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The aim of this thesis is to develop empirical models of the stable catch of brook trout in the Laurentian lakes of Quebec. Current estimators of fish yield are biased and predict poorly. / A model resulting from this study shows that catch increases with fishing effort, but that the rate of increase is smaller at higher effort. For a given effort, the catch-per-unit of effort (CPUE) is greater in larger lakes. Catch is also greater in phosphorus rich lakes and smaller in acid lakes. Catch of bigger fish is associated with a lower CPUE. / No dome-shaped relationship between catch and effort, standardized for the effects of lakes characteristics, could be established. Thus the maximum sustainable yield (MSY) concept provides no guidelines for management of the fishery. However, an early warning of overexploitation is an exceptionally high rate of fishing success. In addition logistic regression based on easily obtained variables can predict the likelihood of stability of the fisheries.

M.Sc. (Zoology) / Suidwes-Afrika is. tn land van kontraste. Dit is tn dorre gebied waar water die paspoort is tot lewe. Die jaarlikse reenval is betrekIik laag (280mm, Dept. Waterwese. 1972) sodatdie natuurlike dreineringsisteme vir die grootste gedeelte van die jaar bree sandslote is. Tydens die kort reenseisoen verander dit egter in bruisende watermassas, wat afloop om uiteindelik in die see, moerasse .of die sand van die Kalahari te verdwyn. Die bou van groot opgaardamme soos die Hardap-, Naute- en Von Bachdam het hierdie watermassas vir mens en dier opgegaar. Die vraag ontstaan egter of water, wat vir mens, dier en op besproeiingslande beskikbaar gestel is, ten volle benut word…

A model of the trophic dynamics among key macroinvertebrate and fish populations in the New River, West Virginia, was developed to help define the roles of predation, competition, and fishery exploitation in the food web of this large, warmwater stream. The fates of production of the harvested crayfish (Orconectes virilis, Orconectes sanbornii sanbornii, and Cambarus sciotensis), hellgrammite (Corydalus cornutus larvae), smallmouth bass (Micropterus dolomieui), rock bass (Ambloplites rupestris), and flathead catfish (Pylodictis olivaris) populations were quantified to evaluate the status of these stocks and to aid specification of the model. Analysis and application of the model were designed to address three research objectives, which were to (1) quantify the roles that predation, competition, and exploitation have in structuring the prey-predator assemblage, (2) evaluate through simulation the effects of multispecies exploitation, and (3) assess through simulation the impacts to the food web of reductions in aquatic insect production associated with the systematic control of black fly larvae in the New River. The trophic basis of production of smallmouth bass, rock bass, and flathead catfish was primarily aquatic insects (in young fishes) and crayfish (in older fishes). Hellgrammites and prey fishes were inconsequential in that regard. Predation by these fishes (primarily smallmouth bass and rock bass) accounted for 76% of the production of ages-1 and -2 crayfish, and harvest by people was equivalent to 5% of crayfish production. Fish predation (primarily by rock bass) and harvest accounted for 14% and 8%, respectively, of the annual production of ages-1 and -2 hellgrammites. Anglers harvested the equivalents of about 91% and 12% of the annual production of fully-recruited smallmouth bass and rock bass, respectively; the extent of flathead catfish harvest was unclear. Error analysis of the model suggested that “bottom-up" (food-limitation) effects were more important than “top-down" (mortality from predation) effects, interspecific and intraspecific competition, and exploitation in maintaining the structure of the prey-predator assemblage in the New River. Exploitation was important in causing transitions in assemblage structure. Simulations of multispecies exploitation demonstrated that production and yield of populations are strongly dependent on food web interactions. Simulation of a 50% reduction in aquatic insect production, similar to that observed following poisoning of black fly Iarvae, predicted substantial declines in insectivores and their predators. Management of New River resources for maxi- mum benefits to people will necessarily involve new approaches to addressing problems in a food web context. / Ph. D.

M.Sc. / Please refer to full text to view abstract

M.Sc. (Zoology) / Please refer to full text to view abstract