The effects of climate variability on zooplankton and basking shark (Cetorhinus maximus) relative abundance off southwest Britain

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.