Techniques of estimating population size, level of fishing, and the degree of dependence of fishing success on environmental factors are examined on the basis of tagging, catch and effort data. A new method is developed to estimate population size from catch, effort, and temperature data when the catchability varies with temperature.The methods of estimation discussed are applied to data collected from a number of lobster fisheries on Canada's Atlantic coast. Analysis confirms a relationship between the catchability of lobsters and bottom temperature. Differences in this relationship are found between areas and between tagged and untagged lobsters within areas. It is suggested that these differences are attributable to the differences in densities as well as to aggregations of lobsters and fishing. The effect of these aggregations on population size estimates is considered.Calculated average catchabilities at comparable temperatures are different for different areas. These differences are correlated with the numbers of trap hauls per day per square miles fished. It is suggested that the differences in the catchabilities might be due to interactions between units of gear not predicted by the customary relationship between catch and effort.