LB 5-29 - Spatio-temporal coupling between fish and zooplankton: Does brook charr track its zooplanktonic prey?

Background/Question/Methods:

Animal movements play a crucial role in ecosystem dynamics, regulating numerous functions from nutrient cycling to habitat coupling. Mobile predators have thus the ability to connect different trophic networks by their feeding behavior and habitat preferences. In this study, we investigate how brook charr locomotion could influence the dynamic of a small boreal lake in Québec, Canada. The lake Ledoux is constituted by three basins, one of which is shallow. Since the brook charr, the only fish species in the lake, does not bear temperatures above 22°C, the thermal barrier in summer prevents them from accessing the shallow basin or littoral areas. Our aim here is to understand how the impact of fish predation on zooplankton is modulate by the presence of a thermal barrier and how individuals select their environment in consequence. Specific aims of the study were (1) to show which features fish will select among habitat and resource variables, distinguishing different categories of zooplankton according to size or taxon by using state-of-the-art statistical tools; (2) show if the fish selection depends on the seasonal variations of the thermal barrier permeability between the littoral and the pelagic zone; (3) and more specifically, determine the inter-individual differences of selection.

Results/Conclusions:

Weekly zooplankton mapping was conducted with an Underwater Vision Profiler 2h before the sunset, to match the a priori peek of fish activity. About 4km of transects at different depth were used to interpolate zooplankton concentration in the entire lake. Acoustic tags were implanted in 21 fish to follow their movements during the same period, every 20sec. Habitat Selection Analysis were adapted in 3D, controlling for thermal preference, to unravel the fish selection processes. We found that fish selected habitat close to the bottom of the water column most of the time. In summer, several fish selected the 6m-isobath to remain close to littoral resource-rich areas while staying in thermal refuge. However, they never went to the shallow basin. When thermal barrier disappeared, they switch from pelagic to littoral areas with a more frequent selection upon zooplankton patches even although some individuals never selected them. In the context of climate change, integrating behavioral response to thermal barrier is crucial to better assess the potential impact of a summer lasting longer in boreal areas.