Effects of reef-top gathering and fishing on invertebrate abundance across take and no-take zones

Abstract

Gathering of molluscs on the reef-top, largely by women, is part of the traditional fishery practised by Bedouin in South Sinai, Egypt. The catch is dominated by Tridacna spp. and Octopus spp. Within the Nabq Managed Resource Protected Area, on the southern Gulf of Aqaba coast, a network of no-take zones (NTZs) was established (in 1995) to promote sustainable management of finfish stocks. Since this zoning also applied to exploitation of invertebrates, surveys of large species of molluscs and echinoderms across selected NTZs and adjacent fished areas were conducted (over 2000–2002) to assess any effects of gathering. Pooled data from all three years showed significantly higher abundances of Tridacna and Tectus dentatus in the NTZs, with greater abundances occurring at the reef edge zone. Size–frequency distributions revealed both a greater size range of Tridacna and a greater mean size of both Tridacna and Te. dentatus, within the NTZs, as compared to the fished areas. The size–frequency distribution of gleaned Tridacna, determined from discarded shells, also showed a smaller size range than did live Tridacna within the NTZs. By contrast, holothurians, which are not currently exploited by the local Bedouin, showed greater abundance in fished areas than in NTZs. Large diadematid sea urchins, along with the non-commercial strombs, Strombus gibberulus and Strombus fasciatus, were also significantly more abundant within fished areas, an effect presumed due to reduced predation pressure from fish as a result of fishing. Within the fished areas, there was a positive relationship between local abundance of Tridacna and of diadematids, thought to be due to high densities of the urchins acting as a deterrent against gathering. The establishment of NTZs has led to significant differences in invertebrate populations between take zones (TZ) and no-take zones. However, since there can be little or no spillover of adults of less mobile or sedentary invertebrates from NTZs to fished areas, any benefit to the fishery will depend largely on the greater larval production and export from protected populations.