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Evaluation of fish density in some Spanish reservoirs

The evaluation of the density of fish populations by means of echosounding techniques has been used thoroughly in the marine environment, but seldom in freshwater ecosystems (rivers, lakes and reservoirs). A few results have been published from Spain (GRANADO-LORENcro & GARcfA-Novo 1984, GRANADO-LoRENcro et al. 1998), and elsewhere (KUBECKA et al. 1992, CADIC et al. 1998, LYONS 1998). In all of these, a relationship can be deduced between the trophic state of the reservoir and the density of fish populations. During a significant drought in 1995, an evaluation of fish populations was conducted in 15 reservoirs of the Júcar Basin Water Authority during June and July, in order to study the minimum water level necessary to mainrain the fish populations. One of them, the Alarcón Reservoir, was fished extensively during the summer with the purpose of reducing the density of the fish and then re-evaluating the populations in September.