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.

Fish Tagging

Wild steelhead smolts were collected from a rotary screw trap in the South Fork Skokomish River (river kilometer, rkm,  13.5) and a weir trap in Big Beef Creek (rkm 0.1) during April and May of 2017 and 2018. Captured smolts were held in flow-through circular tanks for 1-48 (typically < 24) hours prior to tagging. Steelhead smolts were anesthetized using MS-222 infused river water (40 mg/L) and surgically implanted with one of four types of Vemco (now InnovaSea Systems Inc., hereafter InnovaSea; https://www.innovasea.com/fish-tracking/) 69 kHz transmitters: (1) V8-4x (8 mm diameter x 20.5 mm length, 2.0 g, 90 day expected battery life), (2) V9P-6L (9 mm x 31mm, 4.9 g, equipped with pressure sensor, 95 days), (3) V7P-4H (7 mm x 24 mm, 2.0 g, equipped with pressure sensor, 51 days), (4) V7T-4H (7 mm x 34 mm, 2.0 g, equipped with temperature sensor, 51 days), or (5) ‘delay’ V8-4x (8 mm diameter x 20.5 mm length, 2.0 g, 95 days; Table 1). The incision was closed with two interrupted triple knot stitches using sterile monofilament (Ethicon Monocryl 5-0 monofilament, with a 3/8 circle, reverse-cutting 13-mm needle). All transmitters were programmed to ping continuously at random intervals between 30 and 90 seconds. An additional 49 ‘delay’ V8 transmitters were programmed to be off for the first 8 days after release then on for the remainder of transmitter life, and were deployed to test the null hypothesis that the survival rate of tagged steelhead emitting a signal at 69 kHz did not differ from the survival rate of steelhead with silent transmitters. No difference in survival was observed between delay- and continuous-tagged smolts (Rechisky et al. in prep). InnovaSea V7 and V9 pressure sensors recorded the depth (maximum depth = 38 m, but the sensor continued to record max depth if deeper), and temperature sensors recorded the ambient temperature (range = -5 – 35 °C) of the tagged animal when within range of a receiver. Tagged steelhead were held for 20-30 hours after surgery in flow-through 50-gallon tanks then released at the location of capture.

Receiver Deployment

A network of InnovaSea receivers was deployed at various locations along the steelhead outmigration route to record the unique signal of each tagged smolt as it migrated from river mouths to the Pacific Ocean.  Receiver arrays were deployed to estimate survival and provide fine-scale behavior patterns within approximately 250 meters on either side of the Hood Canal Bridge (HCB). Two VR2W receivers were installed at the river mouths (RM) of the Skokomish River and Big Beef Creek to detect tagged smolts entering the marine waters of Hood Canal. An InnovaSea Vemco Positioning System (VPS) comprised of 24 VR2AR and 3 VR2W receivers (69 kHz; capable of decoding the signal from InnovaSea acoustic transmitters) in 2017 (March 5 – August 1), and 28 VR2AR and 14 VR2W receivers in (March 6 – September 13) 2018, was configured and deployed to provide fine-scale spatial information on tagged steelhead at the HCB (Figure 1). The VPS system uses an array of receivers equipped with co-located transmitters to communicate the instrument location to other system receivers.  Receivers were deployed in close proximity to each other to facilitate detection of a single transmission by multiple receivers, which enabled triangulation of each transmitter as it moved through the array. Stationary reference transmitters (5 transmitters in 2017 and 4 transmitters in 2018) were deployed within the system to test the accuracy and precision of the VPS system each year. Four additional VR2AR receivers were deployed approximately 600 meters apart at Twin Spits (TS), 7 kilometers north of the HCB, to determine whether smolts migrated successfully past the HCB. Twelve InnovaSea VR3 receivers spanned Admiralty Inlet (ADM), and a final line of 29 InnovaSea VR3 and VR4 receivers (maintained by the Ocean Tracking Network, https://oceantrackingnetwork.org/) spanned the Strait of Juan de Fuca at Pillar Point (JDF; Figure 1).

Data Processing

All receiver files were downloaded from recovered receivers or via surface modem (VR3s and VR4s) and compiled into a database. Raw data from single line receiver arrays were used for analyses requiring only presence or absence information.  Files from the VPS array receivers were used to generate precise transmitter positions.

InnovaSea processed VPS receiver detection data using hyperbolic positioning techniques (Smith 2013). Briefly, hyperbolic positioning measures differences in transmission detection times between pairs of time-synchronized receivers, then converts the time differences to distance values using the signal propagation speed, allowing for triangulation of a transmitter position. VPS analysis returned the coordinates and date-time of each tagged animal as it moved through the array, coordinates and date and time of the reference transmitters throughout the length of each deployment, and an estimate of accuracy for each position that is unique to each VPS array. To calculate error, the VPS analysis software uses the known positions of the reference transmitters to measure the distance between the triangulated position and the true position (HPEm), then calculates a second error estimate (HPE) that incorporates variation based on receiver array geometry and effects of depth, temperature, and salinity on the speed of signal transmission. However, the true location is only known for reference transmitters and not for animal transmitters, so the relationship between HPEm and HPE can only be examined for reference transmitters then subsequently applied to animal positions (Espinoza et al. 2011, Roy et al. 2014). For both VPS arrays separately, we created bins for all HPE values and plotted them against median HPEm values. Where there was a steep increase in median HPEm we defined a threshold (same for both years) and deleted animal detections with HPE values larger than that threshold. After applying this filter, 96.2% (2017) and 87.6% (2018) of the animal positions were retained. Median HPEm values for identically filtered reference transmitter data were 4.4 m in 2017 and 5.0 m in 2018. These accuracy estimates can be applied to animal position data because HPE was calculated in the same way for both reference and animal transmitters. Processed data from all tagged animals were plotted for further analysis using ArcGIS 10.5.1 (Redlands, California).

All detection times are in UTC and need to be converted to PST for analysis in local time.