Evasive Behavior of Eastern Tropical Pacific Dolphins Relative to Effort By the Tuna Purse-Seine Fishery

摘要

The behavior of five stocks of four species of eastern tropical Pacific (ETP) dolphins, offshore spotted (Stenella attenuata attenuata), eastern spinner (Stenella longirostris orientalis), coastal spotted (Stenella attenuata graffmani), short-beaked common (Delphinus delphis), and striped (Stenella coeruleoalba), were analyzed relative to tuna purse-seine fishery effort. Behavioral data was collected for 1,218 sightings during Southwest Fisheries Science Center (SWFSC) research cruises in the ETP in 1998 and 1999. Using Principal Components Analysis (PCA) and Discriminant Function Analysis (DFA), we created two reaction indices (RIs) based on five discrete field recorded behaviors that indicated a reaction to the research vessel. These field-recorded behaviors included approaching the research vessel, bow-riding, running, low swimming, and school splitting. Fishing effort data were supplied by the Inter-American Tropical Tuna Commission (IATTC). For each dolphin sighting, effort was defined as the number of sets made by the tuna purse-seine fishery within a given number of days and distance away from the sighting. Fifty-six effort windows ranging from 10 70 days and 30 300nm were used in the analysis. The number of sets per window ranged from 0-576. We asked the following three questions: (1) Can 'reaction to the research vessel' be defined by a combination of discretely coded behaviors. (2) What is the relationship between dolphin reaction or any of the component behaviors and fishing effort. (3) If there is a relationship between dolphin behavior and effort, can behavior be used as a proxy of population level effects of the fishery. The PCA identified a major principal component, which we defined as a 'reaction index,' that accounted for 35 56% (depending on stock) of the variance in the behavioral data (RI-PCA). In effort windows of greater distances and greater number of days, we found a significant positive correlation between RI-PCA and fishing effort for offshore spotted dolphins and eastern spinners (the species primarily targeted by the tuna purse seine fishery). This correlation was confirmed by a nonparametric test in which sightings classified by the DFA as having a negative reaction to the boat were associated with significantly more effort (difference in medians of 1 to 61 sets, depending on the effort window chosen).