Patterns of Timing and Size of Wild Coho Salmon (Oncorhynchus kisutch) Smolts Migrating from the Keogh River Watershed on Northern Vancouver Island

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Abstract
Coho salmon (Oncorhynchus kisutch) smolts leaving the Keogh River and its tributaries demonstrated consistent patterns in their timing and size. An initial early peak in the mean size of migrating smolts occurred several weeks before the peak in their timing, and in several years when smolt migration was monitored until July, a late peak in size occurred near the end of the smolt migration. Changes in the mean size of smolts were not related to their origin within the watershed, and varying age compositions were only partially responsible for the two peaks in mean size. Fish overwintering in tributary lakes grew faster than fish overwintering in tributary streams. A greater proportion of lake-origin smolts migrated to sea after only 1 yr in freshwater than stream-origin smolts. It appears that fish that wait until their second spring to smoltify are among the first to emigrate that spring. These relatively large 2 + smolts, combined with fast growing 1 + smolts that leave early during their first spring, cause the first peak in size. The small numbers of large smolts leaving the Keogh River at the ends of several of the smolt runs apparently left then because of minor flow increases in the river. As has been found with releases of coho smolts from nearby hatcheries, fish that left the Keogh River early in the smolt run generally had a more northerly marine catch distribution than later migrating smolts. A greater proportion of fish migrating to sea late in the smolt run were caught in the fishery than fish migrating early. Additional work examining the marine survival of early and late migrating wild smolts is recommended to enable detailed comparisons to be made between the optimal timing and size of wild and hatchery smolts.

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