Managing mangroves with benthic biodiversity in mind: Moving beyond roving banditry

Study Number:

6

Author:

A. M. Ellison

Abstract:

This review addresses mangrove management activities in the broader context of the diversity of the mangrove benthos. Goals for mangrove ecosystem management include silviculture, aquaculture, or ‘ecosystem services’ such as coastal protection. Silvicultural management of mangroves generally neglects the benthos, although benthic invertebrates may affect tree establishment and growth, and community composition of benthic invertebrates may be a reliable indicator of the state of managed mangrove forests. Similarly, mangrove aquaculture focuses on particular species with little attention paid either to impacts on other trophic levels or to feedbacks with the trees. Exploitation of mangrove-associated prawns, crabs, and molluscs has a total economic value >US $4 billion per year. These aquaculture operations still rely on wild-collected stock; world-wide patterns of exploitation fit the well-known process of ‘roving banditry’, where mobile agents move from location to location, rapidly exploiting and depleting local resources before moving on to other, as-yet unprotected grounds. Collection of brood stock and fishing for other external inputs required by aquaculture (e.g., ‘trash fish’) removes intermediate trophic levels from marine food webs, may destabilize them, and lead to secondary extinctions of higher-order predators. Increased attention being paid to the role of mangroves in coastal protection following the 2004 Indian Ocean tsunami provides an opportunity to reassess the relative merits of management focused on short-term economic gains. Managing for ecosystem services may ultimately preserve benthic biodiversity in mangrove ecosystems.

Main Results and Conclusions:

• A benthic community is a group of organisms that live on, in or near a seabed.
• Mangroves play a crucial role in ecosystems: “The mangrove macrobenthos – those species that live in mangrove muds or depend on mangroves for all or part of their life-cycle – encompasses a number of phyla, including Porifera (sponges), Mollusca (molluscs), Arthropoda (crabs, lobsters, prawns, etc.), Annelida (segmented worms), Nematoda (roundworms), Sipunculoidea (peanutworms), Platyhelminthes (flatworms), and ascidians within the Chordata”(3-4).
• The following reasons have been presented against prawn, mollusk and crab aquaculture:

•  “First, prawn aquaculture generally requires destruction of mangroves for construction of rearing ponds (Sathirathai and Barbier, 2001; Barbier and Cox, 2004; Islam and Wahab, 2005)…Second, water pollution from intensive prawn farms negatively impacts adjacent mangrove ecosystems…Third, the area of mangrove required to support a ‘fishery’ of gravid P. monodon spawners for generating prawn ‘seed’ for stocking ponds in which intensive prawn aquaculture is practiced is ∼ 11 times the pond area (Rönnbäck et al., 2003)….Fourth, white-spot syndrome virus (WSSV) is now established in shrimp ponds throughout the world”(7).
• “Although the Matang mangrove forest is managed at a constant 40 000 ha, cockle production in 2004 was only 50% of its historic high of 121 000 tonnes in 1980 and accounted for only 14%of Southeast Asian production (total production value: US $435 Million [FAO, 2006]) in that same year (Fig. 5  top)” (7).
• “Of the seven countries with significant production data in the FAO (2006) database (excluding Brunei Darussalam, Mauritius, and Sri Lanka, each of which did not produce more than 2 t/y between 1950 and 2004), five have passed their peaks of production (Fig. 6): Taiwan in 1983, Thailand in 1986, Singapore in 1993, Malaysia in 1995, and Australia in 1998”(8).

• It is difficult to stop certain forms of aquaculture because there are no economic incentives to aquaculture companies to conserve mangroves: “What is clear is that patterns of P. monodon production are similar to ‘slash-and-burn’ agriculture in rain forests…Such ‘roving bandits’ can persist because they have no connection to local communities and no incentive to manage sustainably a local resource. They cut mangroves, establish prawn ponds, exhaust them before regulators can catch up and respond appropriately, and then move on to another country and repeat the process”(7).
• The following recommendations for mangrove deforestation mitigation are given at the end of the article: “Key changes required include: the termination of economic subsidies for aquaculture; enforced, legal requirements that effluent from aquaculture ponds be treated prior to release into surrounding ecosystems; and restrictions on the ability of ‘ roving bandits’ to convert untitled mangrove forests to private aquaculture operations serving the global marketplace”(11).

Works Cited:

Barbier, E.B., Cox, M., 2004. An economic analysis of shrimp farm expansion and mangrove conversion in Thailand. Land Econ. 80, 389–407.

FAO, 2006. FISHSTAT Plus: Universal software for fishery statistical time series, version 2.3.1. FAO Fisheries Department, Fishery Information, Data and Statistics Unit.

Islam, M.S., Wahab, M.A., 2005. A review on the present status and management of mangrove wetland habitat resources in Bangladesh with emphasis on mangrove fisheries and aquaculture. Hydrobiologia 542, 165–190.

Rönnbäck, P., Troell, M., Zetterström, T., Babu, D.E., 2003. Mangrove dependence and socio-economic concerns in shrimp hatcheries of Andhra Pradesh, India. Environ. Conserv. 30, 344–352.

Sathirathai, S., Barbier, E.B., 2001. Valuing mangrove conservation in southern Thailand. Contemp. Econ. Policy 19, 109–122.