The effects of climate variability on zooplankton and basking shark (Cetorhinus maximus) relative abundance off southwest Britain PETER A. COTTON 1, *, DAVID W. SIMS, 2 SAM FANSHAWE 3 AND MARK CHADWICK 1 1 Marine Biology and Ecology Research Group, School of Biological Sciences, University of Plymouth, Plymouth PL4 8AA, UK 2 Marine Biological Association, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK 3 Marine Conservation Society, Unit 3, Wolf Business Park, Alton Road, Ross-on-Wye HR9 5NB, UK ABSTRACT At small spatial scales basking sharks (Cetorhinus maximus) forage selectively on zooplankton along thermal fronts, but the factors influencing broader scale patterns in their abundance and distribution remain largely unknown. Using long-term sightings data collected off southwest Britain between 1988 and 2001, we show that the number of basking sharks recorded was highly correlated with abiotic factors, principally sea surface temperature (SST) and the lagged effect of SST in the previous month, but only very weakly to zooplankton density. This suggests that the changes in number of basking sharks recorded over large spatio-temporal scales are more closely related to the availability of climate- driven thermal resources than prey availability, whereas the converse is supported by previous studies at local scales. Taken together, these results imply scale-dependent behavioural responses in basking sharks, with small-scale foraging movements linked by broad scale responses to temperature variation. Key words: basking shark, Cetorhinus maximus, Calanus helgolandicus, copepod, climate variability, North Atlantic Oscillation index, sea surface temperature, zooplankton INTRODUCTION A major challenge in spatial ecology is to understand how environmental heterogeneity affects animal movements from fine to broad spatio-temporal scales (Levin, 1992; Bullock et al., 2002). In this context, the patterns of annual movement and distribution of many plankton-feeding marine vertebrates, such as the baleen whales and certain sharks, are of interest because they can have ocean-basin-scale home ranges and survive on patchily distributed prey whose abun- dance is determined by various stochastic processes (Wursig, 1989; Sims et al., 2003b). Furthermore, many whales and sharks have undergone severe population depletion due to historical overfishing (Anderson, 1990; Jackson et al., 2001), so there is an urgent conservation need to understand how trends in abundance and distribution are influenced by natural fluctuations. Some recent studies have focused on trying to understand broad-scale shifts in planktivore distribution by investigating relationships with envi- ronmental factors, such as zooplankton abundance, sea temperature and climate effects (Tynan and DeMaster, 1997; Sims and Quayle, 1998; Tynan et al., 2001; Sims and Reid, 2002). However, many planktivores are long-lived and so investigations of temporal trends need to be similarly long-term for key factors to be identified. The basking shark (Cetorhinus maximus) is the world’s second largest fish species, with a circumglobal distribution in warm-temperate to boreal seas. It has been hunted for at least 200 yr in the northeast Atlantic (Pawson and Vince, 1998) and concerns regarding its population status have resulted in it being listed as ‘vulnerable’ on the IUCN Red List of Threatened Species (IUCN, 2002) and in Appendix II of CITES (UNEP-WCMC, 2003). Little is known, however, about many aspects of the life history and biology of basking sharks, including how their distri- bution may be related to broader scale changes of the marine environment. In this paper, we examine the relationship between sea surface temperature (SST), North Atlantic Oscil- lation (NAO) index and zooplankton density on the *Correspondence. e-mail: p.cotton@plymouth.ac.uk Received 4 December 2003 Revised version accepted 13 July 2004 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 14:2, 151–155, 2005 Ó 2005 Blackwell Publishing Ltd. 151