Can navigation locks be used to help migratory fishes with poor swimming performance pass tidal barrages? A test with lampreys

- The performance of a navigation lock used as a novel fish pass was measured.
- PIT, radio and acoustic telemetry were used in replicated trials at a tidal barrage.
- Lock passage efficiency was high, time to passage low, but attraction efficiency poor.
- Potential of navigation locks as fishways, worldwide, is high.
- Future studies to improve the effectiveness and for different species are encouraged.

Worldwide, tidal barrages reduce aquatic habitat connectivity and limit fish movements, especially for diadromous migrating species. Providing fish passage at these structures is crucial but technically and economically challenging. We measured the performance of a navigation lock, employed as a single-chamber vertical-slot fish pass, at a tidal barrage by the mouth of a tributary of the River Ouse, NE England. In autumn 2015, 265 European river lamprey Lampetra fluviatilis were tagged with Passive Integrated Transponders (PITs) and released in 11 replicate trials (n = 157 in lock, n = 108 immediately below lock). Fifty nine lamprey were double tagged with PIT and acoustic tags and released in the Ouse, 350 m downstream of the barrage. The percentage of lamprey attempting to pass the upstream gates during PIT trials was moderate to high (55 and 93% for lamprey released below, and in the lock, respectively). Passage efficiency, for lamprey attempting to pass the upstream gates, was also high (average of 66% for releases in lock, 78% for releases below lock). Ninety percent of lamprey, released below the lock and attempting to migrate upstream passed the entire lock in <128 min following release. However, acoustic-tagged lamprey displayed poor attraction to the lock under prevailing high river-discharge conditions. Overall, 36% of acoustic-tagged lamprey attempted to pass the barrage, mostly comprising lamprey released at low tide (cf. high tide), generating a high passage efficiency of 76% (16/21). However, 15 individuals passed through the sluices and only one used the lock. Nevertheless, using navigation locks as fishways has the potential to provide increased access between estuarine and river habitats for a range of biota, including those with poor swimming performance, but effectiveness is dependent on managing water discharge routes. Future studies using different operating protocols, especially to improve fish attraction under different environmental conditions and for a range of species, are encouraged.