Is management limiting the recovery of the New Zealand sea lion Phocarctos hookeri?

Abstract

Pup production of the 'nationally critical' New Zealand sea lion Phocarctos hookeri has declined by 48 % since 1998, with fisheries bycatch playing a role in this decline. Current management of the sea lion population involves, amongst other measures, the setting of an annual bycatch limit based on Bayesian modelling of the sea lion population and fisheries information. Success of management scenarios is determined against two criteria, both of which involve keeping the sea lion population at or above 90 % of a modelled carrying capacity (6,987 mature individuals). Due to a lack of information on the pre-sealing abundance of the New Zealand sea lion, it is unclear whether the modelled carrying capacity represents a cap on sea lion recovery. Here, I use published estimates of genetic diversity based on microsatellite loci (expected heterozygosity, H e ) of the New Zealand sea lion and other otariid species to estimate historical effective population size (N e ). I then use existing knowledge of the ratio of N e to census population size (N C ) to determine a historical census population size of these species. Genetical estimates of historical N e suggest that NZ sea lions were considerably more abundant ([68,000 individuals) historically than the current population estimate (11,855 animals). Importantly, the genetical estimate of historical population size suggests that the modelled carrying capacity (6,987 mature sea lions) is likely an underestimation of recovery potential of the species; hence, current management maybe limiting the recovery of the species.

Conservation needs for the endangered New Zealand sea lion, Phocarctos hookeri

“…The other three species of sea lion have total population estimates in the hundreds of thousands, although some populations of these species are considered in decline (Aurioles‐Gamboa & Trillmich, ; Californian sea lions 387,646; Campagna, ; Gelatt & Lowry, ; South American sea lions >250,000; Steller sea lions 143,000). Genetic analysis of NZ sea lions indicates historical abundance of the subantarctic lineage of NZ sea lions was >68,000 (Robertson, ). When the NZ sea lion mainland genetic clade (which became extinct soon after human arrival; Collins et al, ) is added to this population, the species as a whole is likely to have also numbered in the hundreds of thousands, matching the world's other sea lion species.…”

“…However, now >99% of the species breeding population is restricted to the NZ subantarctic islands, Auckland and Campbell Islands, with these remaining animals shown to be a genetic subantarctic clade, distinct from the now extinct mainland clade which became extinct soon after human arrival (Collins et al, ). Although a direct estimate of pristine abundance for NZ sea lions is not available, the present population size is thought to be reduced significantly from pre‐sealing times (Childerhouse & Gales, ), and genetic analysis indicates historical abundance was >68,000 just for the subantarctic clade of the NZ sea lion population (Robertson, ). New Zealand sea lions have experienced an almost continuous decline in pup production since the late 1990s (Chilvers, Wilkinson, & Childerhouse, ).…”

Human‐mediated extirpation of the unique Chatham Islands sea lion and implications for the conservation management of remaining New Zealand sea lion populations

“…Phocarctos sea lions are listed as ‘endangered’ by the IUCN Red List (Chilvers ) and are in decline on the subantarctic Auckland Islands (50% decline in pup production since 1998), with fisheries bycatch and resource competition due to commercial fisheries as likely drivers (Robertson & Chilvers ). Current estimates of anthropogenic mortality based on bycatch and fisheries interaction data, and allowed bycatch limits (1993–2013: average 81 sea lion deaths/year; range: 63–150/year; Robertson & Chilvers ; AEBAR ; Robertson ,b), are up to 15–37% of our modelled annual mortality (410 sea lions/year) required to cause lineage extinction in <50 years (not accounting for other causes of mortality, e.g. predation, disease, old age; Fig.…”

“…The estimates of N e derived above allowed the calculation of census population size ( N c ). The N e : N c ratio is a species‐specific estimate of 0.14 (based off nuclear microsatellite data; Robertson ), multiplied by two to include both males and females. This estimate of 0.14 is similar to that observed in other mammalian species (Frankham ) and likely conservative for otariids.…”

Snares crested penguins Eudyptes robustus population estimates 2000–2013

“…Although it is difficult to compare population estimates from this study with previous studies that counted chicks and not nests (Johns & Miskelly 1986;Tennyson 1987), these studies also report the relative stability of breeding numbers for Snares crested penguins. There are no survey data to determine whether the breeding population has been higher or lower historically and the only way to assess this would be through genetic analysis (Collins et al 2014;Robertson 2015). When making comparison between this study and that of the 2001 Amey Report (Appendix S1) it must be noted that the Report presented results from direct counts and 'corrected' counts.…”