We used daily air and water temperatures from 14 lakes in Ontario to develop and test a simple method for constructing lake-specific empirical models for predicting daily littoral water temperatures. Data requirements for prediction are modest (average air temperature, day of the year) and the method is robust and practical, requiring only a few (15-20) well-spaced water temperature observations to construct a single-year model that can generate reasonably accurate predictions for an entire ice-free season. The ability of a singe-lake model to predict several years of temperature data is significantly improved by explicitly including information on ice-out date in the model. Our multiyear model for Lake Opeongo described most (18 of 22) years well. Years that were not well described were usually coincident with El Nino Southern Oscillation events. Used with caution, the method can be an effective tool for supplementing direct monitoring of littoral water temperatures and for generating historical water temperature estimates when direct estimates are lacking. These capabilities should be of particular use to fisheries biologists studying or managing populations of fish species with critical life stages that are affected by littoral water temperatures.

Impacts of acidification on the major sportfish species in Ontario (lake trout (Salvelinus namaycush), brook trout (S. fontinalis), smallmouth bass (Micropterus dolomieu), and walleye (Stizostedion vitreum)) were estimated from the results of extensive water quality surveys conducted during the period 1978–85. Local impacts, within the 17 000 km2 area most affected by emissions from Sudbury smelters, were also estimated and compared with the provincial values. The estimated numbers of acidic lakes in Ontario in which viable sportfish populations have disappeared were 119 lake trout lakes (5.1% of known lake trout lakes), 34 brook trout lakes (1.6%), 52 smallmouth bass lakes (2.2%), and 14 walleye lakes (0.3%). Most of these affected lakes were in the Sudbury zone, where fish populations, particularly those of lake trout, began declining more than 30 yr ago. The estimated numbers of critically acidic lakes in the Sudbury zone were 94 lake trout lakes, 14 brook trout lakes, 18 smallmouth bass lakes, and 7 walleye lakes. More recent sampling of a subset of Sudbury lakes indicated that substantial water quality improvements have occurred as a result of emission reductions from Sudbury smelters.

We examined changes in annual age-specific growth rates of 0-8-year-old lake trout Salvelinus namaycush in Lake Opeongo, Ontario, after the introduction of cisco Coregonus artedii into the lake in 1948. Growth rates of piscivorous lake trout (ages 3-8) initially increased as the occurrence of ciscoes increased, and then decreased as the abundance of piscivorous lake trout increased and the size of ciscoes eaten decreased. Growth of non-piscivorous 1- and 2-year-old lake trout decreased as the occurrence of insects decreased and the abundance of 1- and 2-year-old lake trout increased. Prey abundance, prey size, and intraspecific competition appeared to be major factors influencing growth rates. As ciscoes came to dominate the diet of adult lake trout, cannibalism by adults on younger lake trout decreased, as did the mortality suffered by young lake trout. The cisco introduction improved the existing sport fishery for lake trout by raising annual maximum sustainable yield from about 0.18 kg/hectare to about 0.48 kg/hectare; howeverm this increase was accompanied by heightened sensitivity of the population to overexploitation.

Creel survey and independent assessment data on the lake trout (Salvelinus namaycush) and smallmouth bass (Micropterus dolomieui) populations of Lake Opeongo were evaluated. Annual estimates of total mortality, fishing mortality, and abundance were generated for each population over the period 1936–83. Large variations in survey efficiency, angler efficiency, fishing mortality, and abundance were identified over this period. We argue that a creel survey, which is expected to provide reliable information on fish population dynamics, requires an overall study design which includes collection of data on the number and relative efficiency of different kinds of anglers and periodic assessment studies aimed at providing independent checks on both survey effectiveness and population behaviour.

We report on a 20-year integrated field, laboratory, and simulation study of the impact of thermal effluent from a nuclear power plant on the smallmouth bass Micropterus dolomieui population in Baie du Doré, Lake Huron. A quantitative model was constructed to forecast the range of likely effects of the plant on the population. The model was based on the findings of 50 years of basic research on smallmouth bass ecology in Ontario and on 15 years of environmental and biological data collected at Baie du Doré before the power plant became operational. The model embodies detailed descriptions of both the natural variation in water temperature at Baie du Doré, and the mechanisms whereby variations in temperature generate variations in cohort strength. Five years of monitoring data, collected after the plant became operational, were used to evaluate the forecasts of the model. Observed effects on timing of spawning and individual growth were generally within the range defined by the forecasts. The thermal effluent also altered the distribution of adult fish and this, in turn, led to an increase in the effectiveness of the angling fishery. Data from the postoperational period were used to refine the assessment model. The revised model was then used to evaluate the long-term effects on the population of the changes observed to date. We describe how the availability of extensive site-specific and species-specific data helped us to deal with technical problems that have plagued other impact assessment projects. We also discuss the contributions of basic research and quantitative modelling to the effectiveness of the study.