Ontogenetic niche partitioning in a facultatively anadromous salmonid: Implications for population dynamics

Abstract

Species management and conservation efforts are often based on range-wide trends, assuming dynamic equilibrium across space and time, even though fine-scale variability may be driving local dynamics. Oncorhynchus mykiss is a globally introduced, facultatively anadromous salmonid that is experiencing demographic shifts characterized by greater proportions of population remaining in freshwater. The degree of niche overlap between age classes of O. mykiss in freshwater environments may dictate how resources are partitioned within populations. We conducted a meta-analysis of age-specific O. mykiss habitat use to evaluate the degree of niche-partitioning between age classes and how age-specific habitat use relates to global in-stream and landscape level habitat variation. O. mykiss used deeper habitats as they grew towards maturity but did not partition habitat based on water velocity or substrate composition. As annual precipitation increased, O. mykiss used deeper and shallower habitats, and as summer air temperature increased, O. mykiss used shallower habitats. O. mykiss of native origin used deeper habitats than nonnative O. mykiss. However, a large proportion (42–99%) of variation in habitat use was associated with study or ecoregion, making climactic predictors unreliable for predictive species distribution or population dynamics models. Although the exact mechanisms driving geographic variability in O. mykiss habitat use are not fully understood, our results boost our understanding of how demographic shifts affect population resilience under climate change. Further research incorporating individual competitive behavior in predictive population models may elucidate the links between resource availability, demographic rates, and long-term O. mykiss population stability.

Keywords

• Rainbow trout
• Steelhead
• Oncorhynchus mykiss
• Population dynamics
• Intraspecific competition
• Habitat use