Coastal infrastructure alters behavior and increases predation mortality of threatened Puget Sound steelhead smolts

Fundamental movements of migratory species can be substantially influenced
by marine habitat disruptions caused by coastal infrastructure. The Hood
Canal Bridge (HCB) spans the northern outlet of Hood Canal in the Salish
Sea, extends 4.6 meters (15 ft) underwater, and forms a partial barrier
for steelhead migrating from Hood Canal to the Pacific Ocean. Spatial
mark-recapture survival models using acoustic telemetry data indicate that
only 49% (2017; 95% CI = 40, 58%) and 56% (2018; 95% CI = 48, 65%) of the
steelhead smolts encountering the HCB survived past the bridge and 7 km to
the next array. We studied fine-scale movements of more than 300 steelhead
smolts to understand how migration behavior was affected across the entire
length of the HCB and to quantify spatial and temporal patterns of
mortality. Individually coded acoustic telemetry transmitters implanted in
juvenile steelhead were used in conjunction with an extensive array of
acoustic receivers surrounding the HCB to obtain approximations of the
path each steelhead took as they encountered the bridge structure.
Steelhead survival past the HCB appeared unaffected by tidal stage,
population-of-origin, approach location, current velocity, or time of day,
but was influenced by week of bridge encounter. Behavioral data from
transmitters with temperature and depth sensors ingested by predators are
consistent with high levels of marine mammal predation. This study
confirms the considerable impact of the HCB on ESA-listed steelhead smolt
survival, and provides detailed information on the behavior of steelhead
smolts and their predators at the HCB for use in planning recovery
actions.