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

Zooplankton samples were taken in the presence (n -68) and absence (n = 65) of surface f e e d ~n g zooplanktlvorous basking sharks Cetorh~nus mavimus off Plymouth, southwest England, from May to mid-August, 1995-96 A prelim~nary 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 O,rl more numerous and 23 % longer in shark samples A second study in 1996 comprising l l 0 samples showed a 75% increase in zooplankton denslty near f e e d ~n g sharks The total number of copepods present per m3 was not slgniflcantly different compared to non-shark samples, but counts of copepod specles numbers showed clear differences between shark and non-shark samples ivhich accounted for the Increase in biomass per m3 observed near feedlng sharks Compared to where sharks were absent, samples taken near feeding sharks c o n t a ~n e d 2 5 t ~m e s as many Calanus hel-goland~cus ~ndividuals per m3 which were also 50% longer together with fewer of the smaller copepods, Temora longicornls and Acart~a clausi (58 and 33% reduct~ons In no m-3 respectively) There were approximately equal numbers of Pseudocalanus elongatus, Centropages typlcus and Lab~docera wollaston~ In shark and non-shark samples These results show basking sharks off Plymouth were found surface feedlng In patches containing abundant large C helgolandicus with fewer numbers of smaller species which together acted to signlf~cantly increase the biomass per m3 where sharks were feeding compared to areas where they were absent On the b a s ~s of this study the hypothesis that basklng sharks iorage actively to locate more pioductive zooplankton patches can be supported KEY WORDS.Basking shark .Zooplankton .Calanus .For