Providing refuges for adult Pacific lamprey Entosphenus tridentatus inside fishways

Abstract

Fishways at hydroelectric dams are unnatural environments that typically present fish with channels of uniform depth, variable water velocity, and lack of cover. Fish retention and overall passage success may be improved by providing rest and sheltering areas inside fishways. We examined adult Pacific lamprey Entosphenus tridentatus use of two specially designed fishway refuges at Bonneville Dam on the Columbia River (northwestern USA). These relatively small boxes (1.1 m × 0.4 m × 0.2 m) provided low-velocity, dark refuge from predation for Pacific lamprey, a largely nocturnal species. Refuge boxes were equipped with antennas to detect entry of lamprey tagged with passive integrated transponder (PIT) tags. We PIT tagged and released 3,247 lamprey, including 599 that were double-tagged with a PIT and a radio transmitter, over three study years (2012–2014). In each year, PIT-tagged lamprey detected at nearby fishway exits had used a refuge: 12% in 2012, 28% in 2013, and 36% in 2014. Median residence time of PIT-tagged lamprey in the refuges for each year was 20.3 h, 16.6 h, and 13.1 h. Lamprey entrance into refuges peaked at 0300–0500 PDT, and they typically exited at around 2000 PDT, suggesting that refuges primarily functioned as shelter from daylight. Probability of refuge use increased with a morphological indicator of sexual maturity (distance between dorsal fins). In the radio-tagged group, refuge users were equally likely to pass Bonneville Dam as non-users. However, refuge users were less likely than non-users to be detected at sites upstream from the dam, perhaps owing to their maturation status or longer mean passage time through our study area (2.0 vs. 0.6 d for non-users). Refuges show promise for improving fish retention in fishways, particularly for nocturnal and/or small-bodied species that seek shelter from light or predation.