Population status of giant clams (Mollusca: Tridacnidae) in the northern Red Sea,Egypt

Throughout their range, giant clams (family Tridacnidae) are increasingly threatened by anthropogenic impacts and natural disasters, but little is known about their population status. In this first assessment of the tridacnid population at Abu Sauatir in the northern Red Sea, a total of 491 m² were surveyed and >200 clams recorded. Tridacna maxima was the only species found. The population's live:dead ratio was 3:1. Overall clam density was 0.08?±?0.008 live and 0.02?±?0.007 dead individuals per 0.25 m². Greatest densities occurred on the reef flat in 1 m depth (live), and on the northern reef slope in deeper waters (dead). On the slope, live clam density decreased significantly, whereas dead clam density increased significantly with depth. Sizes of live and dead individuals differed significantly. Live clams ranged from 1 to 30 cm (median 5 cm). Juveniles ≤2 cm (8.2% of the population) and individuals >11 cm occurred on the reef slope but not on the reef flat. Live clam sizes did not differ significantly between reef sites. Dead clam size ranged from 2 to 15 cm (median 6.5 cm). 2.1% of the empty shells were ≤2 cm long. Dead clam sizes differed significantly between 5 and 10 m depth on the northern reef slope. The low clam abundance (live and dead) in the shallowest and most easily accessible areas of the reef flat, combined with small sizes, strongly suggest artisanal reef-top gathering for meat and shells.