Mismatch between fishway operation and timing of fish movements: a risk for cascading effects in partial migration systems

• Published in: Ecology and evolution Vol. 6; no. 8; pp. 2414 - 2425
• Main Authors: Leeuwen, Casper H. A., Museth, Jon, Sandlund, Odd T., Qvenild, Tore, Vøllestad, L. Asbjørn
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01.04.2016; John Wiley and Sons Inc
• Subjects: Autumn; Biological evolution; Brown trout (Salmo trutta); Downstream effects; eco‐evolutionary consequences; European grayling (Thymallus thymallus); Fish; Fish barriers; Fish ladders; Fish migration; fish passage functionality; Fish populations; Fragmentation; Habitat fragmentation; Hypotheses; Ladders; life history strategy; Migration; Original Research; Residential density; River systems; Rivers; Salmo trutta; salmonids; Spawning; Spring; Spring (season); Thymallus thymallus; Trout; Weirs; ANE, Norway; salmonids; life history strategy; European grayling (Thymallus thymallus); fish passage functionality; eco‐evolutionary consequences; Brown trout (Salmo trutta)
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.1937

Habitat fragmentation is a growing problem worldwide. Particularly in river systems, numerous dams and weirs hamper the movement of a wide variety of species. With the aim to preserve connectivity for fish, many barriers in river systems are equipped with fishways (also called fish passages or fish ladders). However, few fishways provide full connectivity. Here we hypothesized that restricted seasonal opening times of fishways can importantly reduce their effectiveness by interfering with the timing of fish migration, for both spring‐ and autumn‐spawning species. We empirically tested our hypothesis, and discuss the possible eco‐evolutionary consequences of affected migration timing. We analyzed movements of two salmonid fishes, spring‐spawning European grayling (Thymallus thymallus) and autumn‐spawning brown trout (Salmo trutta), in Norway's two largest river systems. We compared their timing of upstream passage through four fishways collected over 28 years with the timing of fish movements in unfragmented river sections as monitored by radiotelemetry. Confirming our hypothesis, late opening of fishways delayed the migration of European grayling in spring, and early closure of fishways blocked migration for brown trout on their way to spawning locations during late autumn. We show in a theoretical framework how restricted opening times of fishways can induce shifts from migratory to resident behavior in potamodromous partial migration systems, and propose that this can induce density‐dependent effects among fish accumulating in lower regions of rivers. Hence, fragmentation may not only directly affect the migratory individuals in the population, but may also have effects that cascade downstream and alter circumstances for resident fish. Fishway functionality is inadequate if there is a mismatch between natural fish movements and fishway opening times in the same river system, with ecological and possibly evolutionary consequences for fish populations. Fishways with restricted seasonal operation times were found to affect upstream spawning migrations of both spring‐ and autumn‐spawning salmonid fish. We discuss possible effects of altered migration timing of fish in a theoretical framework, in which we propose that movement restrictions of migratory individuals in partial migration systems can indirectly affect conditions for resident fish in the same river system.