Assessment of the species Penaeus latisulcatus and Penaeus japonicus in the shallow-water shrimp trawl fishery at the Sofala Bank, Mozambique.

Abstract

Commercial data from 1995 and 1996 of the Pescamar fishing company in Mozambique were used to study the stock dynamics of the two shrimp species Penaeus latisulcatus (marfil) and Penaeus japonicus (tiger). These species are harvested in the Sofala Bank shrimp fishery secondary to the primary species, P. monoceros and P. indicus. The proportion of P. latisulcatus and P. japonicus in the total catches has increased over the last several years, and this has made a need to study their population dynamics by examining mean CPUE, growth parameters, spawning, maturity and morphometric relationships. A new method for estimating mean CPUE, based on a two-stage sampling method over the usual method (total catch divided by total effort) was proposed in this study. The two methods for estimating mean CPUE were compared and it was documented that the new method may give a better reflection of the relative abundance because it smoothes the biases that are inherent in commercial data. Results showed that the mean CPUE was higher for marfil than tiger shrimp during 1995 and 1996. Differences in relative abundance in the catches were attributed to seasonal variations in spatial and temporal distribution. Growth parameters could not be estimated because the commercial data were not representative of all length classes in the population due to gear selectivity. Both marfil and tiger shrimp are multiple spawners and produced several recruitment peaks both in 1995 and 1996. Size-at-maturity for females marfil shrimp was estimated at 42 mm carapace length while females tiger shrimp mature at approximately 43 mm carapace length. While there appeared to be a strong relationship between carapace length and total body weight for both species, no significant differences were observed in these relationships between male and female tiger shrimp while there was a significant difference between male and female marfil shrimp. Generally, females attain a larger carapace length than males in both species.