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 Corey J. A. Bradshaw 1 , Lloyd S. Davis 2 , Chris Lalas 3 and Robert G. Harcourt 4 1 Antarctic Wildlife Research Unit, Department of Zoology, University of Tasmania, G.P.O. Box 252-05, Hobart 7001, Tasmania, Australia. E-mail: corey.bradshaw@utas.edu.au 2 Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand 3 Box 31, Portobello, Dunedin, New Zealand 4 Marine Mammal Research Group, Graduate School of the Environment, Macquarie University, Sydney, NSW 2109, Australia (Accepted 21 September 1999) Abstract We report on the geographic and temporal variation in pup condition in 20 colonies of New Zealand fur seals Arctocephalus forsteri around South Island, New Zealand during three consecutive breeding seasons, 1996±98 (n = 6856 pups) and provide hypotheses to explain the observed patterns. Three alternative data combinations are presented to calculate pup condition from body measurements and identify which combination seems to be best suited for analysing geographic patterns. A morphometric index of pup condition was best estimated by comparing all pups in all years using least-squares linear regression of the log e -transformed measurements of length vs mass. Condition varied signi®cantly among years and colonies, but not between sexes. We also estimated seasonal changes in pup densities at colonies and investigated the relationship between pup density and condition. Mean pup densities among 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 summer and winter densities varied with year. Pup density explained a signi®cant proportion of the variation in pup condition in 1996 and 1998, suggesting that condition is partly density-dependent. After accounting for the effect of density, we found geographic differences in pup condition, and we offer alternative hypotheses to explain these differences. We also show that the 1998 El Nin Äo coincided with a reduction in pup condition. A morphometric index of pup condition seems to be useful as an indicator of spatial and temporal variation in the marine environment. Key words: Arctocephalus forsteri, condition index, density dependence, El Nin Ä o, ENSO, environmental variation, La Nin Ä a, morphometrics, New Zealand fur seal INTRODUCTION Marine productivity is determined by the complex inter- actions of nutrient upwelling, ocean circulation and climatic processes (Knox, 1994). Consequently, primary productivity varies both spatially and temporally (White & Peterson, 1996). Variation in the biological para- meters of higher predators may re¯ect subtle changes in the marine environment occurring lower down the food web (Trillmich & Ono, 1991). Because of their position on top of the marine food web, seals are affected by variation in the intensity and location of food resources in the marine environment with changes in parameters such as growth, survival, condition, reproductive beha- viour, and diet composition (Kirkpatrick, 1980; Doidge, Croxall & Ricketts, 1984; Gentry & Kooyman, 1986; Croxall et al., 1988; Read, 1990; Boyd & Arnbom, 1991). Female fur seals must regularly alternate between feeding at sea and suckling a pup ashore (Bonner, 1984; Gentry & Kooyman, 1986), and they seem to be adapted to exploiting locally abundant and predictable food resources (Costa, 1993; Boyd, 1998). Pup growth rate is partly a re¯ection of food availability and the foraging ef®ciency of the lactating female (Boyd et al., 1994), and has been used to investigate seasonal varia- tions in local food availability within and among populations of different otariid species (e.g. Mattlin, 1981; Doidge et al., 1984; Kerley, 1985; Trillmich & Ono, 1991; Lunn et al., 1993; Bester & Van Jaarsveld, 1994, 1997). The reliance on local food resources makes the measurement of otariid population parameters good indicators of ¯uctuation in local prey availability (Bester & Van Jaarsveld, 1997). However, estimating pup growth requires multiple captures of individuals and contains inherent biases (Trites, 1991, 1993). Multiple captures are time- consuming and this restricts the number of colonies that J. Zool., Lond. (2000) 252, 41±51 # 2000 The Zoological Society of London Printed in the United Kingdom