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

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.

生境破碎化是全球范围内愈发严峻的问题。尤其在河流生态系统中，大量水坝与堰坝会阻碍诸多物种的移动。为保护鱼类的连通性，诸多河流系统中的障碍物均配备了鱼道（fishway，亦称鱼类通道或鱼梯）。然而，鲜有鱼道可实现完全的连通性。本研究假设，鱼道受限的季节性开放时长会通过干扰春季和秋季产卵鱼类的洄游时机，大幅降低其功效。我们通过实证检验了该假设，并探讨了洄游时机受干扰可能带来的生态-进化后果。我们分析了挪威两大河流系统中两种鲑科鱼类的移动情况：春季产卵的欧洲茴鱼（*Thymallus thymallus*）与秋季产卵的褐鳟（*Salmo trutta*）。我们将28年间监测到的4处鱼道的鱼类上行通过时机，与通过无线电遥测（radiotelemetry）技术记录的未受干扰河流段的鱼类移动时机进行了对比。验证结果证实了我们的假设：鱼道开放过迟会延迟春季欧洲茴鱼的洄游，而鱼道过早关闭则会阻碍秋季褐鳟前往产卵地的洄游。我们通过理论框架阐释了鱼道开放时长受限如何会诱导淡水洄游性部分迁移系统中的鱼类从洄游行为转变为留守行为，并提出这可能会在河流下游聚集的鱼类群体中引发密度制约效应。因此，生境破碎化不仅会直接影响种群中的洄游个体，还可能产生级联传导效应，进而改变留守鱼类的生存环境。若同一河流系统中鱼类的自然移动时机与鱼道开放时机不匹配，鱼道的功能便会失效，这会对鱼类种群产生生态乃至进化层面的影响。