Lion de mer de Nouvelle-Zélande

Counts of pups are commonly used as an index of abundance for pinniped populations (Berkson and DeMaster 2011). New Zealand Sea Lion pups are estimated to account for ~65% of the total number of mature females, and additional to this it has been estimated that 20% of all mature females will never breed (Chilvers et al. 2010). Using those assumptions, in 2008-09 New Zealand Sea Lions numbered approximately 9,880 (95% CI, 8,604–11,297; Geschke and Chilvers 2009) based on total pup production of 2,084, with the number of mature individuals estimated as less than 3,000 (Baker et al. 2010). Using 2013/14 data indicating total pup production of 2,189, the number of mature individuals is estimated to be 3,031 (Chilvers, unpublished). There is no historical population estimate but it is assumed that they were more abundant in the past due to the more extensive range they occupied (Childerhouse and Gales 1998).

The majority of pups are born at the Auckland Islands and annual pup production estimates have been made there since 1994/95 (pups are mostly born in December-January). Estimates suggest that the population was largely stable until 1997/98, but has declined since then at a rate of 4%/year (see the Table 1 in the Supplementary Material - see below). The number of pups counted in 2013/14 was 18% less than in 2012/13, and 48% less than 1997/98 (Childerhouse 2014). The decrease in pup production at Auckland Islands has been linked with decreasing numbers of adult females (Chilvers 2012a).

The other location where a substantial number of pups are born is Campbell Island. While there have been some counts made there (Childerhouse et al. 2005; Maloney et al. 2009, 2012; see Table 1) effort has been intermittent with significantly varying methods (including timing of surveys) and the data cannot be used to estimate meaningful trends. Therefore, while pup count results indicate that Sea Lions on Campbell Island are not in decline, the apparent upward trend is not predicted to continue (Maloney et al. 2012). Campbell Island is at the southern limit of the New Zealand Sea Lion range and pup mortality is very high there, apparently due to cold and wet conditions during the pupping season and substrates unsuitable for a breeding colony and early pup survival (Maloney et al. 2012).

A few pups are also born on the south coast of the South Island of New Zealand and on Stewart Island not far offshore (see Table 1 in the Supplementary Material - see below). The pups born at Stewart Island were only discovered in 2010/2011 and since that year regular searches of the most likely places Sea Lions would pup have been undertaken. The apparent increasing trend of Stewart Island pup counts (see Table 1 in the Supplementary Material - see below) is an artefact of better search techniques and areas searched on Stewart Island since 2010/2011 rather than an increase in pup numbers. The number of pups born at those locations has been about 30/year, which is less than 1% of the total pup production.

The mean age of reproduction for female New Zealand Sea Lions is 10.75 years (Childerhouse 2007) hence the generation time is estimated to be 10.75 years with 3 generations being equivalent to approximately 32 years.

The best information that can be used to project future abundance of New Zealand Sea Lions is the trend in pup production at the Auckland Islands. If the pup estimate from 1997/98 (3,021) is projected three generations forward with a decline of 4%/year the number of pups born in 2029/30 is estimated to be 840, which is a 72% reduction. While threats to New Zealand Sea Lions have been identified (fishing related mortality, climate/nutritional stress, disease), they are not fully understood (Roberts and Doonan 2014). Management measures have been introduced to mitigate fisheries interactions, but declines in Sea Lion numbers have not ceased.

A population viability analysis has been undertaken for the Auckland Island population of New Zealand sea lions (Chilvers 2012b). The PVA was only for this population because it makes up three-quarters of the species and has the most reliable population parameter estimates for modelling. The results show that at the current rate of decline in the Auckland Island population, this population could be functionally extinct (less than 1,000 animals within the population) by 2035 (24 years, less than three New Zealand Sea Lion generations). The modelling of the severest known fisheries and bacterial impacts shows that with a probability of 0.982, the Auckland Island population will be functionally extinct in 59 years with a mean annual population decline rate (r) of -0.039 (Chilvers 2012b).

Recent demographic assessment of the decline in the Auckland Island subpopulation has identified the main proximate causes for decline that include generally low pupping rates, declining trends in cohort survival to age 2 since the early 1990s, and low adult survival (age 6-14 years) since 1999 which may account for declining pup numbers at Sandy since the late 1990s (Roberts et al. 2014). Analyses to identify the ultimate causes have been compromised by a short time series mostly covering the period of decline (Roberts and Doonan 2014). However, juvenile (2-5 years) and adult (6-14 years) survival was poorly correlated to estimated fishing related mortality in the squid trawl fishery at the Auckland Islands. Correlative assessment with cohort survival to age 2 was consistent with disease-related mortality impacting survival after 2005. Roberts and Doonan (2014) consider that declines in maternal conditions, variable diet composition, changes in milk quality and pup mass, and reduced pupping rates are consistent with changes in the nutritional status of the subpopulation; however they noted that some of these responses could also occur in response to pup mortality not driven by nutritional stress factors.

See the Supplementary Material for further information about New Zealand Sea Lion pup production at Auckland Islands, Campbell Island, Otago Peninsula and Catlins (mainland New Zealand), and Stewart Island.