DIEL PATTERNS IN AGGREGATIONS OF PELAGIC DOLPHINS AND TUNAS IN THE EASTERN PACIFIC

Data collected by scientific technicians aboard tuna purse seiners in the eastern Pacific Ocean since the early 1970s have allowed us to stu dy the biology and herd dynamics of pelagic dolphins. A pattern of inc reasing group size in the morning and subsequent decline in the late a fternoon or night was evident for spotted, spinner, and common dolphin s, as well as for large yellowfin tuna that associate with dolphins. D iel patterns were also apparent in the formation of mixed-species herd s of spotted and spinner dolphins and tuna-dolphin aggregations. Diffe rent patterns, however, were displayed by bottlenose dolphins and by y ellowfin and skipjack tunas that did not associate with dolphins. It a ppears that these diel patterns are produced by an interaction of pred ation pressure and prey distribution.