Dam, That’s a Wicked Problem

In the early 1990s invasive Sea Lamprey Petromyzon marinus were continuing to wreak havoc on native fish populations of the Laurentian Great Lakes. Enter low-head barrier dams; a no-chemical management solution to keep the vampric lamprey out of spawning tributaries in the Great Lakes. However, such barriers indiscriminately affect non-jumping species and in the late 1990s the level of impact on non-target resident fish species and communities was unknown. In one of the first observational studies examining the impact of low-head dams on non-target fish species Porto et al. (1999) found movement restrictions in some species and an altered species assemblage upstream of the barrier. This research has as much relevance today as it did in 1999 as our societies struggle to balance nature’s and human’s need for free-flowing, connected rivers and invasive species management.

Main review:

Dams are ubiquitous across our waterscapes in North America and they provide essential services such as irrigation, hydropower, and flood control. We’ve also known for hundreds of years that dams act as barriers to fish movement and can have a negative impact on migratory fish populations. In England in the 12th century a King’s Gap was required to allow passage through all water crossing structures to ensure migrating salmon could reach their spawning grounds (Montgomery 2003). Over the intervening centuries we have learned a great deal about how to mitigate the impact of barriers on river health and fish populations and have also found they have an additional beneficial use in invasive species control. Dams such as low-head barriers prevent Sea Lamprey, an invasive species that drastically impacted the fish communities of the Great Lakes, from migrating into tributaries to spawn.  In the early 1990s barriers seemed like an ideal non-chemical solution to prevent the spread and impact of Sea Lamprey. As Porto et al. (1999) explain “It is thought that the lower crest height [of a low-head barrier] will at least allow the passage of fishes with good jumping abilities and that flow and habitat alterations to the stream will be minor.” However, the authors noted that at the time our understanding of the barrier’s impact on non-target fishes was limited.

In order to improve our understanding of the impact of low-head barrier dams on resident fish populations Porto et al. (1999) assessed fish and fish habitat characteristics above and below real and hypothetical barriers in two paired stream systems (2 barrier streams and 2 hypothetical barrier reference streams) in tributaries of Lake Ontario.

The study evaluated fish movement across the real and hypothetical barriers using a mark-recapture model; looking across seasons they asked if fish were more likely to move within a stream segment in the absence of a hypothetical barrier. The authors also assessed the size of fish moving across the real and hypothetical barriers – hypothesizing that only larger individuals of a species would cross a real barrier. The authors reported a very low recapture rate (5%), so analyses were performed on aggregated data between stream pairs and only upstream movement was considered due to low observed downstream movement.

Porto et al. (1999) discovered that low-head barriers were likely impacting resident fish species. Significantly more fish moved across hypothetical barriers in reference streams than fish in barrier streams, and the impact was greater in fall than spring. Interestingly, while few species moved across the hypothetical barrier (7 species), in barrier streams the movement of those species was significantly less. The mean size of fish crossing real barriers was also greater than conspecifics crossing a hypothetical barrier in the reference streams. Finally, in all seasons species richness above the barrier was lower than below the barrier compared to reference streams.  The authors acknowledge that manipulative experiments would provide increased certainty of the results and rule out the influence of interspecific differences between stream types and fish communities in the paired stream design.

This research shone a light on the trade-offs of using barriers as an invasive species control tool; low head barriers will prevent Sea Lamprey movement into spawning tributaries but will likely also reduce species richness and disrupt the size distribution of resident fish populations above the barrier.

It has been nearly twenty years since the Porto et al. (1999) paper was published and lead author Louise Porto has been studying the impact of dams, large and small, ever since. She notes that in today’s world “there are no easy answers when trying to balance the trade-offs of competing interests for use of these resources” and that “the impacts on smaller resident fish are often overlooked…”

In addition to invasive species control and resident and migratory fish concerns, today’s resource managers are also trying to consider environmental flows and cumulative effects. Fishwerks (https://greatlakesconnectivity.org), a mapping and barrier removal prioritization tool, illustrates the extensive challenge resource managers in the Great Lakes basin face as trade-offs continue to be made.

Co-author Robert McLaughlin commented, when asked about how the 1999 study influenced future barrier work, that “At the time, the findings really forced us to acknowledge the trade-offs associated with using even small, in-stream barriers to control Sea Lamprey. It also stimulated a lot of research on Sea Lamprey trapping, because barriers would not be needed if we could increase trapping efficiency sufficiently. So far, that increase has proven elusive. We are now starting to examine ways of selectively passing desired fishes, while blocking invasive species.”

By Rebecca Dolson-Edge


Montgomery, D. R. 2003. King of fish: the thousand-year run of salmon. Boulder, Colorado, Westview Press. ISBN 0813341477.

Porto, L. M., R. L. McLaughlin, and D. L. G. Noakes. 1999. Low-head barrier dams restrict the movement of fishes in two Lake Ontario streams. North American Journal of Fisheries Management 19:1028–1036.

Video credit:David Lawrie

Cover photo credit: Madeleine Lyttle/USFWS

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