School's out, but not for fish that want to pass

Humans have been modifying waterways for millennia, with evidence of the Mesopotamians constructing irrigation channels for crops dating back to 6000 BCE (Rost, 2017) and with small dams in southern Jordan dating back to 7000 BCE (Fahlbusch, 2009). From these humble origins, the last century has seen a global intensification of river modifications, like dams (Zhang & Gu, 2023), resulting in only 23% of large rivers having uninterrupted flow between their source and the ocean (Grill et al., 2019). Predictably, this has been devastating for freshwater habitats and their inhabitants, such as fish (Keijzer et al., 2024).

In an effort to counteract the negative impacts of barriers in rivers and engineer our way out of a problem, fish passes have been heralded as a potential solution to impeded fish migration. Fish passes come in all shapes and sizes, from ladders and baffles to fish lifts, however, the designs are often biased towards salmonids in the Northern Hemisphere and unsuitable for other fish species (Birnie-Gauvin et al., 2019)—sometimes to a benefit, preventing the spread of exotic fish species (Franklin et al., 2021). Not only this, but even for target species, efficacy (of what can be an expensive intervention) can depend on fish size, motivation (Dodd et al., 2024) and specifications of pass design (Baker, 2014).

In this issue, we get a further glimpse into the complexities of fish passage design and how the behaviour of fish can influence their passage. Using the common galaxias (Galaxias maculatus) (Video 1), which is a small-bodied migratory species often used as indicator species for freshwater habitat connectivity in New Zealand, as well as a raceway with a section of high velocity water as a barrier, Crawford et al. (2025) tested whether schooling group behaviour would impact passage performance. What they found was that although both individuals and groups have the same level of overall passage success, groups were able to pass the barrier more quickly. Not only this, but the groups had a lower metabolic rate than individuals that made it past the barrier, demonstrating another benefit of schooling.

These results by Crawford et al. (2025) have important ramifications for improving the efficacy of fish passes, which is much needed, with practical considerations for accommodating group movements (e.g. constructing larger resting pools). Ultimately, to pass with flying colours, schooling is a must!

William Bernard Perry