A Floating Bridge Disrupts Seaward Migration and Increases Mortality of Steelhead Smolts in Hood Canal, Washington State

Abstract

BackgroundHabitat 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.Methodology/Principal FindingsThe 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 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.Conclusions/SignificanceThis 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.

Multi-population analysis of Puget Sound steelhead survival and migration behavior

“…A substantial portion of the HCB to ADM mortality may be associated with the Hood Canal Bridge itself. Detailed behavioral analysis suggests that the Hood Canal Bridge causes migration delays and abnormal movement patterns that may increase predation on steelhead smolts in the vicinity (Moore et al 2013). Although steelhead smolt populations typically experience their highest mortality rates during their initial marine migration segment, habitat-specific conditions can contribute substantially to variation in mortality rates among regions.…”

Size‐Selective Mortality of Steelhead during Freshwater and Marine Life Stages Related to Freshwater Growth in the Skagit River, Washington

“…Conflicting theories exist on whether near-or offshore marine environments exert the greatest impact on mortality of Salish Sea steelhead. Welch et al (2004) provided evidence, using stationary acoustic receivers, that survival was lower in the open-ocean marine environment, and Moore et al (2010Moore et al ( , 2013 provided evidence, also using stationary acoustic receivers, that survival was lower in the nearshore marine environment. More recently, Melnychuk et al (2013) suggested that the use of both stationary acoustic receivers and mobile tracking could improve the detectability of the actual locations in the marine environment at which mortality occurs in steelhead.…”

“…Recent studies of steelhead in the Salish Sea have highlighted the importance of survival in marine environments, but locations and modes of mortality have not been conclusively determined. Steelhead smolts in the Salish Sea do not appear to spend time foraging and growing in local estuarine or brackish environments (Melnychuk et al 2007;Moore et al 2010), but environmental conditions in both nearshore marine waters (Moore et al 2010(Moore et al , 2013Melnychuk et al 2013;Romer et al 2013) and the high seas of the Pacific Ocean influence marine survival and production of many anadromous salmonid stocks in this region (Welch et al 2000;Welch et al 2004;Moss et al 2005;Melnychuk et al 2007;Welch et al 2011;Atcheson et al 2012). Using an 18-year data series, Atcheson et al (2012) found that the growth of steelhead in the high seas varied with oceanic conditions.…”

Where Predators and Prey Meet: Anthropogenic Contact Points Between Fishes in a Freshwater Estuary

“…Docks, piles, and bridges are discrete structures that influence interactions between predatory fish and their prey. The effects of these structures can be compounding: they can disrupt or delay movements of migratory species such as salmon while simultaneously aggregating predators (Moore et al 2013).…”

“…Specifically, ambush predators may benefit from the decreased light and shelter under docks, while roving predators do not. Some studies found that fishes aggregated near docks but not under them (Moore et al 2013). However, docks may also be attractive to predator and prey species alike because they increase habitat complexity and provide shelter.…”