Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 4483
Title: A floating bridge disrupts seaward migration and increases mortality of steelhead smolts in Hood Canal, Washington State
Author: M. E. Moore, B. A. Berejikian, E. P. Tezak
Publication Year: 2013
Journal: PLoS ONE
Keywords: aggregative predator response, habitat modification, migration barrier, steelhead,aggregative predator response,habitat modification,migration barrier,steelhead

Habitat modifications resulting from human transportation and power-generation infrastructure (e.g., roads, dams, bridges) can impede movement and alter natural migration patterns of aquatic animal populations, which may negatively affect survival and population viability. Full or partial barriers are especially problematic for migratory species whose life histories hinge on habitat connectivity. The Hood Canal Bridge, a floating structure spanning the northern outlet of Hood Canal in Puget Sound, Washington, extends 3.6 meters underwater and forms a partial barrier for steelhead migrating from Hood Canal to the Pacific Ocean. We used acoustic telemetry to monitor migration behavior and mortality of steelhead smolts passing four receiver arrays and several single receivers within the Hood Canal, Puget Sound and Strait of Juan de Fuca. Twenty-seven mortality events were detected within the vicinity of the Hood Canal Bridge, while only one mortality was recorded on the other 325 receivers deployed in throughout the study area. Migrating steelhead smolts were detected at the Hood Canal Bridge array with greater frequency, on more receivers, and for longer durations than smolts migrating past three comparably configured arrays. Longer migration times and paths are likely to result in a higher density of smolts near the bridge in relation to other sites along the migration route, possibly inducing an aggregative predator response to steelhead smolts. Synthesis and applications This study provides strong evidence of substantial migration interference and increased mortality risk associated with the Hood Canal Bridge, and may partially explain low early marine survival rates observed in Hood Canal steelhead populations. Understanding where habitat modifications indirectly increase predation pressures on threatened populations helps inform potential approaches to mitigation.


This paper uses acoustic telemetry to calculate behavioral metrics and quantify mortality at major monitoring sites throughout the Hood Canal, Puget Sound, and Strait of Juan de Fuca. Behavioral anomalies and increased mortality are documented at the Hood Canal Bridge, a floating bridge that spans the northern outlet of Hood Canal.

Foci: Describe the interaction between human activities and ecosystem status and resilience.
Characterize ecological interactions (e.g. predation, competition, parasitism, disease, etc.) within and among species to support ecosystem approach to management.