Recommendations to improve Pacific lamprey passage at fishways

 (extracted from Goodman and Reid 2017).


Pacific lamprey. Photo credit: US Fish and Wildlife Service.

Anthropogenic barriers that restrict upstream movement have been recognized as one of the primary threats to migratory lamprey populations worldwide. Improving connectivity to reaches above barriers returns lampreys to their historical range and increases available spawning and rearing habitat. It can also improve population resilience though access to the broader riverscape, including greater dispersion of
spawning and rearing areas, increased diversity of habitat, improved water quality conditions and typically cooler temperatures at higher elevations and varied aquatic species assemblages. The return of lampreys to upstream reaches also reestablishes their lost ecological roles, including as sediment engineers, bioturbators, filter
feeders, cyclers of primary productivity, prey and transporters of
marine nutrients. However, the majority of existing fishways, which are generally designed for other species, perform poorly for lamprey. Incorporation of the needs of lamprey into existing and future fishways will alleviate this threat and help ensure the long-term survival of these species. Lamprey behaviors, passage performance at design elements, and recommendations to improve upstream passage at barriers are directly applicable to other locations and other fishway types. Although not all lampreys share the climbing ability of Pacific lamprey, many of the issues addressed here can be applied to other at-risk migratory lampreys as well. 
Successful design of passage facilities for Pacific lamprey will ultimately include consideration of the behavior of the species, as well as its physical capabilities, and the needs of other species that use the fishway. Every facility will have its unique constraints and characteristics. Many fish passage facilities are already established along migration routes used by Pacific lamprey. Unfortunately, many are actually unintended barriers to lamprey passage. A crucial component of early design planning for retrofitting existing structures should be observation of lampreys attempting to use the existing facility: their timing, abundance, holding areas, and existing pathways (successful and unsuccessful), as well as an assessment of existing design or structural features that impede passage or may cause lamprey mortality. The following considerations apply to designing or modifying fishways to improve passage success and migration time for Pacific lamprey:
Behavioral considerations:

  • Preferred locomotion in low velocity water (< 0.6 m/s) is efficient anguilliform swimming, typically near the bottom where velocities are lowest. A roughened bottom thickens the boundary flow region and decreases velocities.
  • Preferred locomotion to pass high velocities or physical barriers is either 1) burst-attach swimming along a wall at, or just above, the surface, where water drag is substantially lower, or 2) subaerial climbing to pass around barriers.
  • In unavoidable high-velocity flows, when a smooth surface is available (e.g., along the bottom or on a wall), Pacific lamprey will use a burst-attach locomotion mode to move forward. However, this behavior is strenuous and only feasible for short distances. It is typically initiated at velocities over 0.6 m/s and becomes substantially less effective at higher velocities (2.4–3.0 m/s) and under turbulent conditions.
  • Upon passing a barrier, lampreys typically swim down into deeper, presumably calmer water.
  • Activity is primarily nocturnal, with lampreys seeking cover during the day. Therefore, they may not be seen during day, underrepresenting actual activity at a fishway.
  • When presented with a passage challenge, searching behavior allows lampreys to locate pathways without high attraction flows. We observed Pacific lamprey exhibiting considerable exploratory behavior, both locally within a pool and broadly in the fishway, often heading back downstream when a difficult challenge was encountered. Similar searching behavior has been noted at the large Columbia River dams when approaching obstacles to migration.

Preferred design features:

  • Midwater velocities under 0.6 m/s, preferably with near-bottom velocities approaching zero or under 0.1 m/s. Near-bottom velocities can be reduced by increasing surface roughness, while still allowing attachment surfaces.
  • Smooth, wetted surfaces, with shallow flow or spray zones that provide wet climbing attachment surfaces, allowing lampreys to climb around velocity barriers.
  • Large radius curves at corners (≥10 cm) to provide a continuous surface for reattaching during climbing without angular features.
  • Extended platform space beyond corners (horizontal or vertical) to avoid conflicting direction of body propulsion. Once an individual’s head has passed a curve, the propulsive force vector of the tail and body (upward) is no longer aligned with the head direction, making climbing more difficult. Continuing the horizontal platform above a climb out at about 90° to a full body length (ca. 60 cm) allows the lamprey to align its body before redirecting itself into the subsequent pool.
  • In proximity to high velocity areas, the edges of climbing surfaces should incorporate a low fence to keep lampreys from inadvertently falling off. Climbing often involves lateral searching as well as vertical climbing and the climb path is inherently irregular, with reattachment points off the center-line of movement.
  • In consideration to inherently different approaches that lampreys use to pass physical and velocity barriers, it is worth considering a dedicated pathway for lampreys that avoids the need to adapt existing fishway features designed for other fishes. Such a route can often be established at relatively low cost and has the benefit of specifically meeting the needs of lampreys. A separate pathway can also more easily incorporate lamprey-oriented monitoring equipment.
  • If a separate pathway is established for lampreys, it may be worthwhile to adapt the entry of the fishway to prevent entry by
    lamprey, avoiding wasted energetic cost expended in attempting to pass or explore the main fishway. This will ultimately reduce the time spent by lampreys searching for a more suitable pathway.

Design features to avoid:

  • Gaps and orifices, either in or along bottom of weir, which can entrain and trap lampreys with high velocity near-field flows. Gaps can also entrain debris which reduces aperture size and can entangle entrained lampreys, preventing them from passing downstream.
  • Sharp corners and edges that break suction, force lampreys off the climbing surface or prevent reattachment. This includes u-channels used for retaining weir boards and angular lips on walls.
  • Grates or porous surfaces that prevent suction attachment.
  • Gaps and orifices, either in or along bottom of weirs that produce high velocity pathways requiring extended burst-attach swimming. These can cause extended attachment times or approach the limits of burst-attach capabilities, while increasing energy expenditures.
  • High velocity corridors without alternative routes suitable for lamprey passage.
  • It is important to note that a single feature that prevents passage at any point along the pathway can render a fishway ineffective for lamprey. This can be as minor as a single 1 m reach exceeding 1 m/s without attachment points, a U-channel imbedded in the wall and bottom in higher velocity, or a 2 cm angle-iron lip at the top of a climbing surface.
  • Fishway designs should promote through-passage by lampreys by minimizing areas suitable for holding (e.g. complex cover, burrowing substrate and off-channel refuges). Adult Pacific lamprey have been observed holding and even over-summering in the Cape Horn fishway. Onsite fishway management staff also observe adult Pacific Lamprey emerging from fishway pools annually during late summer fishway dewatering and maintenance. Sometimes individuals do not appear until several days after lowering water levels and typically in locations where sand and gravels accumulate or where cracks in the concrete exist. Apparently, these individuals are over-summering to spawn the following spring, as they are observed outside of the typical spring migration period.

Experimental treatments tested for passage performance.

Experimental treatments tested for fish passage performance. Control (A), Gap (B), Bulkhead (C), Culvert (D), Tube (E), and Inverted U (F). The Gap treatment (B) is shown while the ladder water level has been dropped to show the modification that would otherwise be under water. Image scale varies by treatment. Weir boards are 130 cm (A, B, C and F), Culvert (D) is 16.5 cm diameter, and Tube (E) is 10 cm diameter.