Predicting the Water Requirements of River Fisheries

Abstract

The increasing demands that are being placed on water for a range of human uses are leading to growing control over flow in rivers by storage, abstractions, transfers, channel modification and controlled releases. All of these drastically modify natural hydrographs affecting fish in a number of ways. As a result the quantity of the fish caught in many of the world’s rivers is declining and species assemblages are being modified with the disappearance of some native species and the establishment of exotics. Halting or reversing this trend depends in large measure on retaining sufficient water in the river to enable most of the native species to survive (environmental flows). However, environmental flows should not only consist in the amount of water that is retained in a river for fish and other aquatic organisms but should also involve questions of timeliness and form of the modified hydrograph. Many methods have been developed to assess the impacts of changes to flow regimes in rivers but to date most of these have concentrated on small temperate systems typically inhabited by salmonids. They also tend to address total amounts through the setting of minimum flows. Methods for the assessment of impacts in large, lowland rivers are still being developed. Here differences in the nature of the environment through the existence of lateral floodplains add an additional element to the more channel oriented existing models. In some measure methods for the assessment of environmental flows in large rivers may be based on further elaborations of existing methods, but the extensive nature of lowland rivers, their diverse fish populations and the higher degree of interactivity with other users of the river makes broader frameworks necessary to integrate information. Alternative approaches, such as population dynamic modelling, also may be required to assess the impacts of flow changes on fish.