Modelling of microhabitat used by fish in natural and regulated flows in the river Garonne (France)

The aim of our study was to compare the microhabitat used by three fish species: brown trout (Salmo trutta L.), European minnow (Phoxinus phoxinus L.) and stone loach (Barbatula barbatula L.), in natural and regulated flows of a section of the river Garonne (France). Six Artificial Neural Network (ANN) models were set up, one for each fish species in each flow condition. Models were run and tested with 1107 observations obtained by point abundance sampling performed by electrofishing. Each model had thirteen independent environmental variables (distance from the bank, water depth, water velocity, percentage of different substratum fractions defined as large boulders, small boulders, large pebbles, small pebbles, gravels, sand, mud and bedrock, flooded vegetation cover, and presence or absence of 'blockage' which is one or several pieces of wood providing shelter), and one dependent variable (fish density for the considered population). A cross-validation testing procedure (leave-one-out bootstrap) was performed to validate the ANN models. Finally, we used a method based on the first partial derivatives of the network's output with respect to each input to focus on the sensitivity of some of the variables selected. During the training phase, all models were judged satisfactory with Mean Squared Errors (MSE) ranging from 0.40 to 1.93, and Performance Indexes (PI's) from 60 to 89%. After the testing procedure, MSE ranged between 1.53 and 8.23, and PI's between 51 and 80%. With the exception of brown trout in regulated flow, patterns of microhabitat use obtained revealed that fish densities were highly connected to one major influencing variable: water depth for brown trout and stone loach, and water velocity for European minnow, other variables accounting for lower individual contributions. Analysis of the partial derivatives brought into relief some differences when comparing microhabitat use in natural and regulated flows for some of the variables tested, and no differences when comparing others. The results are discussed with regard to the biology and the ecology of each fish species at microhabitat and macrohabitat scales, and according to the relationship between microhabitat utilization and microhabitat availability. (C) 2001 Elsevier Science B.V. All rights reserved.