Selective foraging behaviour of basking sharks on zooplankton in a small-scale front

The basking shark Cetorhinus maximus is the second largest fish species, attaining lengths of up to 11 m. During summer months in temperate coastal waters circumglobally, these sharks filter-feed on surface zooplankton near water-mass boundaries (fronts),; however, little else is known about their biology. Their foraging behaviour has not been investigated until now, although they have been described as indiscriminate planktivores that are unlikely to orientate to specific plankton-rich waters. We have now tracked basking sharks responding to zooplankton gradients. We show that they are selective filter-feeders that choose the richest, most profitable plankton patches. They forage along thermal fronts and actively select areas that contain high densities of large zooplankton above a threshold density. They remain for up to 27 hours in rich patches that are transported by tidal currents and move between patches over periods of 1-2 days. We mapped feeding locations of these sharks in two years; the maps show that these sharks indicate broad shifts in front-located secondary production. Foraging behaviour of basking sharks therefore indicates the distribution, density and characteristics of zooplankton directly. This makes these sharks unique biological 'plankton recorders', with potential use as detectors of trends in abundance of zooplankton species that are influenced by climatic fluctuations of the North Atlantic Oscillation.