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UBC Theses and Dissertations

The effects of intraspecific competition for food on reproduction in the guppy (Poecilia reticulata) and the implications for the stock and recruitment problem Morrell, Michael Rowland


Laboratory experiments were conducted with female guppies to determine the effects of intraspecific competition for food on fecundity, fry size, and gestation period. Two groups of females which had been raised at different levels of intensity of competition were compared. Gestation period was significantly longer in the high competition group. The effects on fecundity and fry size were dependent on the size of the adult female. The largest females in the high competition group did not differ with respect to these parameters from individuals of the same size in the low competition treatment; the smaller females produced fewer but larger fry in each brood under high competition. The total weight of fry per brood produced by females of a given size was not different between treatments. These results are discussed from the standpoint that the reproductive characteristics of the individuals of a population can be viewed as tactical components of a strategy whose objective is to maximize the present value of the number of surviving progeny produced by each individual in the course of its lifetime. The optimal distribution over time of energy devoted to reproductive ends and the optimal distribution at one point in time of a given amount of energy among many or few offspring are expected to vary among populations according to the particular age- and size-specific mortality rates faced by each population. Both magnitude and variability of mortality are important. It is suggested that by measuring the appropriate reproductive parameters of the adult members of a population, it should be possible to make predictions about the shape and expected variability of the stock/recruit relationship.

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