Influence of light intensity on activity and habitat utilization of walleye (Sander vitreus) in two Northwestern Ontario lakes

Abstract

Ecologists have paid considerable attention to how and why organisms select particular habitats. Of fundamental importance is discerning which physical factors of the environment restrict the occurrence of organisms to particular habitats. A recent habitat suitability model (HSM) for walleye (Sander vitreus) hypothesized that light is the primary controlling variable influencing the spatial and temporal dimensions of feeding habitat. To test the HSM and evaluate the optical and thermal characteristics of walleye habitat I used telemetry to estimate the foraging activity of 24 walleye in two lakes with different water clarity, during periods of changing light intensity. The water clarity in the lakes differed (mean Secchi depths = 2.4 m and 4.8 m); however, the thermal environment, although variable, did not differ consistently between the lakes. Walleye in the stained lake occurred in warmer (mean temperature = 17.0 to 19.0 C), shallower (median depth = 3.3 to 7.4 m) water, close to the depth range predicted by the HSM. In contrast, walleye in the clear lake were generally located at depths (median depth = 5.5 to 8.2 m) approximately 8.0 m shallower than predicted by light levels alone (approximately 14.0 to 19.0 m), likely because predicted optimal light levels occurred at depths where the temperature was too cold (approximately 7.0 to 9.0 °C). The individual activity was highly variable, but the general pattern of behaviour was similar between the two lakes. Walleye activity tended to be low in the afternoon and increased as light levels declined at dusk (i.e., mean change in displacement rate = 35 % between 3 and 5 pm, and 550% between 7 and 9 pm). My results support the hypothesis that subsurface light conditions are a key element of walleye feeding habitat; however, other factors, such as water temperature, submerged structure, and prey-fish behaviours also appear to strongly influence walleye behaviour and distribution. Lake-specific changes to walleye habitat may be the result of predicted changes to water clarity due to exotic species, land use practises, and global warming.