Effects of variability in prey abundance on reproduction and foraging in chinstrap penguins (Pygoscelis antarctica) D. A. Croll 1 , D. A. Demer 2 , R. P. Hewitt 2 , J. K. Jansen 3 , M. E. Goebel 2 & B. R. Tershy 1 1 Ecology and Evolutionary Biology Department, University of California, Santa Cruz, CA, USA 2 Southwest Fisheries Science Center, National Marine Fisheries Service, La Jolla, CA, USA 3 National Marine Mammal Laboratory, National Marine Fisheries Service, Seattle, WA, USA Keywords penguin; foraging; Antarctica; krill; seabird. Correspondence Donald A. Croll, Ecology and Evolutionary Biology Department, 100 Shaffer Road, University of California, Santa Cruz, CA 95060, USA. Email: croll@biology.ucsc.edu Received 24 March 2005; accepted 14 November 2005 doi:10.1111/j.1469-7998.2006.00090.x Abstract Life-history theory predicts that adults of long-lived species such as seabirds should optimally balance investment in current and future offspring. However, when trying to optimize investment in offspring provisioning, the most energeti- cally costly component of seabird parental care, adults need to contend with large interannual fluctuations in prey availability and hence the cost of chick provision- ing. Adults faced with this uncertainty can mechanistically balance parental care by adopting a strategy somewhere along the continuum between maintaining constant investment in foraging effort between years and letting chick provisioning fluctuate or holding chick provisioning constant and varying investment in foraging effort. Using ship-based hydroacoustic assessment of prey, time-depth recorders attached to penguins and land-based observations at the breeding colony, we examined how foraging and reproductive effort in breeding chinstrap penguins Pygoscelis antarctica responded to interannual variation in the abun- dance of Antarctic krill Euphausia superba in the vicinity of Seal Island, South Shetland Islands, 1990–1992. Regional measures of krill density varied by a factor of 2.5 (47.0, 23.8 and 61.2 g m 2 in 1990, 1991 and 1992, respectively) and was correlated with annual measures of breeding adult body weight and reproductive performance (breeding population size, duration of chick rearing, chick growth, breeding success and fledgling weight). In contrast, measures of penguin foraging effort (dive depth, dive duration, number of trips day 1 , trip duration, number of dives trip 1 and dive rate) did not differ between years. We conclude that chinstrap penguins reduce reproductive success rather than increase foraging effort in response to decreases in prey abundance in a manner consistent with predictions of life-history strategies for long-lived seabirds. Introduction Food availability is an important factor regulating animal population dynamics through its constraints on life-history traits such as growth, reproductive success and survival (Martin, 1987; Stearns, 1992). Compared with terrestrial birds, seabird life histories are characterized by high adult survival, low fecundity and deferred maturity (Lack, 1968). In general, seabird populations are relatively stable (Furness & Monaghan, 1987), with catastrophic mortality of adults being rare (Cairns, 1987). Lack (1954) suggested that sea- bird populations are regulated by decreases in the produc- tion of offspring in response to decreased food availability rather than adult mortality, as breeding adults trade off current versus future reproductive success (Stearns, 1992). This predicts that adult seabirds faced with large interann- ual fluctuations in prey availability should hold chick provisioning foraging effort relatively constant and let annual reproductive success fluctuate between years. Although numerous studies have shown that seabird repro- duction is linked to large-scale oceanographic events (Kitaysky & Golubova, 2000; Croxall, Trathan & Murphy, 2002), few studies have simultaneously measured reproduc- tive success, foraging behavior and offshore prey abun- dance. Thus, it is unclear whether seabirds respond to interannual changes in prey abundance by trading off investment in foraging for chicks against future reproduc- tive success (Croxall, Reid & Prince, 1999). The ability to conduct such studies has been limited by an inability to manipulate offshore prey abundance, potential prey switching by the seabirds, limited ability to measure foraging effort in seabirds or inability to measure prey abundance and seabird behavioral and reproductive re- sponse simultaneously. Most studies that have examined this question have compared measures of the reproductive success or foraging effort of seabirds with indirect indicators of offshore prey availability (e.g. Bergman, 1978; Lid, 1980; Anderson, Gress & Mais, 1982; Monaghan et al., 1994; Journal of Zoology 269 (2006) 506–513 c  2006 The Authors. Journal compilation c  2006 The Zoological Society of London 506 Journal of Zoology. Print ISSN 0952-8369