Can thermal refuges save salmonids? Simulation of cold-pool benefits to salmonid populations

Abstract

Objective: The literature on thermal refuges has focused on when fish use what kinds of refuge, but we address how refuge availability and characteristics affect population abundance and persistence under warming. Methods: Population benefits of thermal refuges are very difficult to assess empirically, so we used InSTREAM, an individual-based trout population model. InSTREAM represents several ways temperature affects survival and growth, as mediated by behaviors such as deciding when and where to feed. The model’s credibility was established by showing that it closely reproduced 10 observed patterns in how refuge use varied with temperature, time of day, and trout size and in how refuge availability affects populations. We then simulated four levels of refuge availability in four increasingly warm temperature regimes, examining (1) survival and growth over summer periods of prolonged high temperatures and (2) population abundance and persistence over 22 years. Result: In the simulation experiments, summer refuge use acted as a population bottleneck: the number of surviving trout increased with refuge area and decreased with temperature. During prolonged summer high temperatures, mortality was high until abundance dropped to a level sustainable by the refuges. The model predicted reduced but persistent populations under high warming when thermal refuges made up 2% or more of stream area. Conclusion: The concept of salmonids “hanging on” in refuges, losing weight but surviving peak temperatures, was not supported by our experiment. The value of potential refuges to salmonid populations appears determined not just by temperature and area, but also by other characteristics that affect growth and survival, such as the availability of food and habitat for feeding and predator avoidance.