic effects, (vii) effects of contaminants, (viii) indirect effects of fisheries (i.e. resource competition) and (ix) direct effects of fisheries (i.e. by-catch deaths). Of the nine potential reasons examined here, six can be discounted (ii-vii). Bacterial epizootics (i) occur in the NZ sea lion population, but their impact has predominantly increased pup mortality, which is unlikely to cause the severe decline observed, as pup mortality throughout the species is naturally high and variable. 4. The most plausible hypotheses, based on available evidence, are that the observed decline, in particular, the decreasing number of breeding females in the Auckland Island population, is caused by (viii) fisheries-induced resource competition and (ix) fisheries-related by-catch. By-catch is the main known anthropogenic cause of mortality in the species. Competition with fisheries resulting in resource competition, nutrient stress and decreased reproductive ability in NZ sea lions should be a priority area for future research.

An assessment of thermal-image acquisition with an unmanned aerial vehicle (UAV) for direct counts of coastal marine mammals ashore

“…Zealand fur seal are both excellent species on which to focus research, due to their biological similarities and different risk levels (Boren et al 2006;Robertson and Chilvers 2011). D r a f t …”

Extensive variation at MHC DRB in the New Zealand sea lion (Phocarctos hookeri) provides evidence for balancing selection

“…It is the rarest sea lion in the world, with an estimated population of between 8600 and 11300 individuals (Geschke and Chilvers, 2009). The species has suffered a 40% decline in pup production over the last decade (Chilvers, 2009) attributed largely to adult female mortality in fisheries bycatch (Robertson and Chilvers, 2011). In addition, population decline has been compounded by recurrent infection with bacterial epizootics, that have resulted in high levels of mortality of pups especially, but also adults.…”

“…Sea lions were hunted for food by Maori, and the NZSL was extirpated from the New Zealand mainland by 1826, following European settlement and commercial sealing activities (Childerhouse and Gales, 1998). In consequence, the NZSL range is now restricted mainly to the remote Auckland Islands where 71% of pup production occurs (Robertson and Chilvers, 2011) with a colony also present on Campbell Island (Gales and Fletcher, 1999;Childerhouse et al, 2005). Within the Auckland Islands group, Dundas Island and Enderby Island are the primary breeding sites (Figure 1; Gales, 1995).…”

“…Thus, the NZSL is listed as 'vulnerable' by the IUCN (IUCN, 2008) and as 'Nationally Critical' by the New Zealand Threat Classification Status (Baker et al, 2010). Vulnerability is exacerbated by on-going fisheries pressures (Robertson and Chilvers, 2011) and potential future epizootic events, which could further accelerate the decline of this species.…”

Problems with implementation of the New Zealand Threat Classification System, as illustrated by Baker et al. (2010)

“…For NZ sea lions, this 'proposal' has subsequently been cited as the definitive listing (e.g. Robertson & Chilvers 2011). Although Townsend et al (2008, Appendix 2) indicate that a consultation process should occur for new listings before submission of a paper, this is not a public process: there is no transparency.…”