PREDICTING FISH HABITAT USE TO CHANGES IN WATER FLOW: MODELLING CRITICAL MINIMUM FLOWS FOR ATLANTIC SALMON,SALMO SALAR, AND BROWN TROUT,S. TRUTTA

Abstract

To identify possible critical minimum flows, habitat availability and habitat selection by young Atlantic salmon and brown trout on selected stream reaches were studied in a spatially and temporally heterogeneous Norwegian west coast river. Based on direct underwater observations of 1768 individuals across transects on each reach, unimodal habitat suitability curves for water depth, water velocity and substrate were produced for both species. There was considerable spatial niche overlap between the two species, suggesting niche competition. Rather than narrow optima, they selected habitats within broad ranges of values of the measured microhabitat variables. Atlantic salmon used the broader range. Spatial variation in habitat use was attributed to different habitat availabilities, whereas temporal variations were partly explained by varying water flows and temperature. Variation in hydrophysical conditions, i.e. habitat availability, was quantified by measuring in the same transects at different water flows. A new hydraulic simulation model, the RIMOS (RIver MOdelling System), was developed to allow for hydraulic simulation of habitat availability at a spatial and temporal scale relevant to the fish. By comparing spatial variations in habitat use and availability, habitat preference curves were calculated and applied in the model. Modelling of the availability of suitable habitats from extreme summer minimum flows to 30 year high flows identified critical minimum flow levels, below which suitable habitat was drastically reduced. These values depended on the habitat variable, fish species, temperature and spatial heterogeneity in the habitat. The negative effects of reduced water flow on fish habitat suitabilities were most pronounced for water velocities and for Atlantic salmon. Thus through hydraulic modelling combined with fish habitat preference data, it was possible to predict a likely change in species composition. Habitat–hydraulic modelling also indicated that resilience towards reduced water flows depended on the in situ stream structure and was much greater in pool-like stream reaches than in riffle/rapids areas. Modelling also allowed for time series analysis of fish habitat suitability to identify temporal bottlenecks in available spatial niches. Furthermore, some exploratory modelling was carried out to identify landscape phenomena in fish habitat use. Habitats were considerably more fragmented in riffle/rapids habitats than in pools, where three-dimensional connectivity tended to be complete over a wider range of flows. It is concluded that fish habitat selection data combined with hydraulic modelling at a scale relevant to the fish can be a useful tool in stream management. However, there is no such a thing as ‘the’ suitable minimum flow; the effect of reduced flows will vary with stream structure, the hydro-physical variables in question and the fish species. More studies are needed to elucidate possible spatial and in particular temporal variations in fish habitat selection. Care must be taken in aggregating habitat suitability data into single-valued functions.