Preliminary investigation of direct age determination using band counts in the gastric mill of the blue swimmer crab (Portunus pelagicus Linnaeus, 1758) in two salt-water lakes in the Eastern Mediterranean

Abstract

The blue swimmer crab, Portunus pelagicus (Linnaeus, 1758), is an important component of the crustacean fisheries in Egypt. This fishery occurs mainly in two salt-water lakes in the Eastern Mediterranean, the coastal Lakes Bardaweel and Timsah, which is located about 100 km further south. We investigated for the first time the applicability of using growth band counts in thin sections of two gastric mill ossicles (mesocardiac and zygocardiac) as age indicator in the blue swimmer crab collected from the two lakes. There was no significant difference in band counts in thin sections of the two ossicles, although the zygo-cardiac ossicles were easier to process. Three growth bands were observed in the ossicles suggesting three years as the species life span. Meanwhile, size modal analysis suggested that the longevity of the species in the two lakes is only two years. Mean carapace width (mm) of the first two presumed year-classes obtained from the band counts and size modal analysis ( y 1 72.8 and 76.2 mm and y 2 91.5 and 97.8 mm, respectively) were not significantly different in Lake Timsah. In Lake Bardaweel, however, the mean size observed at the first two years was different ( y 1 78.5 and 102.2 mm and y 2 94.5 and 128.0 mm for the band count and size modal analysis, respectively). Sex-specific size-at-band counts (estimated age) was demonstrated for the species in both lakes. The models were fitted by power equations and there was no significant difference in the growth models between sexes in the two lakes. Further validation needs to be done to confirm the annual deposition of the growth bands before using the results on a routine basis.