Original paper

An analysis of microhabitat used by fish in natural and regulated sections of the River Garonne (SW France)

Reyjol, Yorick

Abstract

The aims of the present work were 1) to assess the influence of regulated flow on microhabitat characteristics in a study stretch of the River Garonne and 2) to highlight the fish-microhabitat relationships observed in two types of sites considered independently (natural and reduced flow sites). Three species were considered: brown trout, European minnow and stone loach, split into three size-classes corresponding to age-classes or ecospecies. Sampling was carried out following the point abundance sampling methodology, with seven microhabitat variables being recorded for each sample (distance from the closest bank, water depth and velocity, percentage of three substratum fractions, i.e. boulders, pebbles and fines, percentage of macrophyte cover). Regarding microhabitat, macrophyte cover strongly decreased from natural to regulated sites in conjunction with the percentage of sandy substratum, while the percentage of cover increased for boulders. For fish-habitat relationships, this work confirms the structuring role played by macrophytes as they can notably act as shelter from predators and rest habitat towards hydraulic conditions in both types of sites. It also highlights the importance of maintaining diverse habitat patches (water depth and velocity, substratum composition, macrophyte cover) to ensure a diverse community of fish being present that can find appropriate microhabitat conditions to complete their life-cycles, as suggested by the significant relationships observed here for the different age-classed.

Keywords

fishflow regulationmicrohabitatmacrophytes