Abstract
The giant clam Tridacna maxima has been largely overexploited in many tropical regions over the past decades, and was therefore listed in appendix II of the Convention of International Trade in Endangered Species (CITES) in 1985. In French Polynesia, several atolls and islands harbor the world’s highest stocks of giant clams in very shallow and accessible areas, which are therefore highly vulnerable to fishing pressure. The local fishery authority (i.e., Direction des Resources Marines or “DRM”) implemented several management schemes in 2002 to control and regulate fishing pressure. However, for further decisions DRM was missing a sensitivity analysis on the effectiveness of the possible management actions. Here, we report on the use of a deterministic Viable Population Analysis (VPA) and spatially-explicit age-based population model that simulated the 30-year trajectory of a Tridacna maxima stock under different management approaches. Specifically, given various scenarios of intra-island larval dispersal, we tested which of No-take-Areas (NTAs), rotational closures, size limits, quotas, and restocking schemes would lead to the highest future stocks in Tubuai and Raivavae, two exploited islands of the Austral archipelago. For both islands, stock abundances were estimated in 2004/2010 and 2005/2010 respectively, and natural mortalities were assessed previously only in Tubuai. When compared to field data, the model successfully predicted the 2010 stocks for Tubuai, but proved to be less reliable for Raivavae, where natural mortality rates may well be different from those on Tubuai. For Tubuai, the spatial model suggested that reducing fishing effort (through fixed quotas) and banning fishing below the 12 cm size limit (as currently implemented) were the most effective management actions to sustain T. maxima populations into the future. Implementing NTAs was of poor effectiveness. NTAs increased giant clam stock inside the protected area, but also increased overfishing in the neighboring areas, and were ineffective overall.
Highlights
In 2009, around 80 million tons of marine resources were captured worldwide [1]
The results, detailed here for French Polynesia islands, provide fresh insights into giant clam management that should be of interest to a broad spectrum of Pacific Ocean Islands
The southwestern reef flat’s hoa displayed the greatest densities of giant clams, whereas the northern reef flat harbored less than 0.17 ind.m22 in 2005 and 0.35 ind.m22 in 2010
Summary
In 2009, around 80 million tons of marine resources (e.g., fish, and invertebrates such as clams, sea cucumbers) were captured worldwide [1]. Evaluation of giant clam stock abundance for two Austral Archipelago islands and five Tuamotu Archipelago atolls [14,9]; 2) an assessment of giant clam population dynamics (mortality and growth) and fishing pressure for two atolls (Tatakoto, Fangatau) and one island (Tubuai) [15]; and 3) the promotion of local comanagement actions in agreement with all stakeholders This successfully led to the first No-Take Area (NTA) worldwide designed for giant clam protection, in Tatakoto atoll in 2004 [14]. Building upon the first NTA implemented in Tatakoto in 2004, Gilbert et al [15] suggested an extended network of 9 small additional NTAs for this atoll Another indirect strategy to reduce fishing pressure on natural stock is to promote aquaculture and spat collection [9]. The results, detailed here for French Polynesia islands, provide fresh insights into giant clam management that should be of interest to a broad spectrum of Pacific Ocean Islands
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