Biology Fisheries

The Grit in the Oyster – Pearl Farming in French Polynesia

Cartier LE, Carpenter KE. The influence of pearl oyster farming on reef fish abundance and diversity in Ahe, French Polynesia. Mar Pollut Bull. 2014;78(1-2):43–50. doi:10.1016/j.marpolbul.2013.11.027.

Aquaculture is growing exponentially around the globe, yet there are still various sectors where the full dynamics and interactions with surrounding ecosystems are unknown or not well studied.  Pearl oyster farming is unique in that, instead of focusing on food production, it focuses on the cultivation of pearl oysters for the production of jewels.  This type of aquaculture does not require fertilizer, feed, or closed water systems, and is generally considered to have a low environmental impact.  Tahitian cultured pearls from French Polynesia essentially dominate the industry, which at its peak in 2000 recorded export of raw pearls totaling around $200 million.  As of 2011, French Polynesia had around 10,000 hectares authorized for pearl farming, the equivalent of around 18,690 football fields!

Pros and Cons

There are of course both positive and negative impacts associated with aquaculture.  It is a billion dollar industry that relieves the pressure of overfishing on wild stocks, yet poorly constructed systems can cause habitat destruction, bring resource competition to the native habitat, and introduce invasive species, which can lower biodiversity of the ecosystem.  One of the benefits to oyster farming is that oysters can improve water quality through filter-feeding.  Two of the known potentially negative impacts that could occur via pearl oyster farming are the introduction of invasive species and disease agents from spat non-native to the area of farming, and impacts from the physical number of oysters in the area (e.g. feces deposition, nutrient consumption, and the presence and impact due to the humans farming them).  However, this is not necessarily the case with properly managed farms and more research needs to be done to quantify any potential impacts, whether they are positive or negative.

This Study

In the study presented here, scientists looked at reef fish as an indicator of biodiversity to determine whether or not pearl farming is affecting local species diversity, fish habitats, and/or fish feeding habitats.  Previous studies conducted in the Philippines have indicated that there may be positive effects of pearl farming on reef fish populations, as it provides additional shelter and substratum to fish larvae and juveniles, as well as food for fish via biofouling that occurs on the nets and oysters.

Research was conducted at 16 sites in the lagoon of the Ahe atoll, which is located within the Tuamotu Archipelago of French Polynesia where pearl farming has been conducted for a long time.  The Ahe atoll has many “bommies,” which are isolated patch reefs that provide good habitat for reef fish and are also located near many pearl farms and oyster lines making for good study sites.  This atoll has one pass in and out – the Tiareroa Pass  – and study sites were located both North and South of the pass to sample different environmental parameters, such as water temperature, circulation, and flushing rates.  By talking to pearl farmers about the area, the researchers identified “Impacted” and “No Direct Impact” sites, where the “No Direct Impact” sites were not near any current farming activity and the impacted sites were in places where there was ongoing pearl farming.

Study sites in the Ahe Lagoon, French Polynesia
Study sites in the Ahe Lagoon, French Polynesia

The “Roving Diver Census” or RDC method was used on the bommies to asses both fish density and diversity.  In this method, the diver descends to 20 meters and records what species are observed and their relative abundance (which is basically an estimated count) over the course of 60 minutes while swimming counterclockwise around the bommie and ascending towards the surface after each circumnavigation, ending the survey in the shallow water atop the bommie.



Analysis and Results

Both pearl farming and location relative to the pass were found to have significant effects on fish abundances.   Pearl farms were found to have slightly positive effects, which the authors relate to farms providing habitat and food.   The habitat comes from the nets and baskets that the oysters are grown in and the food from the biofouling that takes place on said nets and baskets.  Pearl farming did not show any significant effects on either fish diversity or community composition, which can also be interpreted to say there were not negative effects or impacts to the fish communities.

The authors conclude that their results agree with past findings: pearl farming can indeed be included in the environmentally friendly economic zone.  The authors also draw attention to the fact that pearl farming is only ecologically sustainable when the proper guidelines are followed.  They encourage further research on oyster predation in these farming regions and look towards methodology improvement in this area of assessing oyster farming impacts so proper monitoring can be conducted.

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