Drivers of density for the exploited giant clam Tridacna maxima: a meta‐analysis

Abstract

Giant clam populations have been over‐exploited throughout their range over the past decades for their meat and shells. Tridacna maxima, commonly known as the ‘small giant clam’, has remained relatively untargeted by fishers in areas where larger species occur (e.g. Tridacna squamosa), and high densities of the species are still observed on some isolated and enclosed reefs of the Central Pacific. However, it is unclear whether reported discrepancies in densities worldwide reflect differences in fishing pressure only or a combination of differences in exploitation levels and environmental forcing. We reviewed T. maxima surveys throughout its range to (i) identify patterns of density at global scale, site scale (e.g. island) and intrasite scale; (ii) discuss the influence of sampling method on density estimates; and (iii) identify the primary drivers of giant clam density along gradients of human pressure and natural forcing. We found 59 studies that reported density estimates for 172 sites across 26 countries in the Indo‐Pacific and Red Sea. At intrasite scale, densities were strongly dependent on sampling protocols and surveyed habitats. At site scale, we found close links between T. maxima density and human population per reef area, suggesting that isolated reefs where exploitation only recently started may be more vulnerable to stock collapse in the future. Density patterns were also found to vary significantly depending on reef type (e.g. atoll, island, continental coastline). We discuss how natural processes and fishing pressure may control population dynamics and densities among sites, and make recommendations for future research.

Growth, Survival and Reproduction of the Giant Clam Tridacna maxima (Röding 1798, Bivalvia) in Two Contrasting Lagoons in French Polynesia

“…maxima ), where light provides them most of the energy necessary for survival, growth, and reproduction, owing to the ability of their symbiotic zooxanthellae to photosynthesize [2]. The geographical ranges of these two widespread species also cover a variety of reef types, including continental islands, open and closed atoll and submerged reefs, all characterized by very different biophysical environments [3]. …”

“…Tridacna maxima is currently classified as lower risk/conservation dependent (i.e., species of ‘least concern’) under the IUCN Red List for threatened species. Densities and stocks can vary locally by several orders of magnitude, depending on past and present fishing pressure and fluctuations in environmental drivers [3]. The increasing commercial interest in T .…”

“…maxima stock and individual density estimates of T . maxima were derived for a number of locations and reef types (e.g., Cook Islands, French Polynesia, Kiribati, Palau, Samoa; see [3] for a review). Specifically in French Polynesia, stock estimates for seven island and atoll lagoons were conducted in situ in 2004/2005 (Tubuai and Raivavae islands, Austral Archipelago; and Reao, Pukarua, Napuka, Tatakoto and Fangatau atolls, eastern Tuamotu [7]).…”

Assembly rules of fish communities in Tuamotu archipelago atoll lagoons: The case of Fangatau, a lagoon dominated by giant clam habitats

“…Noteworthy was the absence of lagoons characterized by the presence and dominance of unusual habitats. In particular, the eastern Tuamotu Archipelago presents several small shallow lagoons that are unique by the abundance of dead and live shells of Tridacna maxima giant clams (Van Wynsberge et al 2016). These previously unstudied habitats could influence a whole range of processes, fish traits, and community features, resulting in specific assemblages.…”

“…Giant clam dominated lagoons have been recently the focus of a large research effort (Andréfouët et al 2005a;Gilbert et al 2006;Van Wynsberge et al 2016). Among them, Fangatau, Tatakoto, Napuka, Reao, Pukarua, Tureia and Vahitahi atolls have all been studied to assess giant clam stocks in the past 10 years (Fig.…”