Methodological considerations concerning the use of length frequency analysis for growth studies in the Norway lobster, Nephrops norvegicus (L.)

The structure of length distributions of Nephrops norvegicus was investigat ed in order to understand the adequacy of length frequency analysis for est imation of growth rates and growth parameters. Samples from two areas were used: the Atlantic Ocean off the South coast of Portugal and the Euboikos G ulf in the Mediterranean. Monthly samples were obtained from October 1993 t o October 1995. For each area, month and sex, the samples were analyzed usi ng the program MIX in order to identify the mean length for each of the gro ups composing the length distributions. The mean values were used to estima te increments between consecutive groups within the same sample. Biological information extracted from the same samples and referring to maturation an d molting cycles, was used in conjunction with the mean values estimated in this work to interpret modal progression and growth rates. Results show th at modal progression analysis is not appropriate because the modes do not p rogress in a manner that allows the following of a particular group. Growth rates based on increments verified within each sample were also investigat ed. This approach proved to be better, although there were still some probl ems due to large year to year variability and the fact that the magnitude o f the increments did not decrease with increasing size This also poses prob lems regarding the fitting of growth curves to these data; in particular wh en deceleration of growth is required, as in the case of the von Bertalanff y growth curve.