Decoding the serotonin-alcohol crosstalk: the role of central serotonergic dysregulation in alcohol use disorder.

Freshwater ecosystems are among the most threatened habitats globally and continue to degrade rapidly (Albert et al., ). Rivers and lakes provide essential ecosystem services including water supply, food, recreation and hydropower, but exploitation has led to widespread physical modification through dredging, channelisation, bank alteration and fragmentation by artificial barriers (Dudgeon, ; Ekka et al., ). Artificial barriers are man‐made structures which disrupt the natural abiotic and biotic conditions. Estimates state that half of all rivers are now fragmented, with free‐flowing systems largely confined to remote regions (Grill et al., ). Thus, the removal of artificial barriers to restore rivers has gained traction over the last three decades, particularly across Europe and North America (Dolan et al., ; Mouchlianitis, ). In Europe, weirs are the most common barrier type and often are the initial focus of barrier removal programmes, due to nonessential uses, being longstanding structures that pose a threat of collapse and their ecological impacts (AMBER Consortium, ; Mouchlianitis, ). The profile of weirs—including height, slope gradient, construction material and crest design—can vary significantly, each differentially influencing flow regimes, sedimentation dynamics and passage potential by biota, especially fish (Shih et al., ). Broad‐crested vertical weirs are one of the most common designs across Europe, where water flows over a flat, wide surface (Badr & Mowla, ). The v‐notch weir is a more modern design which is particularly beneficial for improving velocity and turbulence in low flow conditions and has rapidly grown in prevalence since the 20th century (Li et al., ; Pospísilík & Zachoval, ). While these designs are used for manipulating flow, they can have unintentional and drastically different impacts on fish passage (Baki & Azimi, ). Successful navigation of these structures is highly dependent on the swimming capabilities, energy reserves and body size of fish (Egger et al., ; Shiau et al., ). However, little attention has been paid to the differential effects of barrier types on crossing behaviours, particularly considering keystone species and wider ecological contexts. Migratory freshwater fish have declined by 76% since 1970, with Atlantic salmon () among the most affected species (Deinet et al., ; van Rijssel et al., ). In freshwaters, salmon are threatened by climate change, predation, disease and fragmentation (Dudgeon, ; Smialek et al., ). Targeted efforts to protect populations, such as habitat restoration and stocking initiatives, have been shown to successfully support reproduction and increase population numbers (Lennox et al., ). During the upstream migration to their natal spawning grounds, adult salmon typically need to overcome several artificial barriers, often leading to injury, energy loss, delays in migration and death (Algera et al., ; Smialek et al., ). For those that do make it to headwaters, barriers also alter sediment composition, reducing the suitability of the riverbed for spawning (Newton et al., ; Smialek et al., ). Barriers can further limit small‐scale movements, drastically reducing accessible habitats of river‐resident juveniles (Buddendorf et al., ; Müller et al., ). This restriction may intensify intra‐specific competition, reduce foraging success and affect density‐dependent survival (Chen et al., ; Fernandez et al., ). As a socio‐economically and ecologically invaluable species of the North Atlantic, the restrictions that in‐stream barriers impose on the migration of native Atlantic salmon are a primary motivation behind their removal (Consuegra et al., ).