Maturity and Molt Status of Crustacea: Determining Classification without Prior Criteria

Abstract

Existing methodology for estimation is reviewed for the situation where, a priori, the existence of two or more groups can be postulated but, in contrast with discriminant analysis, there is no sample in which the correct categories are known. Such mixture models are applied to two data sets related to the maturity and molt status of snow crab, Chionoecetes opilio, namely (1) the classification of crab as morphometrically mature or immature on the basis of the chela height – carapace width relationship and (2) the determination of the number of molts of crab, during a known period at liberty, from the relationship of size at release and size at recapture of tagged animals, in the latter example, a theoretical constraint is imposed that links the relationships at the different stages. The solution is obtained by "nesting" an iterative procedure within an EM algorithm. The method permits hypotheses concerning the number of groups to be tested, including the hypothesis that the data come from a single homogenous group, and each individual is assigned a probability of belonging to a group.