We assembled data from stream surveys by the Oregon Department of Fish and Wildlife from seven watersheds that were important steelhead producers, and were dispersed around the state. The surveys covered 528 km of stream and included measurements of over 10,000 channel units (pool, riffles, etc.). Riffles with average depths greater than 0.4 m are rare, and over 70% of pools have maximum depths less than the 0.8m preferred by typically-sized Chinook and steelhead parr . We deduce from these data that suitable habitat for parr rearing will be the factor most likely to limit a watershed’s carrying capacity for salmonids that over-summer in streams. Allen (1968) points out that fish migrate or die when territory needs are not met. The increasing mismatch between territory requirements and availability of suitable habitat in natal streams as parr increase in size indicates why anadromy is an adaptive solution to the problem.
Not that some deeper pools exist, so some fish could be supported at smolt size or larger – just not the numbers that grew to that age with both anadromous and resident juveniles present .