Abstract
The structure of length distributions of Nephrops norvegicus was investigated in order to understand the adequacy of length frequency analysis for estimation of growth rates and growth parameters. Samples from two areas were used: the Atlantic Ocean off the South coast of Portugal and the Euboikos Gulf in the Mediterranean. Monthly samples were obtained from October 1993 to October 1995. For each area, month and sex, the samples were analyzed using the program MIX in order to identify the mean length for each of the groups composing the length distributions. The mean values were used to estimate increments between consecutive groups within the same sample. Biological information extracted from the same samples and referring to maturation and molting cycles, was used in conjunction with the mean values estimated in this work to interpret modal progression and growth rates. Results show that modal progression analysis is not appropriate because the modes do not progress in a manner that allows the following of a particular group. Growth rates based on increments verified within each sample were also investigated. This approach proved to be better, although there were still some problems due to large year to year variability and the fact that the magnitude of the increments did not decrease with increasing size. This also poses problems regarding the fitting of growth curves to these data; in particular when deceleration of growth is required, as in the case of the von Bertalanffy growth curve.
Highlights
The sensible exploitation of a marine living resource requires some basic information that can be used to evaluate the condition of the resource and the management strategies to apply. Part of this information is of a biological nature, such as the estimation of mortality and growth rates
Growth curves are the most common form of expressing growth rates in models used in stock assessment
Growth curves express the average growth rate of a population and usually consist of an equation where size is a function of age
Summary
The sensible exploitation of a marine living resource requires some basic information that can be used to evaluate the condition of the resource and the management strategies to apply. Part of this information is of a biological nature, such as the estimation of mortality and growth rates. The most widely used growth curve in fisheries is the von Bertalanffy growth curve (Bertalanffy, 1938; Beverton and Holt, 1957) This equation has been incorporated in models of yield per recruit and cohort analysis both in the form of length and weight as a function of age, and has the important advantage of having parameters with biological meaning. Application of the length equation requires decelerating growth throughout the life of a population, a condition which is not always met by the data
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