Tidal Fish Passage & Connectivity

From Salish Sea Wiki

Wiki Rules

Link to List of Workgroups Link to List of Topics Link to List of Places

Link to List of Efforts Link to List of Products Link to List of Documents Link to List of Graphics Link to List of Websites

Link to Delta Sites Link to Embayment Sites Link to Beach Sites Link to Rocky Headland Sites

Link to Headwater Sites Link to Lowland Watershed Sites Link to Floodplain Sites

WDFW is working with tribal partners and restoration practitioners to complete a study of fish movement in estuarine habitats, particularly juvenile salmon in tidal environments, such as embayments and river deltas. This study is intended to provide technical background for improvement of tidal barrier assessment and evaluation, as well as design of tidal water crossing structures (culverts, bridges, tidegates, etc.).


Improving fish access to estuarine and coastal ecosystems has been identified as one of the most effective investments for the protection and recovery of Pacific salmon. Several species of federally listed salmon species, including Chinook and chum salmon, depend on estuary and marine nearshore habitats during early life history stages.  Estuaries provide shallow protected waters for juvenile salmon to grow, with abundant food resources and a wide range of salinities for transition to marine waters.  Tidal conditions in some habitats (e.g., low tide mudflats and constricted embayments) may naturally preclude or hinder fish migration into, and movements within, estuaries. Anthropogenic activities in estuaries, such as road building and modification of the drainage network, can also contribute to loss of estuarine connectivity for fish. Habitat restoration with the intent of improving estuarine connectivity for fish has been hampered by a lack of information, even though fish passage criteria for upland water crossing structures under roadways (culverts and bridges) are relatively well-established.

Fish passage dynamics are generally more complex in estuaries than streams and rivers due to the complications of bi-directional tidal hydrology, changing magnitude of tidal inundation and the hydraulic effects on the relatively weak swimming ability of small fish. Unfortunately, protocols for determining fish passage barriers in streams and rivers have limited utility in tidally influenced areas.  Further, we do not understand the extent to which fish behavior is modified by barriers or how to quantify impacts to restricting fish movements in intertidal and estuarine habitat.

Tidal water crossings structures, i.e., culverts, bridges, and tidegates, occur at low elevation in the watershed and therefore can impact fish access to more stream miles than crossings structures higher in the watershed. The process of correcting tidal fish passage barriers to provide full unimpeded passage is currently underway due to legal requirements, failing infrastructure and tidal habitat restoration efforts.  Therefore, correctly identifying tidally influenced sites needing remediation and using design considerations that account for movements of small fish, as well as adult fish, are important aspects of barrier correction. Once replaced, these structures can be in use for 30 – 100+ years before an opportunity to repair or replace comes around again.

Juvenile fish rearing in estuaries may move between habitats for a variety of reasons, including for example, finding food, escaping predation, and seeking specific environmental conditions related to salinity and temperature. Estuary habitat modification and tidal structures that result in increases in water velocity in tidal channels may hinder or prevent fish movements during portions of time throughout the tidal cycle, particularly for weaker swimming fish such as juvenile salmonids.   Frequently, these modifications will allow for fish movement in the seaward direction during outflow events but may prevent fish from maintaining their position in the channel or moving upstream to areas that represent preferred habitat.  Despite the logistical challenges associated with quantifying fish behavior and working in tidal environments, an understanding of when, where, and why fish move within the tidal cycle is key to developing technical guidance for estuarine barrier assessment and fish passage design.

Study Approach & Results

We were interested in understanding juvenile salmon movements in relatively undisturbed, tidally influenced tributaries where culverts, bridges, or tidegates could conceivably be placed in the future. In particular, we wanted to understand fish movement through the tidal cycle to inform the assessment of barriers in estuarine environments, as well as the design of crossing structures and estuary restoration projects that seek to improve tidal connectivity. To do this, we observed juvenile fish behavior in relatively small tidal channels at four estuarine study sites within two major river deltas in Puget Sound, Washington. 

We tagged juvenile Chinook and coho salmon with passive integrated transponder (PIT) tags that allow identification of individual fish and used multiple antennas to detect fish movement within tidal channels throughout a series of tide cycles.  One site was in the Nisqually River delta and the other three were in the Skokomish River delta.  We were interested in studying fish movement in relatively undisturbed tidal channels that experience tidally influenced flows on a regular basis.    We deployed antenna arrays at three sites in 2019 and one site in 2020 and 2021 to assess the frequency and directionality of short movements of juvenile salmonids (20 – 75 feet), to best align study results with fish passage through a typical water crossing structure. By using cross-channel antenna arrays in series, fish detections could be categorized by movement direction.  We focused on the relationship of physical parameters (e.g., water depth and change in water depth) associated with initiation and direction of movement for juvenile Chinook and coho salmon to inform the following questions:

Under what conditions during the tidal cycle are juvenile salmon moving through estuarine habitats?  What factors initiate fish movement?

We found that fish directional movements, defined here as a movement over a minimum distance in a set period of time, for Chinook and coho juveniles occurred over all phases of the tide cycles, from low tide to high slack tide and back to low tide. Fish movements most often occurred when water depth in the tidal channel was between 2-5 feet. Fish movements also occurred at very low tide when the channel water depth was minimal (i.e., residual riverine flow prior to arrival of tidal flow), primarily in the seaward direction, although some movements were against the flow.

We observed a notable increase in fish movements associated with a change in the direction of flow during incoming tide as tidal hydrology “arrived” at the study site. In addition, fish moved relatively frequently and in both directions on low amplitude tides (i.e., lower high tide and higher low tide of the mixed semi-diurnal tide series).  On higher amplitude tides, fish movements were most frequent at the peak rate of change of water depth, both on the flood and ebb tides.  Rate of change of water depth was utilized as a surrogate for velocity but was based solely on changes in depth with respect to time and therefore does not account for channel geometry or the riverine freshwater components of flow and velocity. Patterns of fish movements related to temperature and salinity were less clear.

We also observed fish movements throughout daytime and nighttime hours, with movement peaks at dawn, late afternoon, dusk, and early evening. 

The fish movement patterns we observed were likely not unique to our study sites within the delta.  In addition to presumed fish movements outside of our antenna range, our data suggested that other large scale movements are likely occurring throughout the estuary with fish travelling much longer distances. In 2019, we released tagged fish at two antenna locations in the Skokomish River delta, with 66 Chinook detected at different antenna arrays than the release site, indicating cross-delta movements of ≥0.8 km.

Do fish move with or against tidal hydrology throughout the tide cycle?

We detected fish movements with and against tide direction at all periods of the tide cycle, from low tide to high slack tide and back to low tide. The majority of fish movements across all sites were ≤5 minutes in duration, although duration of movements against the tide direction were typically longer than movements with the tide.  We found that on the incoming tide (hereafter flood tide), Chinook & coho movements were nearly equal in the same and opposite direction of the tide, while on the outgoing tide (hereafter ebb tide), Chinook movements were predominantly seaward (62%), while 57.0% coho movements were landward.

During low slack tide, fish movements were observed in both directions though predominantly seaward (i.e., in the same direction as the residual riverine flow).  Most fish movements at low tide were by Chinook (77.2%), with few movements by coho.

Movements at high slack tide were approximately equal in direction for both Chinook and coho; however, decreases in tag detection rates at higher tidal elevations and salinities may have resulted in an underestimate of fish observations at the highest tides. 

Interestingly, we found movements both with and against the tide direction, suggesting that tidal hydrology at our sites did not overwhelm fish swimming abilities, and that low velocity travel corridors, habitat (logs, vegetation) and/or channel complexity features that provided low velocity resting areas were likely available in the natural tidal channels of our study sites.

How might water crossing structures affect the ability of fish to volitionally access tidal habitats and how does this vary between Chinook and coho salmon?

It is generally understood that while some Chinook and coho in Puget Sound remain in estuarine habitats for prolonged time periods, most Chinook likely move to marine waters within a few months of entering the estuarine waters, whereas coho typically remain in freshwater or low salinity estuarine habitats until the following spring (Healey 1991, Quinn 2005, Koski 2009).  Both estuary residence time and frequency of fish movements are likely important drivers of the rate of encounter between estuarine fish and tidal water crossing structures. While the overall number of tag detections for Chinook was high (likely due to greater numbers of tagged Chinook released), the number of movements/fish were higher for coho than Chinook overall. Chinook were detected for a shorter time after release than coho (up to 10 weeks and 17 weeks respectively).    

In our study, 47.3% of Chinook recorded only a single directional movement on our antenna, while  19% of coho had only a single movement. In contrast, 53.8% of the tagged coho had more than four movements/fish, compared to 15% of Chinook. For Chinook with more than one movement, 53% were in the seaward direction, while 47% of coho movements were in the seaward direction.  Our observations of 9.2 movements/coho as compared to 3.9 movements/Chinook, along with the frequency of movement in the seaward direction for Chinook and landward for coho is consistent with the idea that Chinook were moving to the marine nearshore, while coho were rearing for extended periods at the time and location of our study.  However, we did not PIT tag Chinook less than 55mm fork length due to tag size limitations, so the direction and frequency of Chinook movements may have been affected by use of larger size fish at the upper range of estuarine residence in Puget Sound deltas (Beamer et al. 2005, Ellings and Hodgson 2007).  In other words, PIT-tagged Chinook in this study may be stronger swimmers than the population of untagged juvenile Chinook using estuaries in winter and early spring since smaller fish are thought to reside for longer periods of time in the estuary and also have generally weaker swimming abilities.  Smaller fish that reside in the estuary for longer time periods (e.g., coho subyearlings) may also encounter tidal water crossing structures at a higher rate than fish that leave the estuary after a short residence (e.g., juvenile chum salmon).

Recommendations from this study

We observed substantial use of and movement within estuarine habitats by juvenile salmon, suggesting that fish passage barrier assessment standards in estuaries would be improved by including criteria that reflect the swimming ability and frequent movements of small salmonids as observed in this study.  Equally, design approaches to passage in tidal areas that rely on design approaches developed for fish passage in streams and rivers that were initially intended to address adult fish are clearly missing an important component of the use of such structures by fish.  We documented juvenile salmonid directional movements throughout the tide cycle, both with and against the tidal flow and at very low and high water levels.  Movements during high tides may have been facilitated by an increasing availability of additional pathways as the tide rises, such as multiple channels, flooding of higher order channels and the flooded tidal marsh vegetation.

Providing both fish access to and connectivity with estuarine habitats that are only available at high tides (e.g., higher order tidal channels, marsh surface) are also key elements of fish rearing in tidal environments and are thus important considerations in the design of fish passage structures as well as efforts to restore and protect habitat processes in estuaries.

Partners & Roles

  • Project Lead - Doris Small (retired) & Padraic Smith, PE, Washington Department of Fish & Wildlife
  • WDFW - Tim Quinn, Ilai Keren
  • ESA - Paul Schlenger
  • Nisqually Tribe - Chris Ellings & Sayre Hodgson
  • Skokomish Tribe - Lisa Belliveau, Anthony Battista, Joseph Pavel, Alex Gouley
  • Port Gamble/S'Klallam - Hans Daubenberger & Emily Bishop
  • Skagit River System Cooperative - Eric Beamer
  • Blue Coast Engineering - Jessica Cote & Traci Sanderson

Final Report and Data

  • This project was funded by ESRP Learning Project and the Washington Department of Transportation research program
  • The ESRP project was contracted in 2016 and a final report completed in March 2024.
  • Progress reports and associated PRISM contract and documents are available online PRISM project 16-2282.
  • Final report with study results is available here (to be added).
  • Final project presentation recording will be scheduled in April or May 2024 and available here (to be added).


  • Greene et al. 2017 Effects of intertidal water crossing structures on estuarine fish and their habitat: a literature review and synthesis (to be uploaded)
  • WDFW Fish Passage Inventory, Assessment and Prioritization Manual 2019 (link to be added)
  • WDFW Water Crossing Design Guidelines 2013 (link to be added)
  • WDFW water crossing barrier mapping (link to be added)