Effect of juvenile steelhead (Oncorhynchus mykiss) on age-O and age-1 brown t rout (Salmo trutta) survival and growth in a sympatric nursery stream.

We determined that juvenile steelhead (Oncorynchus mykiss) had little effect on the survival and growth of age-0 and age-1 brown trout (Salmo trutta) in a Lake Huron tributary. Starting in 1989, we surveyed wild brown trout juveniles in two stream sections. Unfed steelhead fry were introduced into one section in May of 1990 and 1991, establishing moderate populations. Assessments continued through 1992. Steelhead had no measurable effect on juvenile brown trout abundance or survival. Steelhead did negatively influence age-0 brown trout growth, but this effect was minor compared with intraspecific interactions and abiotic factors. We attribute much of the brown trout success to their larger size. In June sampling, brown trout were typically 42% larger than steelhead, as would be common in the region. Variability in factors such as spawner-alevin interactions or climate could alter the timing of emergence or initial abundance, causing a more substantial effect on growth and, potentially, survival. Fishery managers need to evaluate these factors for the stocks in question before combining these species.