Geographic and temporal variation in the condition of pups of the New Zealand fur seal (Arctocephalus forsteri): evidence for density dependence and differences in the marine environment

We report on the geographic and temporal variation in pup condition in 20 c olonies of New :Zealand fur seals Arctocephalus forsteri around South Islan d, New Zealand during three consecutive breeding seasons, 1996-98 (n = 6856 pups) and provide hypotheses to explain the observed patterns. Three alter native data combinations are presented to calculate pup condition from body measurements and identify which combination seems to be best suited for an alysing geographic patterns. A morphometric index of pup condition was best estimated by comparing all pups in all years using least-squares linear re gression of the log(e)-transformed measurements of length vs mass. Conditio n varied significantly among years and colonies, but not between sexes. We also estimated seasonal changes in pup densities at colonies and investigat ed the relationship between pup density and condition. Mean pup densities a mong colonies ranged from 4.94 to 7.46 pups/100 m(2) (summer) and from 4.99 to 7.04 pups/100 m(2) (winter) from 1996-98, and differences between summe r and winter densities varied with year. Pup density explained a significan t proportion of the variation in pup condition in 1996 and 1998, suggesting that condition is partly density-dependent. After accounting for the effec t of density, we found geographic differences in pup condition, and we offe r alternative hypotheses to explain these differences. We also show that th e 1998 El Nino coincided with a reduction in pup condition. A morphometric index of pup condition seems to be useful as an indicator of spatial and te mporal variation in the marine environment.