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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


F. The intermediate fisheries restoration model

There is considerable evidence that biodiversity and biotic integrity of ecosystems are still declining despite the considerable investments in pollution control (Somlody et al. 2001). The major reason for decline appears to be one of environmental quality and the continued degradation of hydrological cycles at the basin scale due to urbanisation, channelisation, deforestation, and agriculture. As a consequence, the physical modification of fish habitat should be considered in the context of basin scale processes, and special emphasis should be placed on the upper parts of the basin, or headwater areas. The streams and small- to medium-sized rivers in these areas are:

  • nursery areas for riverine (white) fish,
  • often less-seriously polluted or regulated than larger rivers and streams so their rehabilitation is, in many cases, less difficult and less expensive,
  • generally less modified than larger rivers and, hence, are more economical to restore using currently available rehabilitation techniques.

A two-dimensional approach must be developed and applied to avoid the further decline and to restore inland water fish habitat. Firstly, the extent and types of physical modifications of the fish habitat affecting fish communities must be identified. Secondly, the degree to which basin-scale hydrological and biological processes can be used as a tool for the restoration of physically degraded fish habitats must be determined (Zalewski 2000).

The existing literature on rivers and lakes indicates that the reaction of fish communities to catchment development has a parabolic character when some characteristic of the fish community, such as production or species diversity, is plotted against intensity of development. For example, Zalewski et al. (1998) demonstrated, quantitatively, that the increase of light input to small upland and lowland rivers stimulates the increase of fish biomass, but full opening of the riparian canopy reduces biomass and diversity (see Figure 6.6). Adamek and Jurajda (2001) provided further confirmation of this phenomenon using an integrated index of catchment use intensity. Increasing nutrient concentrations, as a conseqeunce of the conversion of natural lands to developed lands, initially plays an important role in increasing fish biomass and diversity, but further increases leads to declines in both. Further, if the high complexity of riparian ecotones does not reduce the amount of solar energy, the increased water temperatures and the numbers of pathways of energy flow creates appropriate conditions for invasion by new species from downstream. This leads to a phase of species addition, which may increase fisheries yield but often eliminates native species as a result of competition. Such species replacement is mostly due to changes in physical habitats; for example, siltation of the spawning grounds of salmonid species or oxygen depletion in lakes under intensive eutrophication lead to the elimination of coregonids.

This development in river basins, defined by increasing human population density and aspirations, means that under present day economics it is impossible to reverse the evolution of the landscape and to restore physically modified habitats of freshwater fish to the stage of the pristine ecosystem. (Some exceptions do exist, in the form of specially protected areas such as the UNESCO Biosphere reserves, but even these are limited in value and extent). In general, then, there is a need for river restoration activities that cover the whole basin, if the needs of some migratory fish species are to be met. To this end, there are indications in some countries that the economics of farming as well as societal goals for landscape use are changing to favour reduced pressure on the rural environment. Where this occurs, there are opportunities for river and lake restoration.

Fish communities achieve maximum biodiversity and highest productivity at intermediate levels of human disturbances (Colby et al. 1972, Ward and Stanford 1989, Zalewski et al. 1994, 1998, 2001). At such intermediate levels, individual factors such as hydrological variability maintain the ecosystem in a dynamic state of ecological succession (Ward and Wiens 2001). Moderate increases in the nutrient supply increase primary productivity to such a level that the nutrients can be assimilated by the upper trophic levels of the system (i.e., nutrients are transferred to the fish). This level of increase does not detrimentally change the water quality by creating oxygen deficits and accumulating in the bottom sediments. Such an increase may correspond to basins with non-intensive, diversified agriculture and low degrees of urbanisation, buffered by a diversified landscape and land water ecotones (Adamek and Jurajda 2001).

At intermediate levels of energy access, nutrient spiralling is amplified as a consequence of increases productivity at the primary, secondary, and tertiary levels (Zalewski et al. 1994). The density-dependent reaction of fishes to these enhanced trophic conditions has been well documented in the stock recruitment curve (Backiel and LeCren 1978) and in aquaculture, where optimal growth is achieved at an intermediate intensity of feeding (Ruttkay 1996). Thus, the target of restoration of physically degraded habitats should lie somewhere in the range between maximum biodiversity and maximum productivity of fish communities. This model of "intermediate restoration" is currently being successfully applied to the Danube floodplain (Schiemer et al. 2001), suggesting that, if the connectivity of the river system is maintained, the "patchy" restoration of physically degraded fish habitats at the river basin scale might be sufficient to restore some measure of ecological integrity to the system (see Cowx and Welcomme 1998 for a definition of the bead concept as applied to floodplain restoration).

G. The planning process

The selection of the optimal method for implementing a program of stream management based upon the principles of ecohydrology should start with the setting of objectives. In part, these objectives should include objectives for fisheries management. The dynamics of the ecological processes underlying the management measures can be determined through the conduct of appropriate surveys. These surveys will allow identification of the mechanisms that: influence production in the stream, such as regulation by abiotic and biotic factors; determine the present state of native fish population; and, quantify the degree of environmental disturbance within the stream system and its watershed. This is a first step in evaluating the carrying capacity of the water body, and the possible reasons observed impacts affecting fish production. This knowledge allows a decision to be made as to whether stocking is necessary, and, eventually, allows consideration of the most appropriate procedures for stocking (see Figure 6.8).

Fig 6.8.
Decision tree for a stocking strategy based upon minimising risk and amplifying benefits (according to Cowx 1994, 1998, changed)

The second important step is planning the stocking strategy (see Figure 6.9). An evaluation of the carrying capacity and size of the fishery is essential for defining stocking rates and choosing most suitable stocking method(s). There are some general rules, which are helpful in determining optimal stocking decisions:

  1. Yield is related to stocking rate. Usually this relationship may be expressed as a mathematical equation (Lorenzen and Welcomme 2001).
  2. Stocking rate is related to the size of the fishery. Empirical data on the stocking:catch relationship might allow the determination of a linear relationship between stocking density and size of the fishery. This relationship would provide the basis for a compensatory stocking programme (Amarasinghe 1998, Lorenzen and Welcomme 2001). The relationship is more distinct in lentic environments, where creel data can be used to estimate yield.
  3. Yield per unit area is inversely related to the area of the stocked system. In case of territorial salmonids, stocking is usually more efficient in low order streams because of prevailing abiotic regulatory factors (Figure 6.10a). Increasing stream order is connected with higher levels of species diversity, and, in that way, with stronger competition and a greater risk of predation. On the other hand, growth rate is positively related to stream order (Figure 6.10b.). For this reason, the best solution is likely to be a balance between two variables: size-dependent mortality and density-dependent growth, according to the abiotic-biotic regulatory continuum concept (Zalewski and Naiman 1985, Zalewski et al. 1985, 1990, Lorenzen and Welcomme 2001).

 

 

Fig. 6.10.
The survival(a) and growth (b) of introduced stationary fish – brown trout (Salmo trutta m. trutta) – as a function of space (stream order, or distance from the source) and time (according to Zalewski et al. 1985, changed)

Effective stocking must be based upon a consideration of population dynamics and other biological features of the stocked species. Such considerations should include:

  • season - including both the possibility of floods or droughts, and the availability of food organisms,
  • stocking location - some species have strict environmental requirements; for example, fingerlings of the brook trout (Salvelinus fontinalis) strongly prefer habitats with water depths of up to 25 cm and current velocities if 10 to 20 cm s-1 (Scherer et al. 1984), while fry of the nase (Chondrostoma nasusus) prefer habitats with flows ranging from 1 to 10 cm s-1 (Winkler et al. 1997),
  • schooling behaviour - cyprinids usually tend to create shoals, while salmonids are territorial and generally dispersed. In the case of some predator species, taking this behaviour into account will avoiding heavy losses due to cannibalism, as may occur with pike (Esox lucius), for example.

The third step is related to the choice of seed fish. There are two possibilities: natural sources or hatcheries. Some species, such a eel, are best obtained from natural sources, while other species, such as salmonids, may be better obtained from hatcheries Special care should be taken to consider the health, condition, and genetic features of the fishes. While local veterinary authorities can impose ands enforce health and condition standards, the oversight of the genetic pool is still rather dependent on managerís will. This problem should not be overlooked because it can influence the adaptive skills of both native and released stocks, and contribute significantly to the likelihood of survival and the potential development of a naturally sustainable population. Use of hatchery-reared stocks increases the risk of decreased genetic variability and inbreeding. It also increases the risk of hybridisation of domesticated strains and native stocks.

Farmed fishes are adapted to conditions different from those in natural environment. This may adversely affect their ability to survive natural variations in a stream environment, and influence their behaviour. Farmed fishes may have less stamina due to increased energy expenditures, related to increased aggressiveness, which may adversely affect the timing and location of breeding, for example. This directly impacts the likelihood of establishing a sustainable population. To overcome some of these problems, fish should be preconditioned prior to release. Preconditioning techniques include, for example, starving the fishes before transportation, and adapting the fish to the temperature conditions at the release site.

These three steps allow development of a proposal that should also contain provision for the evaluation of the potential impacts on the ecology and environment of the recipient system, and an assessment of the benefits and costs for local biotic community. Independent scientist and managers should assess such a proposal.

 

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