Age and growth of Nephrops norvegicus in the Catalan Sea, using length-frequency analysis

Abstract

Length-based methods were used to study age and growth of Nephrops norvegicus in the Catalan Sea. The width of the class interval was examined and 1 mm was found most appropriate for the carapace length-based analysis. Two direct methods, Bhattacharya's and Cassie's, and one indirect, ELEFAN I, were used for age determination. Cassie's method was found very subjective and inadequate, while Bhattacharya's and ELEFAN I more effective, easy to apply and more objective. The results of Bhattacharya's method showed the best agreement with those derived from captivity studies; seven age groups were well defined for males and four for females. Mature female age groups were difficult to detect by any method, indicating the effect of reproduction on female growth. Growth parameters of the Von Bertalanffy model and of the seasonally oscillating Von Bertalanffy formula, estimated by FISHPARM and ELEFAN I, were quite close to those derived from captivity studies. ELEFAN I seemed to produce the best results and the Von Bertalanffy model was not found to be superior to its seasonal counterpart, a fact that gives reason for futher studies. Pauly's empirical formula for fish, used for the estimation of instantaneous natural mortality of this species gave reasonable values. The similarity between the results of the present work and those derived from captivity studies, gives support to the potential use of length-based analysis for the estimation of N. norvegicus biological parameters; however, many factors should be taken into account in order to achieve representative samples suitable for length-based analysis.