The river, once a continuous highway for fish, is now fragmented by human development. Discover how engineers and biologists are collaborating to reconnect our fractured rivers.
Imagine you are a salmon, driven by instinct to return to the very stream where you were born to spawn the next generation. Your journey is fraught with predators and powerful currents, but the most formidable challenge is not natural. It is a massive concrete wall blocking your path—a dam. For fish worldwide, this scenario is a daily reality. Fortunately, engineers and biologists have collaborated to create a solution: the fishway. But how do we know if these structures truly work? This is the critical science of fishway attraction and passage efficiency, a field where reliable data is the key to reconnecting our fractured rivers.
At its core, fish passage efficiency is a deceptively simple metric. It is the percentage of fish that successfully navigate a fishway out of those that attempt to do so 1 . However, this simple number hides a world of complexity. Scientists break down a fish's journey into critical stages, each with its own efficiency measure:
The proportion of approaching fish that actually cross the threshold and enter the fishway 4 .
A failure at any of these stages means the fishway has failed. A recent meta-analysis highlighted this variability, showing that passage efficiency for cyprinid fish (like carp and minnows) can range from a poor 38% for nature-like designs to a much more effective 68% for vertical slot fishways 8 .
One of the world's most extensive and successful fish passage systems can be found on the Columbia and Snake rivers in the Pacific Northwest of the United States. For over 15 years, scientists have monitored the passage of nearly 27,000 radio-tagged salmon and steelhead across eight major dams 4 .
The results are impressive. The average dam passage efficiency—from arriving in the tailrace to exiting upstream—was found to be 96.6% 4 . These are among the highest passage rates ever recorded for any migratory fish species.
This success is attributed to a sustained, adaptive management program. The fishways are primarily pool-and-weir designs, which create a series of stepped resting pools separated by submerged weirs with small openings 4 . This design is exceptionally well-matched to the powerful swimming capabilities and determined upstream drive (philopatry) of adult Pacific salmon.
Average Passage Efficiency
Columbia River Dams
Radio-tagged Fish Monitored
Years of Research
Stepped pools allow fish to navigate gradually upward while providing resting areas
While the Columbia River data is powerful, detailed studies at single structures provide unparalleled insight into the factors that make a fishway work. Let's examine a crucial experiment conducted at the Songxin vertical-slot fishway on the Heishuihe River in China 6 .
The researchers employed a sophisticated marking and tracking system to follow individual fish on their journey.
First, the team established 14 sampling points along the river downstream of the dam to understand the local fish community. They identified 19 species present 6 .
Selected fish were fitted with Passive Integrated Transponder (PIT) tags—tiny electronic chips that hold a unique identification code 6 .
Antennae were installed at the fishway's entrance and exit. Whenever a tagged fish swam within range of an antenna, its unique code, the time, and its location were recorded 6 .
Simultaneously, the team continuously monitored environmental conditions like water temperature, flow rate, and time of day 6 .
The study yielded rich, quantitative data. The researchers found that only 15.7% of the fish in the downstream river were attracted to the fishway entrance. However, of those that entered, a much more promising 40.4% successfully passed through the entire structure 6 . This stark difference between attraction and passage efficiency highlights where the biggest problems often lie: getting fish to find and enter the structure in the first place.
By correlating the tagging data with environmental conditions, the team could pinpoint the optimal conditions for migration:
| Metric | Optimal Condition for Attraction | Optimal Condition for Passage |
|---|---|---|
| Flow Rate | 6–7 m³/s | 0–0.5 m³/s |
| Water Temperature | 19–20 °C | 17–20 °C |
| Time of Day | Nighttime | Nighttime |
Source: Data adapted from 6
The study also revealed that May was the peak migration period and that fish moved more efficiently at night 6 . Using a statistical model, the scientists identified that diurnal rhythms, release location, temperature, and flow rate were the primary factors hindering passage, while larger body size and higher water levels promoted successful passage 6 .
Modern fishway science relies on a suite of advanced tools to gather reliable data without interfering with the fish.
| Technology | Function | Key Advantage |
|---|---|---|
| PIT Telemetry | Passive detection of tagged fish at specific points. | High read rates, low cost, small tag size allows for tracking diverse species 8 . |
| Radio Telemetry | Active tracking of fish signals over long distances. | Allows researchers to follow fish movements in real-time through large areas 4 . |
| Acoustic Telemetry | Underwater tracking using sound waves. | Ideal for large, deep, or murky river systems where radio waves don't propagate well 4 . |
| Video Monitoring | Direct visual observation of fish behavior. | Provides rich data on how fish interact with specific structures and flow conditions 2 . |
| Computational Fluid Dynamics (CFD) | 3D computer simulation of water flow. | Allows engineers to test and optimize fishway designs virtually before construction, saving time and resources 3 5 . |
Different telemetry methods suit different research needs and environments.
Radio
Acoustic
PIT
Advanced computational tools help predict and optimize fishway performance.
CFD
Statistical Models
Video Analysis
The science of fish passage is constantly evolving. Researchers are now using tools like Cox proportional hazards regression models to predict the likelihood of a fish successfully passing based on a combination of biological and environmental factors 6 . This allows for more proactive management of fishways.
Furthermore, the field is moving beyond a one-size-fits-all approach. The Columbia River system, while brilliant for salmon, is not a universal solution. As one study noted, "There are abundant examples of 'salmonid style' fishways that are ineffective for other fish communities" 4 . The future lies in species-specific and site-specific designs. For instance, long fishways require more resting pools to prevent exhaustion, and structures must be tailored to the swimming abilities and behaviors of local species, not just charismatic migratory salmon .
| Fish Family | Most Effective Fishway Type | Documented Passage Efficiency | Key Considerations |
|---|---|---|---|
| Salmonidae (Salmon, Steelhead) | Pool-and-Weir / Vertical Slot | Up to 96.6% (Columbia River) 4 | High swimming power, strong upstream drive. Designs can focus on managing high-energy flows. |
| Cyprinidae (Carps, Minnows) | Vertical Slot | 68.4% (Average) 8 | A diverse family with varying abilities. Efficient passage is often linked to spawning motivation 8 . |
| Schizothorax (A Mountain Fish) | Low-Velocity Vertical Slot | 13% (in one long fishway), with sections up to 100% | Requires ample resting areas. Passage is negatively affected by obstructions like trash racks . |
Future fishways will be tailored to the specific swimming capabilities, behaviors, and migration patterns of local fish species rather than using a one-size-fits-all approach.
Continuous monitoring and data collection will allow for real-time adjustments to fishway operations based on environmental conditions and fish behavior.
The ongoing work to improve the reliability of fishway efficiency estimates is more than an academic exercise. It is a critical endeavor to mitigate human impact on aquatic ecosystems, one successful fish passage at a time. By combining rigorous science, adaptive engineering, and a deep understanding of fish biology, we can ensure that our rivers remain vibrant, connected lifelines for all the creatures that depend on them.
References will be added here in the final publication.