Keywords

fish passage

Global

Barriers

Dams

fishway

Dams, culverts, and other water diversion/control structures are the most widespread human alteration to riverine habitat globally. While many of these structures often serve critical services (i.e., source of agricultural and municipal water, power generation, flood protection,and sensitive ecosystem protection), they cause changes to local ecology by impeding animal movement and altering the adjoining habitat and associated ecological processes (Nilsson et al., 2005; Liermann et al.,2007). The disruption of fish movement by barriers has contributed to the decline of both migratory and non-migratory fish populations around the world.

Providing fish passage at barriers is a challenging task that must balance competing site-specific biological, environmental, commercial,recreational, societal, and cultural interests. The most common mitigations for fish passage are barrier removal or fishway installation. fishways provide alternative routes around or through barriers that enable fish to move up- and down-stream, facilitating both migratory and nonmigratory movements. Many advances in fishway design have occurred in the last half-century, with most approaches prioritizing upstream movement of commercially important species like Pacific salmonids(Mallen-Cooper & Stuart, 2007). While somewhat effective at passing large, strong swimming, obligatory migrating adult salmonids, many conventional fishways create unnatural conditions that can impose both behavioral and physical challenges that impede passage of different species at varying life stages resulting in less robust energy reserves necessary for up- and down-stream movement. Recent research efforts have begun to focus on mitigating the potential hazards of downstream fish movement including passage through power generating turbines,elevated drops through spillways, or disorienting plunge pools.

Passage solutions for whole fish communities is not straight forward and can potentially lead to competing outcomes. For example, steps to maximize fish passage around barriers (or even to remove barriers all together) creates a “connectivity conundrum” where the desirable outcome of better connectivity within a river or between a river and receiving body of water could also encourage the spread of undesirable species (e.g., naturalized or invasive fishes) and fish disease(McLaughlin et al., 2013). Due to the multitude of fish species requiring some level of riverine movement and physical and hydrological conditions unique to each barrier, solutions must be approached in the context of local ecology. These challenges have led to new and innovative approaches to fish passage like the Whooshh Transport System (Garavelli et al., 2019), eel style ladders (Reinhardt & Hrodey, 2019), fish friendly turbines (Schwevers & Adam, 2020), and even turning to the recycling industry to inspire future selective fish passage tools to address the“connectivity conundrum” (Zielinski et al., 2020). This special topic offers a glimpse into the wide array of unique solutions being developed to enhance fish passage within and between rivers and lake or ocean environments. We summarize key findings from contributions to this topic below.

Pre-cast cone fishways evolved from the need for fish passage solutions at remote sites in Australia, and are used to provide passage for small-bodied fishes in tidal and coastal lowland sites. Cone fishways incorporate high turbulence and velocities in the center of the channel while maintaining low turbulence and velocities on the edges, where small-bodied fishes can rest while ascending. Evaluated across three field sites where the cone fishway replaced or was complimentary to a conventional fishway, cone fishways were shown to provide upstream passage to a wide array of West Australian species including fish as small as 40 mm long (Stuart & Marsden, 2020).

Moser et al. (2020) investigated the ability of small refuges to improve retention and upstream passage of adult Pacific lamprey Entosphenus tridentatus. The refuges were small aluminum boxes (with cobble substrate inside) placed within a channel adjacent to a conventional fishway designed to pass Pacific salmonids. Most refuge users stayed in place for more than 12 hr and were frequently individuals that had advanced maturation. Low-velocity, dark refuges hold promise for improving fish retention in fishways, particularly for small-bodied or nocturnal species.

While much fish passage research is aimed at upstream passage,Castro-Santos et al. (2020) examined effects of plunge pool design on injury and survival of downstream migrating juvenile blueback herring Alosa aestivalis, which are susceptible to greater mortality than salmonids when subjected to conditions typically found in plunge pools. The results of their laboratory study demonstrated that existing plunge pool designs may offer less than optimal protection for downstream migrating clupeids and that mortalities resulting from poor passage conditions could occur over extended periods of time.

A bi-national collaboration between Canada and the United States focused on the protection of downstream migrating American Eel Anguilla rostrata through hydropower dams in the St. Lawrence River(Pratt et al., 2021). Through laboratory evaluations of electricity, flow,sound, and electromagnetic fields to guide downstream eel movement,researchers found only low frequency sound has any promise for silver-stage eel guidance. Additional field work led to the development of sonar identification of downstream migrating eels using a machine learning platform. The lessons learned from this research will help inform the development of future guidance tools for downstream silver-stage eel passage.

Finally, Baumgartner et al. (2020) outlined the development of a fish passage program for the Lower Mekong River, a region facing increased irrigation infrastructure development that impairs fish passage. The case study highlights the need for local community engagement and fish passage solutions set in the context of local ecology. The fish passage program resulted in construction of 14 and planning of up to 26 fishways in Lao People’s Democratic Republic, with many using designs similar to those evaluated by Stuart and Marsden (2020). Overall, the case study summarizes the pivotal elements to local knowledge development and transfer as well as challenges in international research.

We thank the contributors to this special topic for the interesting case studies from around the world. The studies published in this special issue illustrate the unique nature of localized fish passage solutions. We hope that this special topic can inspire future developments for bi-directional fish passage solutions for whole fish communities.