Determination of zooplankton characteristics in the presence of surface feeding basking sharks Cetorhinus maximus

Abstract

Zooplankton samples were taken in the presence (n = 68) and absence (n = 65) of surface feeding, zooplanktivorous basking sharks Cetorhinus maximus off Plymouth, southwest England, from May to mid-August, 1995-96. A preliminary study in 1995 showed zooplankton density (g m-3) to be 46% higher in samples taken near feeding sharks. Calanoid copepods predominated in all samples, but were 84% more numerous and 23% longer in shark samples. A second study in 1996 comprising 110 samples showed a 75% increase in zooplankton density near feeding sharks. The total number of copepods present per m3 was not significantly different compared to non-shark samples, but counts of copepod species numbers showed clear differences between shark and non-shark samples which accounted for the increase in biomass per m3 observed near feeding sharks. Compared to where sharks were absent, samples taken near feeding sharks contained 2.5 times as many Calanus helgolandicus individuals per m3, which were also 50% longer, together with fewer of the smaller copepods, Temora longicornis and Acartia clausi (58 and 33% reductions in no. m-3, respectively). There were approximately equal numbers of Pseudocalanus elongatus, Centropages typicus and Labidocera wollastoni in shark and non-shark samples. These results show basking sharks off Plymouth were found surface feeding in patches containing abundant large C. helgolandicus with fewer numbers of smaller species, which together acted to significantly increase the biomass per m3 where sharks were feeding, compared to areas where they were absent. On the basis of this study, the hypothesis that basking sharks forage actively to locate more productive zooplankton patches can be supported.