| Summary: | Steelhead (Oncorhynchus mykiss) parr captured at a full stream enumeration trap at
the mouth of the Keogh River, northern Vancouver Island, British Columbia comprised
9-35% (mean = 20%) of seaward dispersing juvenile (smolts + parr) steelhead, 1977-1993.
Total parr annually varied from 400-5000 (mean = 1600). I tested two competing hypotheses
to explain this biologically significant phenomenon: parr seaward movement is (1)
premature but directed migration homologous to smolt migration, and (2) downstream dispersal
resulting from upstream, density-dependent interactions among juvenile steelhead.
At the watershed scale, reconstruction of past stream population sizes of steelhead juveniles
suggested exponentially increasing parr dispersal as estimated main stem population
size increased. At the reach scale, steelhead parr dispersed from experimentally stocked
reaches; high initial stocking densities revealed limits to reach carrying capacity. At the
individual trout scale, observations of agonistic behaviour of non-dispersing and dispersing
parr in artificial stream channels showed that parr size was more important than parr
type in determining attack rate. Dispersing parr had both growth rates and body shape
(indexed by condition factor) that were intermediate between smolts and non-dispersing
parr of the same age. Finally, parr dispersal followed the same seasonal timing pattern that
smolt migration followed in 14 years of daily downstream movement data. Experimental
manipulation and observation of naturally occurring patterns produced data supporting
each hypothesis. Therefore, parr seaward movement in the spring at the Keogh River seems
to be a phenomenon blended from indirect density-dependent effects as well as premature
migration.
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