Habitat enhancement
Background
Future inland capture fisheries development will be determined by the perceived need, capability, and political will to implement a set of actions which may be guided by following general approaches: protection, rehabilitation, mitigation and intensification. Fisheries enhancement by modifying the aquatic environment, such as controlled input of nutrients, water level and water flow controls, landscaping reservoirs or canals to increase the area of productive shallows, assisting fish migration and facilitation access to fishable stocks through inputs of fish-aggregating devices are options available -- although not always applicable and for fishery managers.
Habitat enhancement also includes rehabilitation. Rehabilitation aims mainly to restore the system to as near pristine conditions as possible through physical and biotic modifications of a water body. Habitat rehabilitation should be based on an ecosystem approach where key processes are re-established and maintained. Rehabilitation projects may, for example, address: longitudinal and/or lateral connectivity, fish movements, water flow, land-use planning and water resource management for entire catchments or river basins. In this way rehabilitation will benefit a number of aquatic species and therefore help improve inland fisheries. Although the cost-effectiveness of rehabilitation projects have seldom been undertaken, it is clear that habitat protection is often the most cost-effective means to maintain fisheries.
Habitat complexity of rivers and streams that have been canalised may be increased through de-canalization and by restoring meanders and via reconnecting floodplain habitats such as side channels, oxbow lakes and floodplain pools with the mainstream or by recreating them where they no longer exist.A reservoir, lake or even a river system may be modified by isolating bays, side arms or floodplain features by bunds, fences or block nets. The isolated area can then be treated as an intensive aquaculture pond, either to produce stocking material for the main system, or food fish. Physical interventions can be made into lakes and rivers to improve shelter, feeding and breeding grounds.
Objectives of habitat enhancement
Management of aquatic macrophytes, either by partial removal or planting can be used as means of manipulating fish stocks towards desired fish species composition, for example such as preferred by recreational fishers. Improving water quality can be achieved by installing aerators, or by the removal of bottom anoxic water layers. Where major habitat enhancement measures are required fisheries managers will need to collaborate closely with the principal user, such as hydropower or irrigation authority. In those situations, where the objective is improving water quality in a water body serving for water supply or recreation, fishery managers may be requested to use biomanipulation which involve the removal of large quantities of phytoplankton-feeding fish.
Many migratory fishes are among the most important in the recreational fisheries and their presence is therefore often considered desirable. Since these species are among the first to disappear when the water becomes polluted or when migration routes are interrupted by physical structures, rehabilitation projects may in many cases aim directly at restoring populations of migratory species. However, due to the requirements of migratory species to ecosystem integrity many other species may be benefited from this approach as well.With the objective being a reduction in eutrophication of the water body, the indicators of successful enhancement of the habitat are changes in the dominant fish species, reduction in phytoplankton densities, increase in larger zooplankton and return of macrophytes. On the other hand in lakes and reservoirs that are stocked regularly and the stocked population may rapidly exceed the natural carrying capacity of the recipient water, fertilisation is a simple means of increasing the water body productivity. The success of such treatment is demonstrated by increases in yield from Chinese, Russian and North American reservoirs.
Mapping of fish species distribution in relation to aquatic habitat quality has been given preference in Europe, as it retains its importance in view of the European Union Framework Directive. The highest priority be given to the distribution and diversity, with work on habitat quality mapping to follow at a later stage. Where pressures from other users are eased there may be a possibility to restore natural or quasi-natural features to the river. While this is possible, the fully natural hydrological and biological regimes will not re-establish where the catchment has been degraded.