Decade-long data highlight medium-sized, fast-flowing cold-water streams as important recruitment habitats for European river and brook lampreys

European river lamprey Lampetra fluviatilis is a migratory (anadromous) species, spawning in freshwater rivers where it co-occurs with its non-migratory paired species, European brook lamprey L. planeri . Both species are listed under Annex II of the EU Habitats Directive, requiring habitat protection and sustainable management across their distribution ranges. Yet, both species have experienced declines and spatial drivers of larval recruitment remain poorly quantified. Lamprey larvae are known to occupy low-velocity depositional microhabitats, but how these requirements scale across river networks remains unclear. Here, we analysed size-structured populations of lamprey larvae across 177 sites in 32 Latvian rivers using data collected between 2014 and 2025. Higher proportions of smaller larvae, likely corresponding to the youngest age classes, were associated with river reaches of medium-size and small catchment area, moderate and high riverbed gradient and colder temperature. In contrast, larger individuals dominated low-gradient, warm river sections with large catchment area. These findings suggest that the observed longitudinal recruitment pattern may be driven by downstream larval dispersal across river networks and highlight small- and medium-sized, relatively fast-flowing cold-water reaches as important recruitment habitats. However, recruitment potential in these important habitats may be reduced by anthropogenic pressures, particularly migration barriers and channelization. Our findings highlight the importance of protecting and restoring small- and medium-sized streams and mitigating anthropogenic pressures to support the favourable conservation status of European river and brook lampreys.