Light at the End of the Culvert: Understanding Freshwater Fish Response to Light

Light plays a governing role in the life of most freshwater fish, influencing behaviours such as time of peak activity, foraging, predator avoidance, collective movement and migration. These behaviours are intrinsically linked to daily light cycles, exposure to which is of significant ecological importance. However, anthropogenic activity is increasingly impacting natural light regimes. Artificial light at night (e.g. from
streetlights, domestic buildings, industry and bridges) generates unnatural levels of illumination in aquatic
environments when it should be dark, and a high density of engineered river infrastructure (e.g. culverts that divert rivers
under rail or road crossings) limits natural light penetration, creating patches of darkness during the day. In both instances
abrupt, unnatural transitions in illumination are created over large spatial scales, with potential to influence behaviours of
ecological importance for fish. Despite this, current evidence on the behavioural response of fish to light, and especially
abrupt light transitions, is limited. Such information is needed so impacts of anthropogenic alterations to natural light
regimes can be understood and mitigated. This is of importance because human activity has resulted in freshwater
ecosystems being the most degraded of all environments, and consequently, freshwater fish are one of the most threatened
taxonomic groups on the planet.