Cumulative impacts of habitat fragmentation and the environmental factors affecting upstream migration in the threatened sea lamprey, <scp> <i>Petromyzon marinus</i> </scp>

River ecosystems are often fragmented by artificial structures, such as weirs. For anadromous species, these structures can impede access to upstream spawning sites and ultimately lead severe population declines. This study focused on the freshwater migration of sea lamprey, Petromyzon marinus, an species threatened habitat fragmentation across its native range. To quantify cumulative impacts multiple weirs upstream-migrating adults, explore environmental factors affecting migratory movements, passive acoustic telemetry was applied 56 individuals during their in heavily Severn basin, UK. While 89% tagged lamprey passed first weir release site main river, only 4% fifth weir. 85% migrants, extent immediately downstream a Individuals that (n = 50) took 21.6 ± 2.8 days reach most location, experiencing passage times at 15.7 days; delays constituted median 84% total movement times. Multistate models showed rates tidal non-tidal areas increased significantly when river level discharge were elevated. Upstream-to-downstream changes direction frequent weirs, but rare unobstructed sections. The results provided evidence for effect substantially delaying migrants limiting atypical habitat. also demonstrated crucial roles high tides elevated events enabling passage. Although Estuary features conservation designations this reveals barriers inhibiting migration, problem should be addressed assist conservation.