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Life history variation in Neomysis mercedis holmes (Crustacea, Mysidacea) Johnston, Norman Thomas


This thesis examines the relationship between demographic parameters and patterns of energy allocation in a brackish water mysid to test several predictions of life history theory. Populations of N. mercedis subject to different regimes of age-specific mortality allocated energy to reproduction in a manner consistent with predictions based on the maximization of individual fitness. The mechanism by which reproductive allocation was varied was largely through temperature dependent phenotypic variation in the size at maturity which altered size-related components of fitness such as clutch size. N. mercedis in the tidal marshes of the Fraser River delta produced three generations per year which differed in their demographic and life history traits. Summer generations matured at smaller size, carried fewer embryos, had larger eggs, reduced fertility rates, and more even sex ratios than the overwintering generation. Seasonal changes in body size were shown to result from the effects of temperature. Instantaneous per capita birth rates were greatest in the spring and declined to low relatively constant values throughout the summer. Fecundity varied directly with measures of food availability. Size-specific instantaneous mortality rates were higher on small mysids than on larger animals during the spring breeding period but were higher on large animals during the late summer. The mortality rates of neonates were directly related to the abundance of predatory salmon fry and inversely related to food availability, while those of several larger size classes varied inversely with food. Fish predation was generally strongly selective for large mysids. For the mature size classes, the increment in mortality rate with increasing size was positively correlated with environmental temperature, which provided a mechanism through which temperature dependent phenotypic variation in adult size could be selected. The lifetime energy budgets of females from the W, S1, and S2 generations were about 690, 195, and 175 J respectively. The summer and overwintering females differed significantly in their patterns of allocation of assimilated energy. The reproductive effort of the summer females was about 50% greater than that of the W females (12-13% versus 8.5% of the energy budget). My results were in agreement with several of the predictions of life history theory. Reproductive effort was increased when adult mortality rates were high. Reproduction in N. mercedis imposed a real cost in terms of reduced future fecundity and decreased survivorship. Age at maturity decreased and reproductive effort increased during the growth phase of the population. However, no evidence of genetic differentiation was found between the estuarine population and an upriver freshwater population which differed in reproductive characteristics.

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