Do Fish Ladders Work?

Author: Henry Hershey

Editor: Patrick Cooney

What is a fishway?

The purpose of a fish ladder, or fishway, is to help migrating fish navigate past dams that would otherwise block access to spawning habitat. Although they vary in design, the idea behind fishways is to use clever engineering to dissipate the potential energy of water stored behind a dam via a series of stepwise pools that are navigable by fish. Essentially, a spillway is built to allow some of the water to bypass the dam, giving the fish a way up and over. If the velocity or turbulence of the water coming down the fish ladder is too high, fish won’t be able to swim up the ladder. But, with testing and simulation, the fishway can be configured so that fish can “climb” up and continue on their migration upstream of the dam.

Originally, fishways were designed to accommodate culturally and economically important species, like salmon or shad. The fishways were, in general, very effective at restoring runs of these species to their native range that otherwise would have been blocked by newly constructed dams.

Designing modern fishways

Recently, there has been a global shift in natural resource management and conservation toward focusing on the value of biodiversity. Accordingly, fishways are being designed and modified to accommodate more and more fish species. However, a major holdup towards making additional progress has been a lack of consensus on how fishways should be evaluated for successful passage of fish.

On could argue that the biggest question in fish passage research in the last 10 years is:

“How do we know that fishways are working?”

A pool-and-weir fishway at Bonneville Dam in Oregon, USA.

Analyzing Fishways

This year, I published a paper addressing this question. It is a meta-analysis that synthesized the results of 60 different fish passage studies that in total evaluated 75 unique fishways of various designs.

I grouped the fishways into 5 design types: nature like, denil, pool-and-weir, vertical slots, and automatically operated locks/lifts.

Nature like fishways are ones that simulate natural riverine conditions with natural substrate. The most common type is a rock ramp that uses boulders to dissipate the energy of the flowing water.

Denil, pool-and-weir, and vertical slot fishways are all similar in that they use a series of baffles or weirs made of steel and concrete to create a chain of pools that slow down the water for fish.

Finally, locks and lifts are essentially fish elevators that use mechanical energy to move fish upstream.

A nature-like fishway constructed at Lock and Dam 1 on the Cape Fear River in North Carolina, USA. (Tetra Tech)

Passage efficiency

Despite the incredible variation in the size and design of fishways, the analyses revealed that there were no significant differences in passage efficiency between any of the fishway types. Unexpectedly, the slope and height of the fishway were not significant predictors of passage efficiency. One noted difference was that nature-like fishways were the worst at attracting fish to the entrance. Overall, fishways were only about 60% efficient on average.

Design or Species Driven Success?

I dove deeper into the data to try and figure out what was accounting for variation in fishway efficiency. The first thing I looked at was whether certain groups of fishes were just better at passing fishways than others. I found that, as expected, rheophylic species (ones that prefer flowing water habitats) were better than others. Also, large diadromous species (e.g. salmonids) outperformed most of the other groups. Species that prefer lentic (non-flowing) habitats, and non-migratory species performed the poorest. Therefore, the “success” of a fishway may matter a lot more on which species it is designed for rather than how it was designed.

Impact of Non-Standardized Methods of Fishway Evaluation

An important finding of my study was that one of the most influential factors on fishway efficiency was how the evaluation was performed. All of the studies I used in these analyses had the following method in common: capture fish, tag them, and then count how many of the tagged individuals made it past the fishway. However, the location of fish capture was highly influential on the result!

Studies that tested the fishway with fish captured inside or upstream of the fishway reported better efficiency than studies that used “naïve” fish (ones that supposedly never experienced the fishway previously). This highlights the most pervasive problem in fishway evaluation science today. Very few studies conform to standardized methodologies recommended by experts in the field. Thus, if 100 researchers performed an evaluation in their own way at the same structure with the same species, they would all most likely get different answers about efficiency of the fishway.

Despite the lack of consistency in evaluating the efficiency of fishways, progress has been made over time in improving the evaluations. Reviews of fishway evaluations in 2012 found similar problems with unstandardized methodologies; however, since then, more researchers appear to have adopted best practices for evaluations. Hopefully this trend continues and we can continue to make progress reconnecting the rivers of the world.

Continued Reading

Please take a look at the paper for more detail on results, the analyses performed, and especially regarding the process on how I selected papers to be included for the analyses.

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