Abstract
Early marine migratory behaviour and apparent survival of hatchery-reared Seymour River steelhead (Oncorhynchus mykiss) smolts was examined over a four year period (2006–2009) to assess the impact of various management strategies on improving early marine survival. Acoustically tagged smolts were released to measure their survival using estuary and coastal marine receivers forming components of the Pacific Ocean Shelf Tracking (POST) array. Early marine survival was statistically indistinguishable between releases of summer run and winter run steelhead races, night and day releases, and groups released 10 days apart. In 2009, the survival of summer run steelhead released into the river was again trialed against groups released directly into the ocean at a distance from the river mouth. Apparent survival was improved significantly for the ocean released groups. The health and physiological status of the various release groups were monitored in years 2007–2009, and results indicate that the fish were in good health, with no clinical signs of disease at the time of release. The possibility of a disease event contributing to early marine mortality was further examined in 2009 by vaccinating half of the released fish against common fish diseases (vibriosis, furunculosis). The results suggest that marine survival may be enhanced using this approach, although not to the extent observed when the smolts were transported away from the river mouth before release. In summary, direct experimental testing of different release strategies using the POST array to measure ocean survival accelerated the scientific process by allowing rapid collection of data which enabled the rejection of several existing theories and allowed tentative identification of several new alternative approaches that might improve early marine survival of Seymour River steelhead.
Highlights
In recent years, steelhead populations in the Seymour River as elsewhere in the Strait of Georgia and the eastern side of Vancouver Island have declined substantially [1]
Before the advent of acoustic tags and the associated receiver technology, there was very little ability to monitor specific salmon stocks in the marine environment and no practical ability to measure their survival on time and spatial scales exceeding that of the river phase of the migration
Study Location The Seymour River in North Vancouver, British Columbia, Canada, is a medium-sized stream draining an area of approximately 176 km2
Summary
Steelhead populations in the Seymour River as elsewhere in the Strait of Georgia and the eastern side of Vancouver Island have declined substantially [1]. This decline has been best documented on the Keogh River (NE Vancouver Island), where ocean survival (which averaged 15% in the mid 1980’s) has declined to levels well below the 4% generally regarded as being necessary to maintain a stable population [2]. While ocean regime shifts driven by atmospheric factors such as the Pacific decadal oscillation undoubtedly play a role in this decline [3,4], direct comparison of steelhead survival from otherwise apparently similar rivers flowing east into the ‘‘Salish.
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