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The efficient planning of a Petersen‐type mark and recapture experiment requires some knowledge of the order of magnitude of the population size N. Sample sizes M and C of the mark and recapture samples, respectively, may then be ascertained on the basis of a guessed value of N to achieve any desired degree of accuracy with any specified degree of confidence. Restrictions on the sample sizes M and C are that MC must exceed 4 times the guessed value of N, and the total costs of M and C must be equal. Graphs and formulas are given defining sample size to attain preassigned levels of accuracy and precision of population estimation. A method of choosing sample sizes such that experimental costs are minimized is described.

Measures of thermal habitat space were developed by integrating, over time during the summer period, the amount of lake bottom area and pelagic volume with water temperatures within species' optimal thermal niches. These species' specific measures, thermal habitat area, THA (hectares per 10 d), and thermal habitat volume, THV (cubic hectometres per 10 d), were used as predictor variables in regression equations estimating the total sustained yield, SY (kilograms per year), of each of four commercially important species: lake trout, Salvelinus namaycush; lake whitefish, Coregonus Clupeaformis; walleye, Stizostedion vitreum vitreum; and northern pike, Esox lucius. One or both of THA and THV were strongly correlated with SY for each of the four species for a set of 21 large north-temperate lakes. Several other habitat variables were assessed with respect to species' SY: total lake area and volume, mean depth, total dissolved solids, and the ratio of the latter two as R. A. Ryder's morphoedaphic index. The various statistical results are interpreted with respect to the ecology of the four species.