Is the brown shrimp Crangon crangon (L.) population of the Vaccares lagoon(Camargue, France, Rhone delta) an annual population

To study the population dynamics of Crangon crangon in the Vaccares lagoon in the Camargue (Rhone delta), the age of brown shrimp was determined using the method of Tiews (1954). This method consists in counting the number of segments of the outer branch of the first antenna. This number is correlat ed with the number of moults already accomplished by the shrimp. The durati on of the intermoult depends on water temperature. Adapting this model to o ur environmental data, we determined the age of the brown shrimp. Hatching of larvae occurs in the sea in May, and juveniles enter the lagoon from Jun e to September. Our results showed that Crangon crangon in the Vaccares lag oon reached sexual maturity between 6 and 11 months, and individuals are 1 year old when they leave the lagoon. A few adults from the previous year (1 (+)) migrate back to the lagoon the following year, but these older individ uals represent only a very small part of the population, and have only been observed occasionally. The Vaccares brown shrimp population can therefore be considered as annual, in contrast to northern European populations. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS .