Influence of Streamflow and Predators on Habitat Choice by Trout

DOCTORATE DISSERTATION: I examined the effect of flow and presence of large brown trout on habitat choice by juvenile rainbow trout and brook charr. Experiments were conducted in a large stream enclosure with a pool-riffle-pool configuration. Large brown trout, when present, were confined to the upstream pool. Juvenile trout and charr tended to leave the riffle and occupy the pools when flow was low. They also were less likely to be found in the upstream pool when large brown trout were present; juvenile fish remaining in the upstream pool avoided the deep area with cover favored by the large brown trout especially at sunset and at night at high flow. Low flow apparently caused juvenile trout and charr to inhabit pools with large brown trout that they would otherwise avoid. Observations of marked individuals indicated that juvenile fish increased their preference for shallower water of intermediate velocity when flow changed from low to high, even after adjusting for different depth and velocity availabilities. This suggests that depth and velocity preferences were flow dependent. Furthermore, velocity choice of aggressive fish increased more than for non-aggressive individuals, and velocity choice of rainbow trout increased more than for brook charr. Feeding rates of juvenile rainbow trout and brook charr declined both when flow was low and when large brown trout were present, although aggressive fish had higher overall feeding rates than non-aggressive individuals. Feeding rates were likely affected by changes in food availability related to both flow level and changes in local fish density, as well as the aggressiveness of individual fish. This result indicates that microhabitats having water of the same velocity may not always be of the same value to drift-feeding juvenile salmonids. Overall, this study has shown that flow and predator effects on juvenile salmonid habitat choice are extremely complex. Both habitat use and feeding rate were strongly affected by flow, predator presence, diel period, and the aggressiveness level of individual fish. Interactions among these variables were common. Models used to recommend instream flows for regulated streams may not adequately evaluate stream fish habitat.博士学位论文：本研究探讨了水流条件与大型褐鳟（large brown trout）的存在对幼虹鳟与溪红点鲑生境选择的影响。实验于带有深潭-浅滩-深潭（pool-riffle-pool）结构的大型溪流围隔中开展。实验中存在的大型褐鳟均被限制在上游深潭内。低水流条件下，幼虹鳟与溪红点鲑倾向于离开浅滩，转而占据深潭；当存在大型褐鳟时，幼鱼在上游深潭中的出现概率也会降低，滞留在上游深潭的幼鱼会避开大型褐鳟偏好的带遮蔽深水区，在日落时段与高水流夜间尤为明显。低水流条件显然会迫使幼虹鳟与溪红点鲑进入原本会回避的、栖息有大型褐鳟的深潭。对标记个体的观测表明，即便校正了不同水深与流速的可获得性差异，当水流从低转为高时，幼鱼对中等流速浅水区的偏好会显著提升。这说明幼鱼对水深与流速的偏好具有水流依赖性。此外，具有攻击性的个体相较于非攻击性个体，其流速选择的变化幅度更大；而虹鳟相较于溪红点鲑，其流速选择的变化幅度同样更大。无论是低水流条件还是存在大型褐鳟时，幼虹鳟与溪红点鲑的摄食率均会下降，不过具有攻击性的个体整体摄食率高于非攻击性个体。摄食率可能受到与水流水平和本地鱼类密度变化相关的食物可获得性变化，以及个体攻击性的共同影响。该结果表明，对于以漂流性饵料为食的幼鲑科鱼类而言，具有相同流速的微生境并不总能提供同等的生态价值。总体而言，本研究表明水流与捕食者对幼鲑科鱼类生境选择的影响极为复杂。生境利用与摄食率均显著受到水流、捕食者存在、昼夜时段以及个体攻击性水平的影响，上述变量间的交互作用十分普遍。用于为受调控溪流推荐河道内流量的模型，或许无法充分评估溪流鱼类的生境质量。