| Summary: | Harbor porpoise abundance and/or distribution patterns in the Salish Sea have changed in recent decades. Stock assessments and studies on habit use are critical, yet physiological studies are also important to elucidate how porpoises operate in their environment and to identify potential limitations in habitat use. For example, physiological adaptations that allow marine mammals to forage underwater dictate maximum dive duration and depth as well as influence adaptability to changes in prey distribution and other perturbations. Such adaptations in the muscle for diving include elevated concentrations of myoglobin (binds to oxygen and provides muscle oxygen) and enhanced buffering capacity (counteracts pH changes associated with lactic acid accumulation). Data on myoglobin content are used, in combination with data on other body oxygen stores and body size, to calculate aerobic dive limit (ADL, maximum dive duration supported by aerobic metabolism). Previous work has shown that muscle biochemistry varies across species and that a developmental period is required for juveniles to attain mature muscle biochemistry and accomplish adult dive durations. Thus to gain a better understanding of diving abilities and potential habitat limitations in Salish Sea harbor porpoises, we quantified muscle myoglobin content and acid buffering capacity of muscle collected from stranded porpoises spanning all age-classes. We found that at birth, porpoises have 51% and 69% of adult myoglobin content and buffering capacity, respectively. Porpoises achieve adult myoglobin content and buffering capacity levels by 9-10 months and 2-3 years postpartum, respectively. Mass-specific muscle oxygen stores for neonates and calves are only 40% and 77% of that of adults. Limited mass-specific oxygen reserves in combination with small body size constrain the diving capacity of immature harbor porpoises. As a result, the ADL for a 12 kg neonate (ADL: 3.55 min) is only 60% of the ADL for a 50 kg adult (ADL: 5.89 min). The shorter ADLs for younger ...
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