Échantillonnage et modélisation de l’habitat des communautés de poissons de rivière des basses Laurentides

Many large scale studies have identified habitat modification or habitat losses as primary threats for the conservation of freshwater fish communities. In Canada, No Net Loss (NNL) of the productive capacity of habitats is the guiding principle of the Department of Fisheries and Oceans’ policy for the management of fish habitat. To respect NNL, a better understanding of fish-habitat relationships is required, as well as tools to quantify the impact of habitat modifications on fish. Fish habitat use models (FHUM) are tools that can improve our understanding of fish-habitat relationships, predict species occurrences, densities or biomass on the basis of habitat descriptors and quantify habitat requirements. They consist in relationships between biological descriptors of fish and habitat descriptors. The general objective of my thesis is to improve the performance of FHUM for the lower Laurentian streams by suggesting refinements on 2 crucial aspects in the development of these models: a precise description of the fish community and the use of efficient statistical models. In the first chapter, I evaluate the relative performance of electrofishing and visual surveys (snorkeling) for estimating the abundance of combinations of fish species and size classes in rivers. I also assessed the effect of environmental conditions on potential differences between the results obtained using these two sampling methods. Sampling sites consisted in 10 river sections of 20 m in length distributed in the Laurentian region of Québec. Both methods were used while sections were blocked. Three snorkelers that swam the river sections upstream while identifying and counting fish of each species and sizeclasses performed visual surveys. Three-pass electrofishing was performed and abundances were estimated with a maximum likelihood depletion model. Greater abundances of fish were observed by snorkeling than by electrofishing at all sites. Snorkeling species richness was higher (6/10) or equal (4/10) to electrofishing richness. Differences in the fish communities observed by both sampling methods were not related to environmental conditions. The results of our work are therefore contrary to that of most published studies vii that suggested the superiority of electrofishing on visual surveys. Compared to the conditions found in previous studies, our sampling sites had different environmental characteristics (no fallen trees, insignificant cover of large cobble and boulder) but the most striking dissimilarity was in terms of fish communities (dominance of cyprinids and centrarchids instead of salmonids). Behavioural characteristics favouring capture avoidance (schooling) and facilitating underwater observation (curiosity) may be responsible for the superiority of visual surveys in our study rivers. Survey methods should be selected based on fish community composition. In the second chapter, I develop FHUM for complex stream fish communities. In order to simplify the modelling of such communities, as well as improve our understanding of fish – habitat relationships, I used the ecological guild concept and the niche filtering hypothesis to explore the relationships between guilds based on different types of traits (eco-morphological, reproductive, alimentary and taxonomic) and local environmental descriptors, at the coarse meso-habitat scale. Reproductive guilds led to FHUM that clearly outperformed the other 3 approaches, because of the close relationship between preferred spawning grounds and non spawning habitat preferences, and also because reproductive traits are linked to habitat characteristics at the reach or coarse mesohabitat scale. We also defined guilds based on habitat-use and related them to species traits. Traits related to the feeding biology of fishes seemed to be the best at explaining the habitat-use guilds, but our model did not correctly represent the among-guild relationships. Validation of our reproductive trait model on an independent dataset would confirm our finding, which represents a promising way of modelling fish habitat relationships in complex fish communities. In conclusion, my thesis suggests important improvements to FHUM models in the Laurentian streams by giving new insights on the choice of a sampling method that take into account the biological characteristics of the streams targeted, and by using a promising way of simplifying FHUM for species rich communities: reproductive guilds. viii