The Role of Mangroves in Fisheries Enhancement
In 2011 humans caught and consumed 78.9 million tonnes of fish, crustaceans, molluscs and other species groups from the world’s oceans, accounting for 16.6% of the world’s animal protein intake (FAO 2012). This is projected to increase further, to over 93 million tonnes by 2030 (World Bank 2013). Global demand for fish products has increased dramatically over recent decades. Fishing is also an important livelihood, globally providing employment to 38.4 million people of whom 90% are employed in small-scale fisheries (FAO 2012). The importance of fisheries continues to rise as coastal populations are increasing, and rapidly growing economies are driving up demand for fish. While aquaculture is increasing to meet some of this demand, wild capture fisheries continue to be critically important.
This review of the scientific literature provides a deep exploration of the importance of mangroves for wild capture fisheries. While mangroves are widely recognized for their role in enhancing both small scale and commercial fisheries, they are rapidly disappearing. A fuller understanding of this ecosystem service and its value in both social and economic terms will help enhance the sustainable management of both mangroves and fisheries.
The report firstly discusses some of the ecological processes which underpin the key role of mangroves in fisheries enhancement, followed by an exploration of the different mangrove-associated fishery types. As the fisheries value of mangroves is highly site specific, the report explores the drivers and mechanisms which can help to explain for different locations how many fish a mangrove produces, how many are caught by humans, and what the fisheries value is, both in economic terms, as a food supply or through the livelihoods that they support. Decision-makers can use this information to determine where fish productivity is highest, which allows them to make adequate decisions relating to conservation and restoration actions and sustainable fishing. We conclude with management recommendations for maintaining or enhancing the value of mangroves for fisheries for the long-term.
KEY FINDINGS OF THE REPORT
- Fish productivity from mangroves will be highest where mangrove productivity is high, where there is high freshwater input from rivers and rainfall and where mangroves are in good condition.
- Fish productivity will increase with an increase in total area of mangroves, but notably also with the length of mangrove margin since generally it is the fringes of mangroves where fish populations are enhanced.
- Mangroves with greater physical complexity both in terms of patterns of channels, pools and lagoons, as well as the structure of roots which are important areas for shelter and for growth of some bivalves will enhance fisheries to a greater extent.
- Fish catch will be highest close to areas of high human population density that provide the fishers and the markets for the catch. Of course some of these mangroves close to populations are also likely to be under greater threat than those in more sparsely populated areas – they may be degraded, the waters may be polluted, or they may be over-fished and hence less productive. Where such mangroves are secured through appropriate management regimes, and where their fisheries are well managed they are likely to give greatest value. Consequently, conservation and restoration efforts in these areas close to human populations will likely give the greatest return on investment.
How mangroves enhance fisheries
Mangroves enhance fish production via two main mechanisms – the provision of food and of shelter. Mangroves forests are highly productive, with mean levels of primary productivity close to the average for tropical terrestrial forests. Their leaves and woody matter (detritus) form a key part of the marine food chains that supports fisheries. Decomposers of this detritus include micro-organisms such as bacteria and oomycetes, as well as some commercially important crab species. These decomposers process the leaves and woody matter into more palatable fragments for other consumers.
Mangrove productivity is further enhanced by productivity of periphyton and phytoplankton occurring on mangrove trees, in their soils and in the water column, which typically have lower rates of productivity than the trees themselves, but are nutritionally more accessible to consumers.
Moreover, mangroves often benefit from incoming nutrients from rivers and other adjacent habitats. They may also export nutrients, in the form of dissolved and particulate organic carbon, and living biomass, such as planktonic larvae and maturing fish and invertebrates.
Species of interest to the fisheries sector are found at all levels of the food chain, with detritivores such as mangrove crabs, prawns and mullet; filter feeding bivalves, planktivorous fish such as herring and anchovy species, and higher consumers such as some mud crabs and many other fish including snappers and groupers.
It is not only the high productivity of the mangroves that creates value for fisheries, but also their physical characteristics. Mangrove roots and trunks provide a structure that species such as oysters can grow on. Their roots also trap fine particles, creating soft soils ideal for molluscs and crustaceans to burrow in. Mangroves also provide shelter for many species, enabling them to avoid predation and also invest more time in feeding.
Finally, driven by the nutritional and physical benefits, many species use mangroves as nursery grounds. These include species that spend time in mangroves as juveniles before moving to offshore habitats such as coral reefs. Thus fisheries in these offshore habitats benefit from stock replacement from mangroves.
Values of mangrove-associated fisheries
Some 210 million people live in low elevation areas within 10km of mangroves and many of these benefit from mangrove-associated fisheries. The economic values of mangrove-associated fisheries vary widely, reflecting the wide range of different fisheries, economic markets, and levels of utilisation. Besides economic values, mangrove-associated fisheries provide jobs and food supplies for millions of people. In turn this may provide multiplier benefits such as political or social stability. We summarise the different types of mangrove-associated fisheries into four broad classes:
Inshore mixed species fisheries
These are mostly low-income fisheries undertaken in mangroves close to settlements. They include a broad range of fishing techniques, but many are opportunistic and fishers often return with a highly mixed catch of finfish, molluscs and crustaceans. A large proportion of the catch is for domestic consumption, but some is sold, usually in small local markets. The median value for mixed fisheries from our review was US $106/ha of mangrove/year, but variation either side of this was high.
Inshore mollusc and crustacean fisheries
Certain mollusc and crustaceans caught in mangroves generate quite high market values, and although they may still be harvested at local and small-scales, in many cases fishers are operating a targeted fishery and generating income through market sales. The most important of these are a number of crab species, oysters and other molluscs, and some harvesting of juvenile prawns for stocking of aquaculture ponds. Economic valuations are rare in the literature, but the one value we found, for a crab fishery in Micronesia, was US $423/ha of mangrove/year
Offshore commercial fisheries
These fisheries may operate many kilometres from the mangroves, but benefit from the mangroves’ nursery habitat function. This distance makes it challenging to quantify the extent of this benefit. The importance of mangroves is best documented for offshore prawn fisheries, although it is rarely possible to attribute catches to specific mangrove areas. These fisheries can generate high value returns, but much of this value lies in the industrialisation of the fishery, with high volumes of catch for small numbers of fishers. We found two studies giving economic valuations, with values of 24.3 and 1394 US $/ha/year for fisheries in Indonesia and Mexico respectively.
Mangroves are a critical habitat for a number of species that are considered prize game fish, and locations where there are healthy stocks of such fish have become favoured fishing grounds for a fishery that can be both high value and low impact. Calculations of value for these fisheries are challenging, but most efforts have included estimates of financial flows to associated beneficiaries including accommodation, transport, food and fishing guides. For example, Fishing for bonefish, permit and tarpon was worth US $56.5 million to Belize in 2007 and US $141 million to the Bahamas in 2008.
Modelling the drivers of mangrove-associated fisheries
The review work has helped to inform an understanding of the processes which drive value. The value of mangrove-associated fisheries varies greatly between different locations. To understand this variation in value we break down the benefits that mangroves provide to fisheries into three steps:
1. Potential fishable biomass
This is the biomass that would be present in a location in entirely natural condition. The productivity and availability of fish will be strongly linked to the area of mangroves. Further influence comes from the length of mangrove margin as it is primarily at the fringes of the mangrove where fish populations are enhanced. The physical complexity of the mangrove forest may also play a role, both in terms of patterns of channels, pools and lagoons, and also the structure of roots which are important areas for shelter and for growth of some bivalves. Climate, freshwater and nutrients also influence primary productivity in mangrove areas which affects fish productivity.
2. Actual fishable biomass
Most mangrove fish stocks have been influenced by humans, directly through the harvest of fish, and indirectly through changes to the environment. Even low levels of fishing will have some impact on the remaining fishable biomass, while overfishing can greatly reduce fish productivity and potential yields. Mangrove areas in many places are also compromised by pollution, while impacts to the mangroves from harvesting and clearance directly impact primary productivity and thus influence fish production.
3. Fished biomass
The amount of fish actually being caught is demand-driven, but that demand can be understood and modelled in relation to coastal population sizes, the influence of markets, of economic drivers, cultural traditions and so on. Fished biomass represents one core measure of value, but it is also a key component of other values measured in terms of money, jobs, food security and other metrics.
Recommendations for proper mangrove management
The tremendous value of mangroves for fisheries, in both social and economic terms, provides a strong incentive to secure mangroves for the long-term through proper management of both mangroves and mangrove-associated fisheries. We outline three broad classes of management which need to be considered:
Avoiding mangrove loss
Maintaining mangrove areas is almost always the most cost-efficient way of ensuring value flows over time. Critical to success are both the establishment of clear and effective regulatory frameworks, and the establishment and recognition of tenure and use rights, ideally at local or community levels. Protected areas, established for conservation purposes, already include over 25% of the world’s remaining mangrove forests. However, other mechanisms – ranging from nation-wide regulations on mangrove clearance, to controls ensuring sustainability of mangrove silviculture – can also be effective. In all cases such regulations are most effective when mangrove ownership is clearly established and where communities are fully aware of the benefits they derive from adjacent mangroves.
Restoring natural mangroves
Where mangroves have been degraded or lost they can still be restored, enabling the return of ecosystem services relatively quickly. Critical to successful restoration are understanding the causes of loss in order to ensure these can be prevented in the future, and ensuring that the communities or owners of mangroves are supportive of restoration. Where these conditions are met, the main focus of restoration should be restoring growing conditions – tidal flows, freshwater inflow and sediments. These alone may be enough to allow natural mangrove recovery, but in some cases mangroves may need to be planted to commence or enhance recovery.
Fish stock management is a core principle of ensuring continued supplies and in attempting to maximise yield and/or profits. A large body of management interventions have been developed, alongside the science to inform such management.
Despite this, understanding of management of mangrove-associated fisheries per se remains limited. Key fisheries management tools include regulating access to the fishery, through ownership or licence; regulating fishing methods, for example to prevent wasteful bycatch or damage to the seabed; spatial controls such as the closure of certain areas permanently or seasonally to allow survival of breeding populations or key nursery areas. Market mechanisms, such as sustainable fisheries certification can provide important incentives in some fisheries to encourage and ensure implementation of such measures. Lastly, since aquaculture is a major driver of mangrove conversion, it is imperative to simultaneously work towards more sustainable aquaculture so that the fisheries enhancement function of mangroves is not jeopardized.
Education and communication are key tools in all these management interventions, to build public and political support. Many fishers are unaware of the key role mangroves may play in supporting fisheries, even those far offshore. Raising awareness can simply involve disseminating the key facts, but new ideas, including the more accurate quantification of value and explanation of key underlying drivers may greatly increase openness and enthusiasm for improvements in mangrove management.