Exclusion barriers

  A. Cumulative effect

  1. Figures below show the potential effectiveness of using 2 and 3 exclusion barriers to block non-native trout intrusions.

  2. Barrier placement would be at the downstream extent of intended isolation of cutthroat trout in headwaters [1].

  3. In the figures, intrusion beyond barriers is expressed as breakthroughs per 1000 barrier encounters by non-natives.

  4. It probably is unreasonable to expect a barrier to be 100% effective, including because of structural deterioration over time.

  5. By using serial barriers, however, such as 2 or 3, at that downstream extent, barrier effectiveness appears considerable.

  6. For example, 3 barriers with 90% effectiveness each, in combination means barrier intrusion of 1 per 1000 encounters.

  7. By comparison, only 1 barrier having 90% effectiveness can mean an estimated 100 intrusions per 1000 encounters.

Cutthroat | Decision | Nominee | Barriers | Example

Overview, cumulative effectiveness of barriers

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Closer view, cumulative effectiveness of barriers

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  B. Context​

  1. For context, exclusion barriers were 70-100% successful in 86% of 92 studies about blocking aquatic invasive species [2].

  2. High-flow events and bedload movements caused failures at barriers to protect cutthroat trout at a Colorado stream [3].

  3. Creating isolated habitat in headwaters has the advantage of no problem with intrusion from upstream.

  C. Evaluation

  1. ​With 2 or 3 barriers, backpack electrofishing could be used between them to monitor and evaluate barrier effectiveness.

  2. It would be less difficult and likely more successful than assessing everywhere upstream, and could be done regularly.

  3. It also could help distinguish between upstream removal process and barrier effectiveness.

  4. That is, non-native trout observed upstream but not between barriers could suggest incomplete upstream removal.

  5. Non-natives between barriers, however, would indicate barrier problems.

  D. Creative use

  1. As an example of options, creative use has been made of a culvert at Wildcat for barrier installation, shown below.

  2. The concrete floor and walls provide for stable, virtually unerodable anchoring of the device.

  3. Construction materials are 3 planks of 2X6-inch lumber, polyurethane-sealed and attached with 2 metal brackets.

  4. The concrete floor prevents a scoured pocket from forming just downstream from which trout could launch leaps.

  5. This application has only 1 barrier, but more could be installed along the roughly 30-ft length of the culvert.

  6. Spacers could be used between planks for narrow passages of water to minimize flow pressure against the barrier.

  7. Many tributaries in the study area enter culverts on their way to the main stem.

  8. This simple, durable approach for barrier installation may meet needs for separating trout populations at other tributaries.

  D. Opportunities

  1. Nine of the 12 tributaries at which temperatures and water quality have been measured in this study have culverts.

  2. Those culverts are at perennial, trout-bearing streams, and so are potential candidates for use in separating populations.

  3. In descending outfall elevation, culverts are at Snow Spur, Slate, Coal, Scotch, Wildcat, Tenderfoot, Priest, Taylor, and Stoner.

  4. Some culverts are rectangular concrete structures, for example, at Wildcat, as shown below.

  5. Others have concrete walls and ceiling, but rock bottoms, as at Priest, near its outfall, shown below.

  6. Still others use round, corrugated steep pipe, for example, as at a high elevation at Coal, also shown below.

  7. Any of those configurations likely would enable simpler barrier placement than in open stream channels.

  8. Being at roads, they would be relatively convenient for monitoring and maintaining.

  9. Being inside resilient structures, they would be less susceptible to damage from erosion than barriers in open channels.

Looking upstream and downstream at the Wildcat exclusion barrier

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Downstream_edited.jpg

Culvert with rock bottom at Priest

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Round, steel culvert at Coal

Coal High_edited.jpg

Cutthroat | Decision | Nominee | Barriers | Example