Matrix difference equations have been used to model the discrete time dynamics of a variety of populations whose individual members have been categorized into a finite number of classes based on, for example, age, size, or stage. Examples of such models include Leslie's age-structured model and the Usher model, a size-classified model which has been applied to trees, corals, sea turtles, copepods, and fish. These matrix difference equations can incorporate virtually any type of nonlinearity arising from the density dependence of fertility and survival rates and transition probabilities between classes. Under a fairly general set of assumptions, it can be shown that the normalized class distribution vector equilibrates, and thus an asymptotic or limiting equation for total population size can be derived. In this research we assume the existence of a dynamically modeled resource in limited supply for which the members of the species compete, either exploitatively or through interference. The existence and stability of population size equlibria or cycles is then studied by means of bifurcation theory. Several biological considerations are addressed, including the Size-Efficiency Hypothesis of Brooks and Dodson, the effects of changes in individual physiological parameters on the size and competitive success of a species, and the effects of delays on the viability of a species.

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I assessed the relationship between growth of fish in situ, food consumption, prey type and fish community descriptors in 12 perch (Perca flavescens) populations exhibiting a 1.8 to 10-fold range in growth rates. / Perch growth rates consistently decreased as fish numerical density increased (r$ sp2$ = 0.60) and increased as feeding levels increased (r$ sp2$ = 0.30). I found no significant relationship between the quantity of food consumed by perch and fish numerical density. Prey quality explained from 50 to 95% of the observed variability in growth but was related to fish numerical density in only one of the 3 perch age classes studied. / I conclude that non-exploitative interactions (operating through increased activity costs) is a more viable explanation for the among-population variability in fish growth rates than is exploitative competition (yielding decreases in the total quantities of food consumed and/or prey quality).

Little information has been published regarding the American plaice, Hippoglossoides platessoides platessoides (Fabricius), a subarctic species which is distributed from Cape Cod to Labrador. In recent years plaice has risen rapidy in commercial importance in eastern Canada. In 1940 only 3 million lb were landed compared to a total of more than 74 million lb in 1960. American plaice in 1961 made up 64 per cent by weight of the total Canadian flounder landings, excluding halibut. [...]

Size fluctuations in amphibian populations at single breeding sites are related to variations in recruitment from the larval stage. However, overall patterns of abundance should be related to the pattern of recruitment success at the many sites where breeding occurs over a landscape. Recruitment patterns for six amphibian species, Ambystoma laterale, A. maculatum, Bufo americanus, Hyla versicolor, Pseudacris crucifer and Rana sylvatica, were investigated at multiple sites within the 10 km 2 Mont St. Hilaire nature reserve during 2001 and 2002. Amphibians bred at eighteen sites but recruitment occurred at only 8 and 11 sites in each year, respectively. Hydroperiod and predation were the most important determinants of recruitment. Four species suffered multiple recruitment failures both years. However, when sites were considered collectively, recruitment was successful for all species because of asynchronous recruitment patterns across heterogeneous sites. Amphibian population dynamics may be stochastic at any single site but recruitment success at multiple sites is a plausible predictor of abundance and trends in regional population persistence.