Responses of Two Brown Trout Populations to Different Regimes of Flow and Temperature: Application of a Population Dynamics Model

Two trout populations (Salmo trutta) have been studied since 1994 for the first one and 2003 for the second, in two sections of a French Mediterranean river, about 10 km away. This part of the Roya river, where two hydroelectric power plants are implanted, has been largely monitored during this period : flow and temperature were recorded, the habitat suitability depending of the flow was estimated (according to Phabsim protocol adapted to French streams), spawning grounds and redds were measured and genetic analyses were conducted on the trout populations.

Both populations showed fluctuations in the young of the year stock from a year to another. Some differences in the trout population structure of each site were also observed, with especially a deviation in terms of growth: length of trouts is higher, they survive longer and female ripeness is later (at three years old) downstream than upstream. A model of the dynamics of the trout population (MODYPOP) has applied to these two populations. It allows to formulate different hypotheses which explain the dynamics of these two neighbouring populations characterized by different structures.

The temperature explains the growth differences between these two populations at a low distance from each other but with high difference of temperatures, not only during growth period but also during winter. The impact of flooding in the post-emergence period is considerable on all two populations. Autumn floods during spawning period are very frequent on these sites; their impact is most often masked by the spring floods that occur after. The genetic analyses pointed out also that the upstream population is highly impacted by stocking with young fishes (in this section, trouts are essentially the atlantic strain) while the downstream population has conserved its Mediterranean strain character. The warm temperatures, associated with populations of different strains, explain a better growth downstream, maybe thanks to better adaptation of the local strain than the introduced one. However, the effect of hydroelectric power plants on the connectivity between the trout populations remains to be studied in more details.

These applications have enabled determining the relative importance of changes in population density due to different types of events. The comparison is helpful to a discussion of the effect of temperature, especially in the context of climate warming.