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Tuna purse-seiners in the eastern tropical Pacific (ETP) capture yellowfin tuna by chasing and encircling herds of associated dolphins. This fishery has caused mortality in 14 dolphin species (20 stocks) and has led to significant depletions of at least three stocks. Although observed dolphin mortality is currently low, set frequency remains high and dolphin stocks are not recovering at expected rates. Mortality of nursing calves permanently separated from their mothers during fishery operations may be an important factor in the lack of population recovery, based on the recent discovery that calves do not accompany 75%–95% of lactating females killed in the purse-seine nets. We assessed age-specific potential for mother–calf separations and subsequent mortality of calves by reviewing and synthesizing published data on physiological and behavioral development in delphinids from birth through 3 yr postpartum. Results indicate that evasive behavior of mothers, coupled with the developmental state of calves, provides a plausible mechanism for set-related mother–calf separations and subsequent mortality of calves. Potential for set-related separation and subsequent mortality is highest for 0–12-mo-old dolphins and becomes progressively lower with age as immature dolphins approach adult stamina and attain independence. [ABSTRACT FROM AUTHOR]

Morphological and physiological development impacts swimming performance throughout ontogeny. Our investigation of the ontogeny of swim performance (mean and maximum swim speed) and swim effort (stroke amplitude and tailbeat frequency) of independently swimming bottlenose dolphins (Tursiops truncatus) demonstrated that swimming capabilities are extremely limited in calves. Mean and maximum swim speeds of 0–1-month-old calves were only 37% and 52% of that for adults, respectively, and levels similar to those of adults were not achieved until one year post-partum. Limitations in swim speed were associated with an inability to achieve mature thrusting capabilities, as stroke amplitude and distance covered per stroke remained significantly lower than adult levels during the first-year post-partum. Although calves were expected to require less thrusting power to propel their smaller bodies through water, size-specific stroke amplitudes of 0–3-month-olds (23–26% of body length) were smaller than those of dolphins ⩾10 months post-partum (29–30% of body length). As a result, swim speed standardized by body length was significantly slower for 0–3-month-old dolphins compared with dolphins ⩾10 months post-partum. These results suggest that other factors, such as underdeveloped physiology, act synergistically with small body size to limit independent swim performance in dolphins during ontogeny. [ABSTRACT FROM AUTHOR]