An analysis of meteorological and hydrographic records collected at South Bay, Ontario, over the past nine years shows that when the monthly mean surface water temperatures for successive years are plotted against the monthly mean air temperatures, the general relation can be described by two linear regressions. One of these lines corresponds to the May–July period in which the temperatures rise, while the other covers the August–November period in which they fall. A quite comparable picture is also obtained for Lake Opeongo, Ontario, and Lake Mendota, Wisconsin. The latter bodies differ markedly from South Bay in morphometry and location. Lakes Opeongo and Mendota have continental situations, whereas South Bay is enclosed in a comparatively small land mass which in turn is surrounded by very large water masses.

972. Effects of eutrophication on salmonid communities in oligotrophic lakes. J' Fish' Res. Bd. Canada 29:975-983.Oligotrophic lakes respond to progressive eutrophication by a sequence of predictable events. Increased nutrient loads and subsequent increased plant production result in alterations in the abiotic environment, including changes in the color and transparency of the water, increased turbidity, oxygen depletion in the hypolimnion, and increased chemical stratification. The physico-chemical changes precipitate biotic changes among the phytoplankton, littoral algae, zooplankton, and benthos. The salmonid community may respond initially with an increased body growth rate in various taxa and a higher incidence of parasitism, but later inhibition of natural reproduction occurs, and finally, the taxa are replaced by others that can survive in the changed environment.A relation between natural nutrient loading (expressed in terms of a morphoedaphic index) and yield (both quantitative and qualitative) is proposed as an aid to determining the natural successional status of a lake. Knowing the natural baseline of a particular lake the fisheries managers can judge the nature and size of responses due to cultural nutrient loading and then alter the rate of cultural nutrient loading to modify the ecological effects, or they can use biological engineering to capitalize on the present conditions'Among the most important effects of eutrophication is the increased vulnerability of sedentary discrete stocks to changes in other stresses such as fishing. Corsv, P. J., G. R. SraNcr,nn, D. A. Hunuv, .qNo A. M. McCoMers. 1972. Effects of eutrophication on salmonid communities in oligotrophic lakes. J. Fish. Res. Bd. Canada 29:975-983.

Selectivity curves for three species of fish taken in South Bay, Manitoulin Island, and in Georgian Bay during 1954–59 with gangs of nylon gillnets differed with respect to mode, skew, and kurtosis. A graphic method was developed to describe one aspect of body shape, namely the relation between girth and distance from the snout, which would affect selectivity. Surprisingly, the shapes of mature (but not gravid) and immature perch and suckers of both sexes could be described by the same curve. Hence, differences in body shape did not account for differences in the selectivity of these species. In contrast, the taper for gravid female perch was markedly different from that for nongravid, as might be expected. Catches in which the positions where caught were carefully identified showed that most of the fish were caught either on the opercle or near the position of maximum girth. That the fish could be caught at different positions accounted for most of the skew in the selectivity curves based on maximum girth. Selectivity curves plotted on the basis of girth where caught, rather than on maximum girth or length, show that efficiency of capture was maximal when the girth of the fish was 1.0–1.2 times as great as the perimeter of the mesh. Efficiency was negligible at girth/perimeter ratios smaller than 0.8 or 0.9 and it declined rapidly at ratios greater than 1.2. However, a few fish were taken at ratios up to 1.5 or 1.6.

In reviewing the literature dealing with the influence of physical, chemical and biochemical factors on the development and decline of phytoplankton pulses, this paper first outlines the seasonal cycle of phytoplankton pulses which occurs in many parts of North America and then presents evidence that this cycle is caused chiefly by the seasonal change in water temperatures resulting from the change in solar radiation. The way in which this cycle is sometimes obscured by irregular changes in phytoplankton numbers caused by local conditions of weather or nutrient supply is next illustrated. The climate, weather and chemical conditions are then resolved into the components light intensity, duration of illumination, temperature, concentration of nutrients, ionic balance and pH, and these are classified according to specific effects on the growth and survival of plankton algae as limiting, controlling and lethal factors. The roles of factors originating within the organisms (i.e., autotoxins and antibiotics) are also considered. The discussion of laboratory investigations leads to certain criticisms of present culture methods and to some reinterpretation of observations obtained in some of these investigations. Finally, this paper illustrates the phenomenon of acclimation as it applies to phytoplankton, and it stresses the need for adequate prehistories of culture stocks in order that the results of different investigations can be validly compared.

The methods by which the curve of gill‐net selectivity has been determined by various workers are discussed. An empirical demonstration of the applicability of the normal curve of probability to the distribution of size classes of lake whitefish (Coregonus clupeaformis) in catches from experimental gangs of gill nets fished in South Bay, Lake Huron, from 1954 to 1958 is presented. A plot was made of the numbers of fish caught in each length class against the logarithms of the ratios of the girth of the fish to the perimeter of the mesh. An adjustment was made to allow for the presence of unequal numbers of the various length classes in the population fished. The curves for all length classes concerned when superimposed were found to be essentially the same and could be described by the normal frequency distribution with the mode at 0.100 and a standard deviation of ± 0.0514, expressed as logarithms of fish girth to mesh perimeter. The efficiencies of nylon and cotton nets are discussed on the basis of the literature and of experiments in which gangs with alternate cotton and nylon nets were fished in South Bay. These experiments indicate that the nylon nets take about three times as many whitefish as the cotton, but take fish of essentially the same size composition.