For the past year, I have had numerous opportunities to speak about the conservation of giant clams. Here, I decided to write a four-part series about the conservation issues surrounding giant clams to commemorate my TED talk in April 2017! For the first post in this series, I shall share the history of giant clam mariculture and explore some of the country case studies in implementing it as a conservation solution.
Part 1: Sustainable Mariculture
Mariculture refers to cultivation of marine fish and other marine life for food. In this instance, giant clams are considered food resources for coastal communities! Giant clams have been traditionally harvested for their flesh, especially the adductor muscle (i.e. the muscle that holds the two shells together). Unfortunately, there was a surge in interest and demand for their meat between the 1960s and 1980s that saw a large-scale harvesting across the Pacific Islands (Pearson, 1977; Shang et al., 1991). This included commercial harvesters and illegal poachers, and as a result, population numbers drastically reduced. In extreme cases, populations of the larger species (Tridacna gigas and Tridacna derasa) become locally extinct – which means that their numbers are either extremely low or none present. 😦
Brief history of giant clam mariculture technology
Compared with other marine molluscs (such as oysters and mussels), the technology of culturing giant clams is fairly recent, with rapid developments since its initiation in the 1980s. Beginning in the late 1970s and early 1980s, pioneering regional programmes, based mainly in Palau and Australia, conducted extensive research and provided technical training for giant clam mariculture in numerous South Pacific nations. It was reported that there are at least 34 giant clam hatcheries in 25 countries, and hundreds of giant clam ocean nurseries and reserves (see TEDxKamuela talk by Gerald Heslinga). Although it is not certain if all of them remain functioning today.
Mariculture remains a feasible technique for mass production of clam individuals for various purposes – food, aquarium markets, and restocking onto reefs. Most of the giant clam farms operate commercially with the goal of efficient production of primary products (meat and shells) for various markets (Lucas, 1997). For example, the Tridacna Mariculture Development Center (TMDC) is an active culture centre based in Yap (Micronesia) that produced almost 2.5 million juvenile clams under 6 months in 2014.
Rearing giant clams have no apparent damaging environmental effects, e.g. the presence of introduced clams is unlikely to disrupt the natural reef ecosystem (Lucas, 1997). But there remains the possibility of inadvertently introducing exotic parasites, diseases and other biota (Newman & Gomez, 2003), especially if broodstock are imported. Nevertheless, clam hatcheries could function as a means to support conservation and facilitate sustainable harvesting (Heslinga, 2013). Such activities can also provide livelihood opportunities in localities where there are few alternatives (e.g. remote atolls in French Polynesia; remote locations in Solomon Islands; outer lying islands in the Marshall Islands).
Status of mariculture in restocking efforts?
Efforts to reverse the global decline of giant clam populations face many challenges. For example, concerted efforts to produce juvenile giant clams have led to the restocking of coral reefs in at least 30 countries (Teitelbaum & Friedman, 2008). However, in general, the success of these initiatives is neither well-studied nor well documented. Furthermore, restocking clams face the following issues:
- The survivorship of restocked clams varies widely within and among localities, with the main causes of mortality being predators, storms, poaching, and the lack of continuous husbandry (Heslinga, 2013).
- Restocking hatchery-reared juvenile clams is that they may be less genetically variable, which can increase vulnerability to parasites and diseases (Benzie & Williams, 1996).
- High mortality rates, coupled with the high costs and intensive labour of rearing clams to reach escape size (typically ~25 mm for which giant clams are less vulnerable to predators), may explain the waning enthusiasm and funding for restocking giant clams in some areas since the late 1980s (Bell, 1999; Bell et al., 2005).
- Restocking giant clams requires long-term commitment and monitoring, as they are very long-lived animals that requires at least 10 years to reach maturity.
- To date, firm evidence that restocked clams have produced local recruitment of juveniles is generally lacking or poorly documented, probably owing in part to the remoteness of the areas under study, and the difficulty and expense of conducting authoritative surveys.
On a brighter note, my colleagues in the Philippines just published their report on recruitment of juvenile Tridacna gigas from restocked clams (Cabaitan & Conaco, 2017). This is great news! As this case study confirms that giant clam restocking programmes potentially can achieve reproductive sustainability, and can eventually provide viable breeding stock to support local hatchery programs. However, the take-home message from this four decades of endeavour showed that the most reliable approach is to implement well-managed, financially sustainable giant clam hatcheries and well-defended ocean nurseries (Heslinga, 2013).
The pioneering concepts of culturing and restocking giant clams remains useful as a platform for improvements using the technologies and ideas of today. For example, scientists designed a methodology to encourage recruitment of giant clams onto the reefs of Cooks Island. One other thing that I’ve learnt from our own Singapore experience is that techniques on the manuals may not always work out! The transfer of technology only works to some extent, while the rest of it has to be modified and tailored to the local environmental conditions. An example for Singapore is that the escape size for restocking tends to be much older and larger so as to allow them to better cope with the sediment rain. Much of the time is spent on streamlining the techniques for each site!
Mariculture has definitely proven its potential for producing large quantities of juvenile clams. But this brings me to ask the question:
Is conservation of a species about increasing their numbers? Is this sufficient as a solution to conserving a species?
Given that there are now numerous other biological factors that ‘defines’ a species, perhaps we now need to more carefully consider the concept of species conservation. In the next part, I will discuss how population genetics can shed light on genetic diversity – which is now considered a distinguishing feature of species on a molecular level.
Bell (1999) Reducing the costs of restocking giant clams in Solomon Islands. Coral Reefs 18: 326.
Benzie & Williams (1996) Limitations in the genetic variation of hatchery produced batches of the giant clam, Tridacna gigas. Aquaculture 139: 225-241.
Cabaitan & Conaco (2017) Bringing back the giants: juvenile Tridacna gigas from natural spawning of restocked giant clams. Coral Reefs.
Heslinga (2013) Saving giants. E-book on Blurb.
Lucas (1997) Giant clams: mariculture for sustainable mariculture. In: Conservation and the use of wildlife resources, M. Bolton (ed.), pp. 77-95.
Newman & Gomez (2003) On the status of giant clams, relics of Tethys (Mollusca: Bivalvia: Tridacninae). Proceedings of the 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000, pp. 927-936.
Pearson (1977) Impacts of foreign vessels poaching giant clams. Australian Fisheries 36: 8-11.
Shang et al. (1991) Report on a market survey of giant clam products in selected countries. Center for Tropical and Subtropical Aquaculture Publication #107.
Bell et al. (2005) Chapter 2 – Restocking initiatives. In: Advances in Marine Biology Vol. 49, Southward et al. (eds.), pp. 9-41.
Teitelbaum & Friedman (2008) Successes and failures in reintroducing giant clams in the Indo-Pacific region. SPC Trochus Information Bulletin #14: 19-26.