Cumulative impacts of habitat fragmentation and the environmental factors affecting upstream migration in threatened sea lamprey Petromyzon marinus.

Abstract

1. Riverine ecosystems are often fragmented due to man-made structures, such as weirs. By impeding access to upstream spawning sites, the effects of these structures on anadromous species can be severe, ultimately leading to population declines. 2. This study focused on the freshwater spawning migration of sea lamprey Petromyzon marinus, a species threatened by habitat fragmentation across its native range. To quantify the cumulative impacts of multiple weirs on upstream-migrating adults, and explore the environmental factors affecting migratory movements, passive acoustic telemetry was applied to 56 individuals during their spawning migration in the heavily fragmented River Severn basin, UK. 3. While 89% of tagged sea lamprey passed the first weir upstream of the release site on the main river, only 4% passed the fifth. For 85% of migrants, the upstream extent of migration was immediately downstream of a weir. Individuals that passed weirs upstream of the release site (n = 50) took 21.6 ± 2.8 days to reach their most upstream location, experiencing cumulative passage times at weirs of 15.7 ± 2.8 days; these delays constituted a median of 84% of total upstream movement times. 4. Multistate models revealed that weir passage rates by sea lamprey in tidal and non-tidal areas increased significantly when downstream river level and discharge were elevated. Upstream-to-downstream changes in direction were frequent downstream of weirs, but rare in unobstructed river sections. 5. The results provided evidence for a cumulative effect of multiple weirs on sea lamprey movements, substantially delaying upstream migrants and limiting their spawning to atypical habitat; they also demonstrated the crucial roles of tides and elevated discharge events in enabling weir passage. While the Severn estuary features conservation designations for sea lamprey, this study reveals that barriers are inhibiting upstream migration, an issue which should be addressed to assist their conservation.