Life at the top: lake ecotype influences the foraging patterns, metabolic costs and life history of an apex fish predator

1.We used acoustic telemetry and acceleration sensors to compare population-specific measures of the metabolic costs of an apex fish predator living in four separate lakes. We chose our study species and populations to provide a strong test of recent theoretical predictions that optimal foraging by an apex fish predator in a typical aquatic environment would be consistent with feeding to satiation rather than continuous feeding. We chose four populations where the primary prey type differed along a body size gradient (from small invertebrates to large planktivorous fish) and along a thermal accessibility gradient (from easily accessible coldwater pelagic prey to less accessible warmwater epilimnetic and littoral prey). We expected that these gradients in prey type would evoke distinctly different activity gradients depending on whether predators fed to satiation (e.g. less frequent ‘rest’ detections where primary prey are smaller/less accessible) or fed continuously (e.g. fixed level of ‘rest’ detections under all prey conditions).
2. Our study organism was a fall spawning, coldwater visual apex predator (lake trout). Therefore, we focused our study on diel (early night, dawn, day, dusk, late night) changes in metabolic costs associated with summer feeding behaviour. The duration (~20 days) and fine temporal scale (~30 minutes) of our behavioural data provided a uniquely detailed picture of intra- and inter-population differences in activity patterns over a critical period in the annual growing season.
3. In all populations, diel shifts in activity were qualitatively consistent with that expected of a visual predator (e.g. resting state detections were most frequent at night).
4. Between-lake differences in daytime thermal experience were qualitatively consistent with between-lake differences in the location of primary prey (e.g. excursions to warm habitats were common in lakes with epilimnetic/littoral fish as primary prey and relatively rare in lakes with pelagic coldwater invertebrates/fish as primary prey).
5. Daytime activity patterns were consistent with feeding to satiation rather than continuous feeding: (a) individuals in all four populations exhibited clearly delineated bouts of resting behaviour and active behaviour; (b) the frequency of resting bouts and the resultant overall cost of daily activity were strongly associated with the size and accessibility of prey – in lakes with smaller and/or less accessible prey, predators rested less frequently, exhibited marginally higher costs when active and had higher overall daytime activity costs. Within each lake, similar changes in activity occurred concurrently with diel changes in prey accessibility/relative density.

1. 我们采用声学遥测（acoustic telemetry）与加速度传感器，对栖息于4个独立湖泊的顶级鱼类捕食者的种群特异性代谢成本开展对比分析。本研究选取目标物种及种群，旨在对近期理论预测开展严谨验证：典型水生环境中顶级鱼类捕食者的最优觅食策略，应符合饱食后停止摄食的模式，而非持续摄食。我们选定的4个种群，其主要猎物类型沿体型梯度（从小型无脊椎动物至大型浮游食性鱼类）与热可及性梯度（从易获取的冷水浮游猎物，至较难获取的暖水湖上层及沿岸猎物）存在差异。我们推测，猎物类型的这两类梯度将催生显著不同的活动梯度：若捕食者以饱食为摄食终点（例如当主要猎物体型更小/更难获取时，"休息"检测的频率更低），则活动模式会异于持续摄食的情况（例如所有猎物条件下"休息"检测的水平均保持固定）。
2. 本研究的受试生物为秋季产卵的冷水视觉型顶级捕食者——湖鳟（lake trout）。因此，我们将研究聚焦于与夏季摄食行为相关的代谢成本昼夜变化，涵盖深夜、黎明、日间、黄昏及深夜等时段。本次行为数据的采集时长约20天，时间分辨率达约30分钟，这使得我们得以在年度生长季的关键窗口期，详尽刻画种群内与种群间活动模式的差异。
3. 所有种群的活动昼夜偏移模式，均与视觉型捕食者的预期表现定性一致，例如"休息"状态的检测频次在夜间达到峰值。
4. 不同湖泊间的日间热暴露差异，与主要猎物的栖息位置差异定性一致：例如，以湖上层/沿岸鱼类为主要猎物的湖泊中，捕食者前往温暖生境的活动较为常见；而以冷水浮游无脊椎动物/鱼类为主要猎物的湖泊中，此类活动则相对罕见。
5. 日间活动模式符合饱食摄食而非持续摄食的特征：其一，4个种群的所有个体均呈现出清晰划分的休息行为与活动行为时段；其二，休息时段的频次及由此产生的每日活动总能耗，与猎物体型及可及性显著相关——在猎物体型更小且/或更难获取的湖泊中，捕食者的休息频次更低，活动时的能耗略高，且日间总活动能耗更高。在单个湖泊内部，活动模式的相似变化与猎物可及性/相对密度的昼夜变化同步发生。