Fishes typically occupy a species-specific temperature range, with their occupied depth being related to the lake’s temperature profile. When a fish’s preferred temperature range coincides with the thermocline, the location of their preferred thermal habitat is influenced by the rise and fall of internal waves, leading to possible changes in fish depth. These internal waves are common in large, stratified lakes, yet we do not know how they affect the spatial distribution and behavior of freshwater fishes. We conducted nighttime hydroacoustic surveys in a large, deep embayment of a large thermally stratified lake to observe whether pelagic fish respond to vertical oscillations of the thermocline caused by internal waves. The coldwater pelagic fish in our study (primarily cisco, Coregonus artedi) typically occupied a narrow vertical band approximately 5–8 m thick and temperatures between 10.8 ± 0.8–13.6 ± 1.6 °C (fishes sized 106–500 mm), just below the thermocline (centered around 15–17 °C). Importantly, the upper bound of fish depth varied in response to vertical thermocline movements associated with internal waves, suggesting fish respond to changes in their physical environment on timescales commensurate with basin-scale internal wave periods (hours to days), to remain within their preferred thermal habitat. Dissolved-oxygen levels were typically above avoidance thresholds of these fish, thus not likely exerting a strong influence on fish location. Our findings emphasize the need to account for internal waves when designing hydroacoustic and netting surveys, as thermocline movements can influence where fish are located.

鱼类通常占据特定物种的温度范围，它们占据的深度与湖泊的温度剖面有关。当鱼类的首选温度范围与温跃层重合时，其首选热栖息地的位置会受到内波起伏的影响，从而导致鱼类深度可能发生变化。这些内波在大型分层湖泊中很常见，但我们不知道它们如何影响淡水鱼类的空间分布和行为。我们在一个大型热分层湖的大型深海湾进行了夜间水声调查，以观察中上层鱼类是否对内波引起的温跃层垂直振荡做出反应。我们研究中的冷水中上层鱼（主要是cisco、Coregonus artedi) 通常占据约 5–8 m 厚的狭窄垂直带，温度在 10.8 ± 0.8–13.6 ± 1.6 °C 之间（鱼体长 106–500 mm），正好位于温跃层下方（以 15–17 °C 为中心）。重要的是，鱼深度的上限随着与内波相关的垂直温跃层运动而变化，这表明鱼在与盆地尺度内波周期（数小时到数天）相称的时间尺度上对其物理环境的变化做出反应，以保持在其首选范围内热栖息地。溶解氧水平通常高于这些鱼的回避阈值，因此不太可能对鱼的位置产生强烈影响。我们的研究结果强调在设计水声和网调查时需要考虑内波，因为温跃层运动会影响鱼的位置。