Assessing the Habitat Requirements of Stream Fishes: An Overview and Evaluation of Different Approaches

With the widespread decline and endangerment of freshwater fishes, there is a need to clearly define habitat requirements for effective species management and habitat restoration. Fish biologists often infer habitat requirements on the basis of correlative habitat associations in the wild. This generates descriptive models that predict species presence or abundance at a hierarchy of scales: distributional (macrohabitat) models predict the presence/absence of species at large scales, capacity models predict the abundance at the reach or channel unit scale when a species is present, and microhabitat models predict the distribution of individual fish at smaller spatial scales (e.g., instream habitat suitability curves for velocity, depth, and substrate). However, relationships based on habitat associations in the wild rarely give definitive insight into the absolute requirement for a particular habitat (i.e., necessity of a habitat for the persistence of individuals and populations). The assumption that habitat selection accurately reflects the fitness consequences of habitat use is rarely validated; more rigorous assessment of habitat requirement usually involves manipulative experiments or measurements of fitness (individual growth, survival, or reproductive success) in different habitat types. Bioenergetic habitat models offer a promising mechanistic alternative to correlative habitat suitability models for drift‐feeding fish and have the potential to predict habitat‐specific growth rates on the basis of swimming costs and energy intake. Once smaller‐scale habitat requirements of individuals are well defined, the final step is to determine when and how the requirements of individuals limit populations. Extrapolating smaller‐scale habitat requirements to the population level requires either large‐scale (ecosystem) manipulations of habitat, adaptive management, or habitat‐explicit population models. For species with distinct ontogenic shifts in habitat requirements, the concept of optimal habitat ratios may be useful for identifying limiting habitat factors and defining baselines for habitat restoration. Defining optimal habitat configurations for different species may also provide a basis for predicting how habitat change differentially affects species with contrasting habitat needs.