Life history plasticity and population regulation in sea otters

We contrasted body condition, and age-specific reproduction and mortality b etween a growing population of sea otters (Enhydra lutris) at Kodiak Island and a high-density near-equilibrium population at Amchitka Island, Alaska. We obtained data from marked individuals, population surveys, and collecti ons of beach-cast carcasses. Mass:length ratios indicated that females (but not males) captured in 1992 at Amchitka were in poorer condition than thos e captured at Kodiak in 1986-1987. In 1993, the condition of females at Amc hitka improved in apparent response to two factors: (1) an episodic influx of Pacific smooth lumpsuckers, Aptocyclus ventricocus, from the epi-pelagic zone, which otters consumed; and (2) an increase in the otters' benthic in vertebrate prey resulting from declining otter numbers. Reproductive rates varied with age (0.37 [CI = 0.21 to 0.53] births female(-1) yr(-1) for 2-3- yr-olds, and 0.83 [CI = 0.69 to 0.90] for females greater than or equal to 4 yr old), and were similar at both areas. Weaning success (pups surviving to greater than or equal to 120 d), in contrast, was almost 50% lower at Am chitka than at Kodiak and for females greater than or equal to 4 yr of age was 0.52 (CI = 0.38 to 0.66) vs 0.94 (CI = 0.75 to 0.99), respectively. Six ty-two percent of the preweaning pup losses at Amchitka occurred within a m onth of parturition and 79% within two months. Postweaning survival was als o low at Amchitka as only 18% of instrumented pups were known to be alive o ne year after mother-pup separation. Adult survival rates appeared similar at Amchitka and Kodiak. Factors affecting survival early in life thus are a primary demographic mechanism of population regulation in sea otters. By m aintaining uniformly high reproductive rates over lime and limiting investm ent in any particular reproductive event, sea otters can take advantage of unpredictable environmental changes favorable to pup survival. This strateg y is consistent with predictions of "bet-hedging" life history models.