Seasonal oscillation in shore attendance and transience of New Zealand fur seals

Abstract

The New Zealand fur seal (Arctocephalus forsteri) appears to show regular shore attendance in the form of seasonal oscillations. This phenomenon should be quantified to properly interpret counts of fur seals >1 year old (i.e., non-pups). Here we test the predictability of peaks in the annual shore-attendance oscillation on Otago Peninsula using an autoregressive sine model and >2 years of intensive survey data. We predicted that the peak in fur seal numbers ashore would lie between 14 January - 4 April (1996) and 8 January - 2 April (1997), although this low predictability is undesirable when attempts are made to monitor population trends. Estimating population size from counts of non-pups also requires knowledge of the rate of turnover of individuals. Therefore, we tested the hypothesis that tagged animals from other colonies are immigrants to Otago Peninsula. With sightings on Otago Peninsula of fur seals tagged elsewhere in New Zealand, we used a Monte Carlo approach to simulate the expected frequency of single and multiple sightings of individuals. We found that the observed frequency of multiple sightings was significantly less than predicted by the model (P < 0.0001), indicating that tagged animals were transients. We also discovered that the sex ratio of tagged animals varied with breeding colony of origin (G1= 52.07, P < 0.0001), suggesting that the impetus for emigration differs among colonies. We concur with the view that counting pups is the only way to estimate the relative abundance of New Zealand fur seals. In addition, we showed that counts of non-pups cannot be used to estimate population size because an unknown proportion of individuals is transient. However, counting of pups does not address the issue of estimating relative abundance for locations with large numbers of nonbreeding individuals and few or no breeders. With few or no pups it is impossible to estimate relative abundance using counts of pups.