Reservoir Fish Stocking: When One Plus One May Be Less Than Two

Abstract

Fisheries management in Brazilian reservoirs is based (since the 1970’s) on stocking and construction of fish passes. Low landings of the fisheries and the precarious conservation status of native populations in the Upper Parana River basin indicate how useless these practices were. Failures in most stocking programs conducted may be explained by the negligence of basic assumptions for implementation (clear goals, scientific foundation and evaluation of results). In spite of the common sense support, decision makers should consider that, for any management actions involving biomanipulation, there are relevant environmental risks related to the origin and selection of broodstock and production of fries, and to the releasing of reared fish. Among the latter should be mentioned introduction of associated non-native species (pathogens and parasites), genetic degradation of native stocks (bottleneck effects, loss of genetic variability and fitness, domestication), imbalances and changes in community structure. For an environmental friendly and economical and societal desirable stocking, the decision process should consider information on the receptor ecosystem, target species, uses and users of the resource, legislation and risks for biodiversity conservation. Therefore, the first aspect to be considered is the need for stocking and identification of environmental constrains to it. The ability to produce fish with genetic quality equivalent to native stock and with unaltered ability to spawn in nature (the main challenges in the stocking process) should also have decisive roles in determining whether a stocking program should be implemented. Size, quantity, season and site of releasing should be based on the life cycle, distribution and structure of natural populations, whereas evaluation and monitoring should be considered as integral and indissoluble parts of the stocking process. Habitat management and fishery control should be considered as alternatives or complements. Impoundments are sources of impacts on biodiversity and the success of stocking in such environments appears temporary. Ideally, the success should be quantified by the ability of stocked fish to reproduce in nature and to contribute to the genetic variability of the population. For ethical conservation reasons stocking cannot be only evaluated through fishery landings.