Habitat effects on recruitment, population limitations, and recovery potential in an endangered stream fish.

Abstract

Elucidating the habitat-related processes driving variation in juvenile recruitment and population fluctuation is crucial for the conservation of freshwater fishes, particularly for species at-risk, where recruitment variation increases at small population size. We used a combination of instream enclosure experiments and habitat use observations to investigate between-stream variation in recruitment of juvenile Nooksack Dace (Rhinichthys cataractae sp. cataractae), a federally endangered stream-dwelling minnow endemic to the lower Fraser Valley of British Columbia, Canada. There was no evidence for a recruitment bottleneck associated with high water temperature or low dissolved oxygen in spawning riffles at the egg incubation stage, or during a two week juvenile growth experiment in enclosures, although larval dace growth was positively related to temperature. However, immediately post-hatch dace showed a very strong preference for shallow (< 5cm deep), fine substrate marginal shoal habitat with no overhanging or instream vegetation cover, which was associated with well-developed pool-riffle channel structure. Shallow marginal rearing habitat was absent in the stream experiencing recruitment failure, where encroachment by invasive reed canarygrass (Phalaris arundinacea) created homogenous steep-bank habitat with heavy instream vegetation cover, implicating loss of critical marginal rearing habitat as the recruitment bottleneck.