Effects of zooplankton density and diel period on surface-swimming duration of basking sharks

The surface-swimming duration of basking sharks (Cetorhinus maxinius) was determined in the western English Channel from May to July over a five-year period. Surface duration showed marked variation from 0.17 to 1.45 h over the short time period from late May to mid-June. Stepwise multiple regression was used to identify any relationships between surface-swimming duration and time of day, daily sea surface temperature (SST), and zooplankton density. There was no support for any effects of SST or two measures of zooplankton density (median and maximum densities). However, there were significant relationships for surface duration with time of day and minimum zooplankton density (r(2)=0.50) indicating basking sharks respond to the abundance of prey and its temporal availability. This suggests that the probability of sighting sharks at the surface will vary depending on the diel period and the surface abundance of zooplankton.