The spatiotemporal movement ecology of shortfin eels and entrainment risk of yellow eels at a flood relief pump station with gravity bypass

Ecological Solutions and Evidence (British Ecological Society) Vol. 6(4) October-December 2025

1. Land drainage and flood protection infrastructure such as pump stations, tide/flood gates and levees are used to mitigate flood hazards and maintain productive land. However, such infrastructure fragments aquatic ecosystems, often altering the hydrological, geomorphic and physico-chemical characteristics of waterways.
2. To better understand the risks to eels in the vicinity of flood relief pump stations, we undertook a passive integrated transponder tag study evaluating the movements of 332 shortfin eels (Anguilla australis) across 2.5 years at a pump station with a gravity bypass in New Zealand.
3. We observed clear diel and seasonal patterns in eel movements, with increased eel activity at dusk and during late spring and summer. In addition, we observed a unimodal relationship between daily eel activity and water temperature with peak activity occurring between 14°C and 15°C.
4. The gravity bypass was the primary route used by eels for both downstream (45%) and upstream (20%) movements, while direct passage through the pump was uncommon, occurring in only 28 individuals. Among more than 12,000 recorded encounters with the flood pump and gravity bypass, the most frequent outcomes were avoidance of the bypass and obstruction at the pump (42% and 58% of the encounters across individuals, respectively), whereas direct or delayed entrainment through the flood pump occurred rarely (13.5% and 15.2% of the encounters across individuals, respectively).
5. No eels were entrained in the flood pumps during the autumn downstream migration due to low flows throughout this period that precluded the need for active pump operations. In contrast, we observed eel entrainment during winter and spring, when the pumps were most frequently operating, suggesting that non-migratory (yellow) eels may have been affected.
6. Practical implications. These results highlight that the risk to eels of pump entrainment and mortality extend beyond the main period of downstream migration that has been the focus of most studies. Pump station design and operational management must allow for bidirectional movements of eels to provide effective functional connectivity.