Hydrological and biotic forces constrain brown trout (Salmo trutta) population dynamics, but tests of their role across numerous streams are uncommon. In 30 French stream reaches, using 5–8 samples (1 per year) each, we investigated whether the year-to-year seasonal hydrology influenced annual trout densities within reaches, and whether the relationships were shared by all reaches. We also searched for intraspecific interactions between and within cohorts. Trout data were age class (0+, 1+, and adults) densities. For each year, hydrology was described using 13 variables, each computed for a reproduction, emergence, and growth period related to the biological cycle of trout. We used analyses of covariance (ANCOVA) to test how trout densities at year n – 1 and hydrology at year n influenced trout densities at year n. High flows during emergence significantly reduced the 0+ densities, consistently across the 30 reaches. Then, 1+ and adult densities were linked, respectively, to 0+ and 1+ densities from the previous year. Analyses also revealed density-dependent survival mechanisms for the 0+ cohort, suggesting intracohort competition. Therefore, hydrology constrains trout dynamics only during the critical emergence period, after which intracohort interactions regulate the 0+ density. Such mechanisms, validated across 30 environmentally different reaches, seem to be fundamental to trout population dynamics.

N. 2003. Synchrony in brown trout, Salmo trutta, population dynamics: a 'Moran effect' on early-life stages. -Oikos 100: 43-54.Synchrony among populations (i.e. spatial covariation in temporal fluctuations of population size or growth rate) is a common feature to many animals. Both large-scale autocorrelated climatic factors (the 'Moran effect') and dispersal between populations are candidates to explain synchrony, although their relative influence is difficult to assess. Only a few investigations have reported patterns of synchrony among freshwater populations, and even fewer directly related these patterns to an environmental variable. In the present study, we analysed the spatio-temporal patterns of fluctuation of 57 brown trout populations widespread across France, each sampled continuously during 5 years. We compared the respective influence of connectivity and stream distance within basins (i.e. that potentially allow a basinscale dispersal) and environmental factors (hydrological and air temperature variables, available for 37 sites) on the synchrony of brown trout cohort densities (0 + , 1+ and adults). A series of Mantel tests revealed that the degree of synchrony was not related to connectivity or stream distance between sites, suggesting no effect of dispersal at the basin-scale. The degree of synchrony among sites for the 0 + fish was significantly related to the degree of hydrological synchrony (based on high flows during the emergence period). For all three age classes, the synchrony in the temperature patterns did not explain synchrony in trout dynamics. Our results allow us to discuss the respective influence of dispersal and climatic factors on the spatio-temporal patterns of trout dynamics at the basin scale. F. Cattanéo and N. Lamouroux, Cemagref, U.R. Biologie des Ecosystèmes Aquatiques, 3 bis Quai Chau6eau, CP 220, FR-69 336 Lyon Cedex 09, France (catta-neo@lyon.cemagref.fr). Alternati6e address for FC: U.R. Hydrologie-Hydraulique, same address as abo6e. -B. Hugueny, IRD, Ecologie des Hydrosystèmes Flu6iaux, Uni6. Lyon I, 43 b6d du 11 No6embre 1918, FR-69 622 Villeurbanne Cedex, France.

No abstract

General relationships between organisms and their habitat, consistent across spatial scales and regions, suggest the existence of repeatable ecological processes and are useful for the management of stream networks. From published data, we defined four guilds of European fish species with contrasting preferences for microhabitat hydraulics within stream reaches. At the scale of stream reaches and across 139 French sites (590 460 fishes sampled), we analysed how fish guild proportions were related to reach hydraulics (proportion of pools vs. riffles %POOL; median discharge by unit width Q50/W). The strongest correlations were observed between two fish guilds and %POOL ( p < 0.001, r 2 ! 0.41) and between one fish guild proportion and Q50/W (p < 0.001, r 2 ¼ 0.10). These reach-scale relationships were consistent across six large French basins, and consistent with the analyses made at the microhabitat scale. Therefore, microhabitat preferences for hydraulics are strong enough to generate consistent reach-scale community responses to hydraulics across regions, despite the influence of other filters such as temperature, nutrient levels or history. The distribution of basic geomorphic features (pools, riffles) in streams and their modification (by dams, weirs and dikes) can modify the proportion of fish guilds by up to 80%, probably contributing to the longterm decline of riffle-dwelling species in Europe.

With 5 figures and 4 tables