Estimating the Intermolt Periods in Asynchronously Molting Crustacean Populations

Abstract

SUMMARY In some crustacean populations, including many tropical lobsters and crabs, molting occurs asynchronously throughout the year. That is, at any time of year, the animals are uniformly distributed throughout the intermolt cycle. We present methods for finding maximum likelihood estimates of the size-specific intermolt period for this situation when mark-recapture data are available consisting of initial size, time at liberty, and whether the animal molted while at liberty. Alternatively, the intermolt period can be determined by the same estimation procedure when wild animals are brought into captivity and observations are made on which animals molt within some fixed time period or before a fixed date. The method is applied to data on spiny lobster (Panulirus argus) to estimate intermolt period as an exponential function of length. Crustacean growth is a discontinuous process that is often described in terms of two components: the change in size at molt and the duration of the intermolt period (Botsford, 1985). Information on these processes is used to construct a model of growth in size over time for use in fishery assessment models. Both growth components vary with the size of the animal. These can be studied by observing animals held in captivity though the results may not be representative of growth in wild populations. An alternative is to estimate the components of growth from mark-recapture data. In this paper, we focus on the problem of estimating the intermolt period under the assumption that the individuals in a population at any time are uniformly distributed throughout their intermolt period. We use the method of maximum likelihood to derive a procedure for estimating the intermolt periods of tropical and subtropical decapods from mark-recapture data when seasonal changes in growth are negligible. The procedures can also be used to analyze laboratory data on the proportion of animals molting when animals are held for periods of time sufficiently short to minimize the effects of captivity. In Section 2, we consider mark-recapture studies and develop the estimators for intermolt period as a function of body size and other covariates. The growth of spiny lobsters