Part 2: Protected areas and development
3.2 The critical role of protected areas in freshwater fisheries
The failure to recognise the full significance of the freshwater capture fisheries has meant that there has been a corresponding lack of attention given to the contribution of
protected areas as an essential management strategy in the fishery sector. A range of protected area types contributes to the fisheries sector in important ways.
Protected areas established primarily for biodiversity conservation contribute to fisheries productivity through the maintenance of hydrological functions and contain many rivers and streams that are important fish habitat.
3.2.1 Maintenance of the hydrological system through protected areas
The extensive forests contained within protected areas in the region make a very significant contribution to development through their influence on hydrological factors such as maximum flood heights, dry season flows, water quality, and annual rainfall. These benefits are critical to fisheries sector productivity. They are important to many other sectors as
discussed in other chapters of this report, especially the maintenance of water resources, and of a wide range of ecosystem services and products to agriculture, transport, hydropower, industry and urban settlements.
The threats to the hydrological system in the region include forest loss and land use changes and emphasise the need for an effective protected areas network managed with hydrological functions in mind.
Box 3.1: Impacts of sediment on aquatic ecosystems
A part of the sediment released into water bodies by erosion will remain in suspension, while the rest will be deposited. Both forms of sediment can have significant impacts on aquatic life.
Suspended sediment affects aquatic life by:
• abrading and damaging fish gills, increasing risk of infection and disease;
• scouring of peri phyton (plants attached to rocks) from the stream;
• causing shifts in fish community toward more sediment-tolerant species (including introduced carp);
• reducing feeding efficiency of species which depend on sight for feeding;
• reducing light penetration causing reduction in plankton and aquatic plant growth;
• reducing filtering efficiency of zooplankton in lakes and estuaries;
• adversely impacting aquatic insects which are near the base of the food chain; and
• increasing stream temperature through absorption of heat into sediment particles, reducing the amount of dissolved oxygen in the water, and possibly also confusing migration patterns in any species which respond to seasonal temperature changes.
Suspended solids can also have detrimental impacts on reproductive success, though the effects vary from species to species. Turbidity can affect spawning time, place of spawning, and spawning behaviour. It can also affect juvenile freshwater and warm water fishes at sub-lethal levels by reducing sight feeding distance, disrupting activity and respiratory patterns, and/or changing migration and orientation responses (Wilber 1983).
However it would not be safe to assume that any land use change leading to an increase in total suspended solids (TSS) would necessarily have significant detrimental effects on fish stocks. TSS concentrations in the Mekong mainstream are ten times higher in the wet season than in the dry (Schouten in press) suggesting that the aquatic biota is somewhat adapted to extreme changes in sediment load. However very large increases in sediment can have very detrimental impacts - as has been demonstrated by the impacts of commencement of operation of the Theun-Hinboun (Lao PDR) and Yali Falls (Vietnam) hydropower schemes.
Conversely, changes in land use significantly reducing the TSS and nutrient loadings can have negative effects on the ecology and thus on fisheries. The discharge of the Mekong waters forms a nutrient-rich plume supporting high primary production and related high fisheries catches in the Mekong Delta and along the south coast of Vietnam. Reduction in the volume or concentration of this discharge would be likely to have significant negative impacts.
Deposited sediment affects aquatic life by:
• physically smothering the benthic aquatic insect community which forms an important part of the food chain;
• reducing survival rates for fish eggs or even causing total smothering of eggs;
• destruction of fish spawning areas;
• “imbedding” of stream bottom reduces fish and macro-invertebrate habitat value;
• depleting dissolved oxygen levels in lakes or streams by increasing sediment oxygen demand; and
• smothering freshwater mussels or impeding their feeding processes.
Sedimentation is also affecting smaller wetlands on the edges of the floodplains and in the foothills of the mountains bordering the catchment (e.g. Numa Shams and Mahfuzuddin Ahmed 1996).
Source: Adapted from Anon 1997
3.2.2 Threats to the hydrological system
Many resource managers assume that there is a clear and straight forward link between loss of forest cover and hydrological impacts such as increased flood heights, reduced dry season flows, reduction in water quality and lower annual rainfall. For example, a Mekong watershed forestry strategy and action plan states that:
“The ongoing losses in forest cover in the Lower Mekong River Basin (LMB) over the past decades and the emerging adverse trends in the watershed are issues of international concern. Widespread
deforestation is causing increasing water runoff, soil erosion, siltation in rivers and wetlands, increasing rates of occurrence and severity of floods, landslides and droughts, loss of biodiversity, fisheries depletion, and damage to agricultural and irrigation systems. These impacts are often transboundary in nature, especially affecting downstream areas. The physical and natural resources of the LMB watersheds provide goods and services to communities, including: protecting water sources;
minimising the risk of floods by attenuating run-off, protecting urban environment and infrastructure (premises and transport facilities) (ARCADIS Euroconsult 1999).”
Yet, the relationship between forest cover and watershed hydrology is not as clear or direct as is normally supposed. While a great deal of anecdotal material suggests such linkages (e.g. floods in Thailand 1988, China 1998, and Vietnam 1999 were all widely reported as being the result of excessive logging - and large- scale poorly managed logging had in fact occurred in watersheds in those countries), there is
considerable scientific debate as to the exact nature of the hydrological effects of alteration of forest cover.
This debate has been thoroughly canvassed in recent literature reviews by Aylward (in press) and Calder (2000) (Box 3.2).
While logging per se may or may not have significant impacts on water flows and water quality, the land uses following logging in the region have definitely led to adverse hydrological impacts. Logging requires a network of roads and tracks into the forest area. Others then use these roads to carry out activities, which typically include illegal logging of remaining trees, charcoal-making, and clearing for agriculture.
This sequence has been observed throughout the region in upland forests and in the flooded and other riverine forests bordering the Mekong and its main tributaries.
Forests eventually converted to some form of agriculture can be classified as undergoing disturbances of high intensity with potentially important effects on stream flow and sedimentation (de Graaf 2000).
However, it is not only the sedimentation effects of the post-clearing land uses that impact water quality.
Such impacts also result from pesticide and fertiliser run-off from agriculture, industrial pollutants, and leachate from mining tailings, for example.
3.2.3 Loss of forests in the region – protected areas a last resort?
The health of the region’s forests is linked through a variety of pathways to the welfare of the fisheries sector including the hydrological benefits. Despite their substantial contribution to development, over the last five decades the forests of the region have been significantly degraded in terms of both area and quality as detailed in Chapter 6 on forests. Between 1960 and 2003, extensive logging and deforestation expanded from northern and north-eastern Thailand (1960s to late 1980s) and the western highland regions of Vietnam (1980s to mid 1990s) to Lao PDR (mid-1980s to present) and Cambodia (mid-1980s to present).
Present forest cover is about 40 per cent of the region,8 down from 56 per cent in 1970, though the loss of forest quality has been significantly higher than the loss of forest area. Causes of forest loss or
8 Forest cover in the Chinese part of the Mekong Basin is 38 per cent (Xinhua News Agency, Beijing 21/2/01, quoted in BBC Summary of World Broadcasts, 28/2/01.
deterioration include commercial logging (legal and illegal); slash and burn cultivation; land encroachment for settlement, farming or infrastructure development; and wood cutting for fuel (Sverdrup-Jensen 2002).
Box 3.2: Effects of logging on hydrology
Clearing or degrading large areas of forest cover is likely to result in some or all of the following impacts but the extent, and sometimes the direction, of the impact depends on local variables:
• increased erosion, tending to rise dramatically when the leaf litter layer is removed or
destroyed, and/or there is repeated disturbance of the soil (e.g. by burning, frequent weeding, or overgrazing);
• increases in sedimentation rates, resulting from changes in vegetative cover and land use;
• nutrient and chemical outflows generally increase;
• overall water yield is generally inversely related to forest cover (i.e. more cover results in less outflow) - contrary to popular belief and extensive anecdotal reports;
• seasonal flows may increase or decrease, depending on various factors specific to each situation, but, in the case of deep soils, dry season flows diminish more rapidly with forest disturbance;
• groundwater recharge may increase or decrease, generally in a similar fashion to seasonal flows;
• peak flow (i.e. immediate flood height) may increase (sometimes dramatically) under certain conditions, including where the forest clearing or subsequent land use reduces infiltration, e.g.
urban settlement, mining, extensive road construction, poor cultivation practices (including some shifting cultivation), frequent burning or overgrazing; and
• local precipitation is probably not significantly affected by changes in forest cover, at least up to a scale of 10 sq. km. though above this, and certainly at large scales such as the whole Mekong catchment, there may be a reduction in the amount of rainfall as a result of reduction in forest cover.
Source: Alward in press and Bruijnzeel in press
As a result of these factors, protected areas now represent the most secure form of forest protection in the four lower Mekong countries and in general contain the least disturbed forests in the region. The extent of forest protection provided by protected areas is substantially greater than is suggested by the 18 per cent of the region under some form of protected area status. The majority of these protected areas are in the upper catchments of the river systems, and constitute more than 50 per cent of the area of forested upper catchment - and are thus responsible for a significant percentage of the hydrological benefits that forests provide to development in the region.
3.2.4 Development benefits from maintaining the freshwater system
The maintenance of forest cover in protected areas can provide the following benefits to freshwater systems:
• protection from erosion;
• protection from increased sedimentation and its associated impacts;
• maintenance of nutrient levels in water bodies;
• in certain circumstances, protection from dramatic increases in flood heights (peak flows);
• in certain circumstances, protection of dry season flows;
• in certain circumstances, maintenance of ground water recharge; and
• maintenance of rainfall levels where large areas of forest are involved.