Antoine Gilbert

Extraordinarily high giant clam density under protection in Tatakoto atoll (Eastern Tuamotu archipelago, French Polynesia)

The remarkable abundance and dominance of the tridacnid clam Tridacna maxima in Eastern Tuamotu archipelago (French Polynesia) has been recorded since at least Salvat (1972). For instance, densities at Reao and Fangatau atoll reached 224 and 136 ind m, respectively (Salvat 1972; Andréfouët et al. 2005). These values were qualified as ‘‘extraordinary’’ compared to other parts of the world (Green and Craig 1999). In April 2004, during a stock assessment survey in Tatakoto atoll (17 20¢S, 138 20¢W) based on the methodology explained in Andréfouët et al. (2005) we recorded with 0.25 m quadrats, a density of up to 544 ind m on the slopes of ‘‘mapiko’’ (i.e. natural accumulation of dead and live clam shells in Tuamotu language) (Fig. 1). About 10% of the 282 surveyed quadrats contained above 200 ind m. Such densities are possible with a negative size-density relationship that highlights very compact aggregates of small individuals. Since Tatakoto atoll increasingly exports clam meat towards the Tahiti market, in agreement with the population, the Service de la Pêche (Fishery Services of French Polynesia) has classified as no-take-area the 0.5 km of lagoon area where the highest clam densities occur (Fig. 1). This is not only to protect the local stock, but also the overall sustainability of this small fishery since other studies in progress reveal that this area,

Best Management Strategies for Sustainable Giant Clam Fishery in French Polynesia Islands: Answers from a Spatial Modeling Approach

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.

First observation of the giant clam Tridacna squamosa in French Polynesia : a species range extension

The most widespread species of giant clam, Tridacna maxima, was believed to be the only giant clam species occurring in French Polynesia (Laurent et al. 2002). In mid 2006, while surveying T. maxima in Tubuai Island (Australes archipelago, French Polynesia, 23 20¢S, 145 20¢W, Gilbert et al. 2006), the larger fluted giant clam, Tridacna squamosa was identified (Fig. 1b). The specimen pictured, and several others, were found on the outer slope of the barrier reef at depths of approximately 20 m. Further genetic analysis, based on the sequencing 431 bp of 16S mtDNA confirmed the morphological identification after comparison with samples in Genebank. This result enlarges the geographical range of T. squamosa to between Polynesia (145 W) and East Africa (Rosewater 1965). Other recent observations made on high latitude islands of French Polynesia (Galzin et al. 2006) have changed the previous biogeographic patterns described for different taxa. This new occurrence confirms the potential importance of anti-tropical zones in the dispersal of species throughout the Pacific Ocean (Randall 1995). We suggest that several species may have colonised the remote eastern islands in the Pacific though high latitude islands like the Australes, before colonizing more tropical areas.

Alert thresholds for monitoring environmental variables: a new approach applied to seagrass beds diversity in New Caledonia.

Monitoring ecological variables is mandatory to detect abnormal changes in ecosystems. When the studied variables exceed predefined alert thresholds, management actions may be required. In the past, alert thresholds have been typically defined by expert judgments and descriptive statistics. Recently, approaches based on statistical power were also used. In New Caledonia, seagrass monitoring is a priority given their vulnerability to natural and anthropic disturbances. To define a suitable monitoring strategy and alert thresholds, we compared a Percentile Based Approach (PBA) and a sensitivity analysis of power (SAP). Both methods defined statistically relevant alert thresholds, but the SAP approach was more robust to spatial and temporal variability of seagrass cover. Moreover, this method characterized the sensitivity of threshold values to sampling efforts, a useful knowledge for managers.

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

Shell growth, reproduction, and natural mortality of the giant clam Tridacna maxima were characterized over a two-year-period in the lagoon of the high island of Tubuai (Austral Archipelago) and in the semi-closed lagoon of Tatakoto (Tuamotu Archipelago) in French Polynesia. We also recorded temperature, water level, tidal slope, tidal range, and mean wave height in both lagoons. Lower lagoon aperture and exposure to oceanic swells at Tatakoto than at Tubuai was responsible for lower lagoon water renewal, as well as higher variability in temperature and water level at Tatakoto across the studied period. These different environmental conditions had an impact on giant clams. Firstly, spawning events in the lagoon of Tatakoto, detected by gonad maturity indices in June and July 2014, were timed with high oceanic water inflow and a decrease in lagoon water temperature. Secondly, temperature explained differences in shell growth rates between seasons and lagoons, generating different growth curves for the two sites. Thirdly, local mortality rates were also found to likely be related to water renewal patterns. In conclusion, our study suggests that reef aperture and lagoon water renewal rates play an integral role in giant clam life history, with significant differences in rates of shell growth, mortality and fertility found between open versus semi-closed atoll lagoons in coral reef ecosystems.

Endangered New Caledonian endemic mushroom coral Cantharellus noumeae in turbid, metal-rich, natural and artificial environments

Since its description in 1984, little attention has been paid to the New Caledonian endemic mushroom coral Cantharellus noumeae (Fungiidae), an IUCN Red-listed, endangered coral species. Our study presents the first ever quantitative assessment conducted on C. noumeae populations for two contrasting sites in the same turbid bay. Sites differed by their substrates of artificial or natural origins. Metal concentrations of superficial sediment were measured. C. noumeae was found in high densities in metal-rich and turbid environments at both locations, reaching up to 288 individuals per 50m(2). It was 3.5 times more abundant on natural rock than on artificial substrates. Recruitment was also higher proportionally on rock (47% vs 7-14%). The composition of the associated coral communities included 30-37 species occurring in low densities. Our findings clarify the environmental niche of this species and its colonization potential, in order to eventually better characterize its conservation status.

Nickel and ocean warming affect scleractinian coral growth

The sensitivity of corals and their Symbiodinium to warming has been extensively documented; however very few studies considered that anthropogenic inputs such as metal pollution have already an impact on many fringing reefs. Thus, today, nickel releases are common in coastal ecosystems. In this study, two major reef-building species Acropora muricata and Pocillopora damicornis were exposed in situ to ambient and moderate nickel concentrations on a short-term period (1h) using benthic chamber experiments. Simultaneously, we tested in laboratory conditions the combined effects of a chronic exposure (8weeks) to moderate nickel concentrations and ocean warming on A. muricata. The in situ experiment highlighted that nickel enrichment, at ambient temperature, stimulated by 27 to 47% the calcification rates of both species but not their photosynthetic performances. In contrast, an exposure to higher nickel concentration, in combination with elevated temperature simulated in aquaria, severely depressed by 30% the growth of A. muricata.

Localised and limited impact of a dredging operation on coral cover in the northwestern lagoon of New Caledonia

We report here an interannual survey (2006-2012) of coral cover in the northwestern lagoon of New Caledonia, to assess the impact of an important dredging operation (August 2008-February 2010) associated with the construction of the largest nickel mining site in the Pacific. A BACI (Before-After Control-Impact) analysis failed to detect any significant interaction between period (before, during, and after dredging) and the category of the stations (impact vs. control). Among the 31 stations surveyed, only seven showed decreasing coral cover during the study period, mainly due to a decline in Acroporidae. However, the relationship between the dredging and this decrease was highly plausible only for one station, situated 0.9km from the dredging site. High hydrodynamism in the study area, the abundance of resistant corals and efficient protective measures during the dredging operation might explain these localised and limited impacts.

A massive subtidal aggregation of hermit crabs in Surprise Atoll lagoon, New Caledonia

Many hermit crab species can form large aggregations, sometimes numbering thousands of individuals (e.g., Gherardi and Vannini 1992). Research suggests that such clustering behavior can reduce predation risk and positively influence feeding, reproduction, and shell exchange. While clustering is well documented among lineages inhabiting terrestrial and intertidal zones, it remains poorly studied at subtidal depths where the greatest hermit crab diversity exists. The limited work that has examined this behavior across multiple habitats found clustering to be rare or entirely absent among species occupying subtidal depths (including in six Dardanus species; Barnes and Arnold 2001, but see Ramsay et al. 1997). Here, we report clustering behavior in the subtidal hermit crab species Dardanus scutellatus (H. Milne Edwards, 1848) that formed a massive aggregation of tens of thousands of individuals from 10 to 15 m depth (Fig. 1). Observations were made at approximately 0830 hrs, November 24, 2013 in the remote Surprise Atoll of Entrecasteaux Reef (New Caledonia), on a gently sloping lagoon bottom of sand-covering pavement near 18.4775 S, 163.0835 E. Though our survey was very limited (<25 min, coveringapproximately 250 m), we did not visually locate the end of this cluster. We also saw no obvious signs of a significant food source that might have attracted these crabs. No other benthic fauna was observed, but sea snakes swam through the area. To our knowledge, such a large subtidal aggregation has never been formally reported, although scientific divers recorded a similar event 15 December 2009 in a similar habitat on Beautemps-Beaupré Atoll (Laboute, pers. comm.). The novelty of this observation emphasizes that our knowledge of clustering behavior in hermit crabs remains incomplete.

Enhancement of coral calcification via the interplay of nickel and urease

Corals are the main reef builders through the formation of calcium carbonate skeletons. In recent decades, coral calcification has however been impacted by many global (climate change) and local stressors (such as destructive fishing practices and changes in water quality). In this particular context, it is crucial to identify and characterize the various factors that promote coral calcification. We thus performed the first investigation of the effect of nickel and urea enrichment on the calcification rates of three coral species. These two factors may indeed interact with calcification through the activity of urease, which catalyzes the hydrolysis of urea to produce inorganic carbon and ammonia that are involved in the calcification process. Experiments were performed with the asymbiotic coral Dendrophyllia arbuscula and, to further assess if urea and/or nickel has an indirect link with calcification through photosynthesis, results were compared with those obtained with two symbiotic corals, Acropora muricata and Pocillopora damicornis, for which we also measured photosynthetic rates. Ambient and enriched nickel (0.12 and 3.50 μg L-1) combined with ambient and enriched urea concentrations (0.26 and 5.52 μmol L-1) were tested during 4 weeks in aquaria. We demonstrate in the study that a nickel enrichment alone or combined with a urea enrichment strongly stimulated urea uptake rates of the three tested species. In addition, this enhancement of urea uptake and hydrolysis significantly increased the long-term calcification rates (i.e. growth) of the three coral species investigated, inducing a 1.49-fold to 1.64-fold increase, respectively for D. arbuscula and P. damicornis. Since calcification was greatly enhanced by nickel in the asymbiotic coral species - i.e. in absence of photosynthesis - we concluded that the effect of increased urease activity on calcification was mainly direct. According to our results, it can be assumed that corals in some fringing reefs, benefiting from seawater enriched in nickel may have advantages and might be able to use urea more effectively as a carbon and nitrogen source. It can also be suggested that urea, for which hotspots are regularly measured in reef waters may alleviate the negative consequences of thermal stress on corals.