Recent Salmon Declines: A Result of Lost Feeding Opportunities Due to Bad Timing?

As the timing of spring productivity blooms in near-shore areas advances due to warming trends in global climate, the selection pressures on out-migrating salmon smolts are shifting. Species and stocks that leave natal streams earlier may be favoured over later-migrating fish. The low post-release survival of hatchery fish during recent years may be in part due to static release times that do not take the timing of plankton blooms into account. This study examined the effects of release time on the migratory behaviour and survival of wild and hatchery-reared coho salmon (Oncorhynchus kisutch) using acoustic and coded-wire telemetry. Plankton monitoring and near-shore seining were also conducted to determine which habitat and food sources were favoured. Acoustic tags (n = 140) and coded-wire tags (n = 266,692) were implanted into coho salmon smolts at the Seymour and Quinsam Rivers, in British Columbia, Canada. Differences between wild and hatchery fish, and early and late releases were examined during the entire lifecycle. Physiological sampling was also carried out on 30 fish from each release group. The smolt-to-adult survival of coho salmon released during periods of high marine productivity was 1.5- to 3-fold greater than those released both before and after, and the fish's degree of smoltification affected their downstream migration time and duration of stay in the estuary. Therefore, hatchery managers should consider having smolts fully developed and ready for release during the peak of the near-shore plankton blooms. Monitoring chlorophyll a levels and water temperature early in the spring could provide a forecast of the timing of these blooms, giving hatcheries time to adjust their release schedule

Stomach sample composition for wild and hatchery coho smolts in the Campbell River estuary during springs of 2007–2009.

<p>Stomach sample composition for wild and hatchery coho smolts in the Campbell River estuary during springs of 2007–2009.</p

The mean stomach fullness of wild and hatchery coho salmon smolts in the Campbell River estuary and near-shore area from 2007 to 2009, with standard error.

<p>The mean stomach fullness of wild and hatchery coho salmon smolts in the Campbell River estuary and near-shore area from 2007 to 2009, with standard error.</p

Health parameters of early and late release coho from the Seymour River Hatchery during April 2007, expressed as group mean (SD).

<p>The results for the Quinsam smolts can be found in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012423#pone.0012423-Chittenden1" target="_blank">[18]</a>.</p

The study area, including the Seymour and Quinsam Rivers, Discovery Passage and the Strait of Georgia (SOG), Indian Arm and Burrard Inlet.

<p>Release locations are denoted by an asterix (*).</p