The influence of zooplankton communities on the feeding behavior of whale shark in Bahia de La Paz, Gulf of California

Abstract

The whale shark is the world’s largest filter feeding shark and holds international protection as an endangered species. It is known to form seasonal aggregations linked to blooms of zooplanktonic organisms. The aim of this studywas to investigate whether whale shark foraging behaviors are related to the density and/or concentration of available prey items. Zooplankton samples were obtained between October-2016 and March-2017 from Bahia de La Paz in theabsence (n =32) and presence (n = 12) of foraging whale sharks. Results indicated that zooplankton biomass was 1.6 times significantly greater (t = -3.21, p < 0.05) when sharks were observed feeding and the mean number of zooplankton individuals of all taxonomic groups in the presence of feeding whale sharks was 2.5 times greater than in the absence of feeding, although it was not significantly different (t = -1.70, p > 0.05). The dominant taxonomic groups were copepods and chaetognaths with no significant differences found in the mean number of copepods (t = -1.81, p > 0.05), chaetognaths (t = -1.70, p > 0.05), euphausiids (t = -0.12, p > 0.05) and decapods (t = -1.80, p > 0.05), between the presence and absence of feeding whale sharks. In conclusion, whale sharks feeding behaviors occur during higher concentrationsof zooplankton biomass and the species may potentially target dense patches of both copepods and chaetognaths.