Environmental factors governing the distribution of the bottlenose (Tursiops truncatus) and the spotted dolphin (Stenella attenuata) in Golfo Dulce, South Pacific, off Costa Rica

Abstract

This study, conducted from June 1996 to July 1997, was directed at determining the abiotic environmental factors governing the distribution of the bottlenose dolphin (Tursiops truncatus) and the pan-tropical spotted dolphin (Stenella attenuata) in Golfo Dulce. The spotted dolphin was associated with significantly deeper zones (t-test = -9.761; p < 0.001, n = 202) and with higher salinity (t-test = -3.538; p = 0.001; n = 202) than the bottlenose dolphin. The combi- nation of environmental variables in the distribution areas of each species in Golfo Dulce was different, both spatially (F = 9.724, df = 12; p < 0.001) and seasonally (F = 9.735; df = 12; p < 0.001). Of all the environmental variables considered, Multiple Discriminant Analysis showed depth to be the main factor differenciating the distribution areas of the two species. The relationship between group size in each species, depth, and the location of their distribution areas in relation to the shore were analyzed and evidence regarding these aspects and variations in foraging strategies and prey composition was proposed to explain the differences found in their spatial distributions. Seasonal variations in salinity in the distribution areas and their inverse relationship with the time of the year were analyzed in relation to their possible combined effects on prey composition during different seasons in order to explain the seasonal variations in distribution patterns.