The importance of mangroves to prawn fisheries, particularly banana prawns (Udang Putih)

• Fisheries Research Institute, Malaysia
• University Sains Malaysia, Malaysia
• Griffith University, Australia

Policy-makers in Malaysia and elsewhere long regarded mangroves as unproductive, and extensive areas were therefore cleared for agriculture, aquaculture, industry or residential purposes. However, mangroves and seagrasses are important as nursery habitats for the young of many fish and invertebrates, and prawns in particular are dependent on their nursery areas. Catches of many prawn species in Malaysia and of the banana prawn in Australia are known to be larger in areas with greater mangrove growth.
There was a need to investigate the commercial catch statistics for prawns in Malaysia and to examine the relationship between prawn catches and the extent of mangroves. Some of the prawn species caught in Malaysia did not depend on mangroves and it was not known whether the size of the prawn catch was a good representation of prawn numbers in a region.
The project topic was important because prawn fisheries are among the most valuable fisheries in Australia and Southeast Asia. On the west coast of peninsular Malaysia they account for 40% of the value (although only 5% of the biomass) of the total marine catch.

The main purpose of this project was to study the effects of mangrove removal on the productivity of estuaries in Malaysia, and in particular the consequences for prawn fisheries.

The project was divided into three parts. The first examined the impact of mangrove loss on the productivity and environment of the Sungai Merbok estuary of Kedah in Malaysia. Within this single estuary system, unmodified mangrove creeks were compared with similar creeks where most or all of the mangroves had been removed and converted to aquaculture ponds or other uses. The scientists made detailed measurements of environmental parameters, water quality and the abundance of juvenile prawns.
The second part involved the use of nets and beam trawls to sample newly arrived prawns and resident juveniles in the shallows of both 'control' and 'modified' creeks. Data on juvenile banana prawns from CSIRO studies of the Embley River estuary of north-eastern Australia were also analysed and compared.
The third strand of the project examined statistics for past catches in Malaysia at district, state and regional levels and determined historical trends. Such data were used as part of the assessment of prawn stocks. Preliminary assessments already commenced were developed further.

The project provided information beneficial to fisheries managers in both countries on the actual and potential impacts to fisheries resulting from mangrove loss on a large scale.

The study found a positive relationship between commercial banana prawn catch and extent of mangrove in all states of Malaysia. Loss of mangroves could be expected to lead to a reduction in prawn catch. It also found a relationship between the extent of shallow coastal water and commercial catch of white prawns, affirming that maintenance of the shallow fringes of estuaries is important to the fishery.

Researchers found little conclusive evidence that mangrove loss altered nutrient concentrations in mangrove waterways, but nitrogen pollution was evident near populated areas. They found no evidence that secondary production (fish, prawns and other biota) had been impacted in any major way, but populations of benthic organisms were lower, there were fewer species and changes in species composition in areas of total mangrove removal.

Project reviewers commented on some problems with the sampling designs for the fish and prawn studies and suggested alternative analytical approaches to help address these inadequacies. While the research did not find conclusive evidence that juvenile prawn abundance is influenced by certain levels of mangrove removal within estuaries, nevertheless scientists did establish a positive relationship between historical prawn catch and the extent of mangrove forest at State level.

An analysis of the commercial prawn fishery in Matung Estuary, undertaken as part of a linked PhD study, will provide valuable guidance to Malaysian fishery managers, particularly in relation to the design of appropriate catch monitoring systems. This study will also serve as a useful model for shrimp fisheries in Malaysia and elsewhere.

The establishment of a GIS database was considered a feature of the project outputs: this will provide a useful tool for explaining and presenting the project results to resource managers and the community at large in both Malaysia and Australia.

This project was included in an adoption study of projects completed in 2001-2002, which is available as an ACIAR publication.