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

Shannon Balfry,Alexandra Eaves,Jenny L A Jensen,Cedar M Chittenden,David Ewart,Sonja Saksida,David Welch,Brian Smith,R Scott Mckinley,Steven J Bograd,Stephen Vincent,Shannon Anderson,Elan Downey

doi:10.1371/journal.pone.0012423

Abstract

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.

Highlights

Increasing surface temperatures in western North America during the last half-century [1] have led to earlier spring freshets [2], [3] and advancing plankton blooms in near-shore and open ocean areas [4]–[6]
During periods of high marine productivity (2007: .500 phytoplankton?ml21 and .2,500 zooplankton?m23; 2008 and 2009: .500 phytoplankton?ml21 and .1,000 zooplankton?m23), very few coho salmon were captured in the estuary
Coho salmon smolts released within a week of the near-shore marine productivity blooms had higher survival to adulthood than those released both before and after

Summary

Introduction

Increasing surface temperatures in western North America during the last half-century [1] have led to earlier spring freshets [2], [3] and advancing plankton blooms in near-shore and open ocean areas [4]–[6]. This trend towards an earlier productivity bloom may favour earlier out-migrating smolts and species [3], as smolts migrating out to marine feeding areas at the time of the plankton blooms may be more likely to reach a critical body size before summer, allowing them to survive the winter [4], [7], [8]. Matching the timing of the smolt migration with the zooplankton bloom may increase the population’s overall survival

Methods

Results

Conclusion