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Seaward movements of Keogh River steelhead parr : density-dependent dispersal and premature migration Burrows, Jeff A.


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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