The CHase Encirclement Stress Studies (CHESS) are part of a comprehensive research program designed to investigate the status of dolphin stocks that are involved in tuna purse seine fishing operations in the eastern tropical Pacific Ocean (ETP). The tuna industry has used the association between tuna and dolphins to fish in the ETP for over five decades, and stocks of spotted dolphins (Stenella attenuata) and spinner dolphins (Stenella longirostris) were depleted by high historical levels of mortality in tuna purse-seine nets. While changes in the fishery during the last three decades have greatly reduced the observed mortality of dolphins, there continues to be concern that the fishing methods used may be causing stress and having a negative impact on dolphin population recovery.
The 1997 International Dolphin Conservation Program Act (IDCPA) requires the U.S. Commerce Department to conduct population assessments and stress studies to determine if the tuna purse seine fishery is having a significant adverse impact on any depleted dolphin stock in the ETP. Population assessments were conducted from 1998-2000 as part of the Stenella Abundance Research Project (STAR). The mandated stress studies include a review of relevant stress-related research(Curry 1999), collection and analysis of necropsy samples from dolphins killed in the commercial fishery, a review of historical demographic and biological data from the affected dolphin stocks, and an “experiment involving the repeated chasing and capturing of dolphins by means of intentional encirclement” (CHESS).
The Chase Encirclement Stress Studies involved the use of a research vessel and a chartered tuna fishing vessel during August-October 2001 to repeatedly capture and release individual dolphins and collected biological samples for evaluating stress. The research program incorporated several complementary research projects that addressed different ways in which chase and capture stress may manifest itself in individual dolphins involved in tuna purse seine operations. Research techniques for evaluating stress included a) analyses of single and repeat blood samples, b) molecular analyses of chronic stress from skin samples, c) measurement of dolphin surface and body temperatures to investigate heat stress, d) satellite tagging and tracking, e) documentation of reproductive status and, if it occurred, cow/calf separation between successive chases, and f) behavior studies.
The goal of the chase/recapture experiment was to provide scientific data on physiological indicators of stress in chased and captured dolphins, and, if possible, to estimate a range of consequences for the individual dolphin’s survival and reproduction. Population-level conclusions about the potential for significant adverse impacts of fishery-induced stress were based on the complete results of all studies mandated in the IDCPA (see above).