Behavioral Contributions to Separation and Subsequent Mortality of Dolphin Calves Chased by Tuna Purse-Seiners in the Eastern Tropical Pacific Ocean

摘要

The observation that fewer nursing calves than lactating females tend to found killed in tuna purse-seine sets in the eastern Tropical Pacific Ocean (ETP) (Archer et al. 2001) implies that at least some calves become permanently separated from their mothers as a result of the chase procedure. The long-term dependence of dolphin calves on their mothers implies that at least some of these separated calves may subsequently die due to loss of their mother. Such unobserved mortality could be contributing to the apparent lack of recovery of the dolphin populations in the ETP. This report qualitatively examines behavioral characteristics that might contribute to calf separation during chase and subsequent unobserved mortality by relating observed chronologies of dolphin mother-calf relationships from birth through about two years of age, to activities involved in chase and encirclement during tuna purse seine sets in the ETP. Behavior of calves at all ages will likely focus on attempting to maintain close physical association with their mothers during times of stress (i.e., perceived threat of chase by tuna-seiners). Behavior of mothers during times of perceived threat appears likely to focus more on remaining associated with the rest of the adults in the school rather than remaining closely associated with their calves. Because stress reactions of dolphin schools involve increased swimming speed, the disparity between calves and mothers with respect to physical size and stamina may contribute significantly to likelihood of separation, especially for smaller calves, faster chase speeds and longer chase durations. Calves that do become separated from their mothers likely face increased risk of mortality due to predation and/or starvation, due to the behavioral propensity of calves for continuous acoustic signaling during separation (thus increasing risk of perception by predators) and due to the behavioral propensity of adult female dolphins to avoid adopting orphaned calves.