Determinants of Productive Capacity for Stream Salmonids

Trout growth and production are controlled by (1) the area and quality of habitat for sequential life history stages, (2) the availability and production of invertebrate prey, and (3) stage-structured population dynamics, in particular, the degree of recruitment limitation associated with serial habitat bottlenecks or stochastic disturbance events like floods or droughts. These controls are influenced by stream habitat structure, water chemistry (which controls primary production), and flow regime, as modified by riparian and watershed-scale influences. Production is optimized when channel structure maximizes both the production (flux) of drifting invertebrates and the efficiency with which trout can harvest drifting prey, and when habitat heterogeneity minimizes the occurrence of limiting habitat bottlenecks for critical life history stages. While habitat structure and prey abundance set maximum potential habitat capacity, recruitment acts as a control on whether maximum production is realized; stochastic events like floods that result in egg or juvenile mortality may limit production below capacity. Range contraction and declining production are associated with a warming climate, increasing eutrophication, and habitat impacts that degrade channel complexity (loss of riparian forest, watershed development, flow regulation). Effective protection of productive capacity requires moving beyond generic policy prescriptions to implementation of controls on cumulative development impacts at watershed scale.