Change-in-ratio and index-removal methods for population assessment and their application to snow crab (Chionoecetes opilio)

Change-in-ratio and index-removal estimators are presented in a general form suitable for fisheries studies. We also show how to combine the two approaches in a single estimator. It is necessary to sample the population before and after the fishery, and to determine the total harvest and its composition, in order to use these methods. We use the methods to estimate the population of legal-size snow crabs (Chionoecetes opillio) in St. Mary's Bay, Newfoundland, before and after the fishery, and to estimate the catchability coefficient and exploitation rate. It is also possible to estimate the abundance of pre-recruits but this requires the assumption of equal catch ability of all animals, a condition that may not be met. These methods have been largely neglected by fishery scientists; however, they seem to be ideally suited for studies of many populations of large crustaceans.