Evasive behavior of spotted and spinner dolphins (Stenella attenuata and S. longirostris) during fishing for yellowfin tuna (Thunnus albacares) in the eastern Pacific Ocean


Heckel, Gisela, Kim E. Murphy, and Guillermo A. Compeán Jiménez
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This study examines the question of whether the evasive behavior of northeastern offshore spotted dolphins (Stenella attenuata) during fishing for tuna (by the Mexican fleet) varies in geographic areas of the eastern Pacific Ocean (EPO). It also investigates whether evasion differs between northeastern offshore spotted and eastern spinner dolphins (Stenella longirostris orientalis). Observations recorded in the database of the Mexican Programa Nacional de Aprovechamiento del Atún y de Protección de Delfines (PNAAPD) from 1992 to 1995 were analyzed. The calculated evasion index was the estimated percentage of dolphins that evaded capture in relation to the herd’s estimated initial size in each set. Evasion index by set was averaged in 2 × 2 quadrants and then used to draw a contour map. Three areas were outlined with low (25%), medium (44.44%), and high (71.80%) median evasion indices. These areas were significantly different (P<0.0001) according to the Kruskal-Wallis nonparametric multisample test, thus indicating a spatial pattern in evasive behavior of northeastern offshore spotted dolphins during fishing operations of the Mexican fleet. Spatial patterns in evasive behavior might be related to the dolphins’ learning capacity, hence experience of individual dolphins or herds with tuna purse-seining in the EPO should be estimated to demonstrate this. To be representative, future research should utilize available historical fishing effort data for the international fleet. Furthermore, a multivariate approach to this issue is necessary. One of the investigated areas (mouth of the Gulf of California) was further analyzed regarding differences between two stocks of dolphins. Evasion indices for eastern spinners were significantly different from those for northeastern offshore spotted dolphins (P<0.0001, Kolmogorov-Smirnov two-sample test). This difference may correspond to different evasive strategies used by the two stocks to evade capture in the net, such as evasion under the net and dispersion (division of herd into subgroups during the set). Eastern spinners apparently evaded more frequently than northeastern offshore spotted dolphins by diving under the net. During the three set stages of tuna fishing (before chase, during chase, and during encirclement), eastern spinner dolphins dispersed less often than spotted dolphins, behavior that may permit them to coordinate their evasive movements more effectively than northeastern offshore spotted dolphins. Evasion over the net was rarely observed in either stock.