Seabird Interactions with Dolphins and Tuna in the Eastern Tropical Pacific

Bird flocks associated with dolphins in the eastern tropical Pacific are described from observations obtained during eight cruises that took place from January to March of 1976, 1977, 1979, and 1980. In the northern tropical waters between latitudes 5"N and 30"N, 43% to 53% of bird flocks co-occurred with dolphins. In equatorial and southern subtropical waters between latitudes SON to 12"s and in the central Pacific less than 8% of the flocks were associated with dolphins. In northern tropical waters about 70% of dolphin schools associated with flocks were composed of spotted or spotted plus spinner dolphins; conversely, 59% of spotted dolphin and 96% of spotted plus spinner dolphin schools co-occurred with bird flocks. Most large schools of these dolphins were associated with birds, and thenumber and diversity of bird species increased with dolphin school size. The average species composition of birds in dolphin-associated flocks of northern tropical waters was: boobies 4 I.%, Wedge-tailed Shearwaters (Puffinus pacificus) 3 1.4%, jaegers 12.8%, Sooty Terns (Sterna firscata) 6%, frigatebirds 3.6%, and others 4.5%. Positive statistical associations were found among these bird species, which are explained by common attraction to food made available by feeding yellowfin tuna. In the southern latitudes and in the central Pacific, flocks were dominated by Sooty Terns, and few flocks were associated with dolphins. These flocks appeared to be associated with skipjack rather than yellowfin tuna. INTRODUCTION Flocks of seabirds accompanying dolphin schools (order Cetacea) are a conspicuous sight in the eastern tropical Pacific (ETP). Tuna fish-ermen search for these flocks to locate dolphin schools and the yellowfin tuna (Thunnus al-bacares) that swim with them. These flocks are usually associated with only a few of the many species of dolphins found in the ETP. It is only those few dolphin species, in turn, that are commonly associated with tuna. The dolphin-associated tuna are primarily yellowfin and, less frequently, skipjack Katsuwonus pelamis (Hammond 1981). In this paper, we describe the characteristics of seabird flocks that asso-ciate with dolphins and also describe the na-ture of the joint aggregations of birds, dol-phins, and tuna. We present evidence for a major discontinuity in the pelagic commu-nity-between multispecies flocks associated with dolphins and yellowfin tuna, on the one hand, and single species flocks associated with small tuna, on the other.