Upstream distribution and dam passage of native Sea Lamprey (Petromyzon marinus) following improved basin connectivity in the Penobscot River, Maine

Sea Lamprey are a native species in Maine’s rivers and have been in steep decline due to habitat fragmentation. Recent river connectivity improvements in the Penobscot River increased habitat access, but the effects of these changes are still being assessed. We used eDNA and radio telemetry to assess the current habitat use and dam passage for Sea Lamprey in the Penobscot River. We then compared these results with previous studies and historical data to quantify changes in Sea Lamprey distribution through time. Our results show that the distribution of Sea Lamprey decreased by 84% between the 1800’s and 2007. Restoration efforts have increased the species’ distribution by 45% between 2007 and 2023, but the species remains limited to only 23% of its historical range. Extant dams continue to prevent the expansion of Sea Lamprey. Many Penobscot River dams lack fish bypasses and passage performance is highly variable among dams with bypasses (0%-96% overall passage success between 2020 and 2023). Improved passage at dams remains an important direction for restoration of these native fish.