Inter-annual variation in movements and passage of seaward migrating European eels at a shrouded Archimedean screw pumping station

The construction of less damaging (here called fish-friendly) pumping stations has taken place in recent years, but it is unknown if they provide efficient and timely passage to migratory fish, such as European eel (Anguilla anguilla (L.)). The pump is often the only downstream passage route and operation, i.e., fish passage opportunity, is temporally variable depending on precipitation and prevailing river levels. Once the pumps are operating, eels must also consider the pumping station an attractive downstream passage route. Here, the movement of seaward migrating silver European eel (n = 59) upstream of a fish-friendly shrouded Archimedean Screw Pump (ASP) was assessed during three migrations (December–March in 2018/19, 2019/20 and 2020/21) with highly contrasting hydrology using acoustic telemetry. The overall passage rate was low (36.8 %) and minimum passage time was 65.2 days during a year with very little pump operation (2018/19), with seven eels tagged with long-life transmitters passing the pumping station the following year (2019/20). Furthermore, the median number of approaches to the pumping station was seven, with 36.8 % (n = 7) approaching more than 10 times. By contrast, passage rate was high (95.0 %), maximum passage time was 2.7 and 34.0 days (minimum = 3 min in both years) and all but one eel passed during the first approach during the two wettest years (2019/20 and 2020/21). Eels were almost exclusively nocturnal, regardless of pump operation, with 96.1 % of total approaches occurring between sunset and sunrise and no eels passed downstream during the day. Ultimately, limited eel passage opportunity during dry periods and a reluctance to pass when operational curtailed the effectiveness of these pumps to provide efficient and timely passage. Thus, measures are required to align pump operation with the timing of eel migration, especially in dry years, and reasons for retreat from the pumping station during operation must be identified and alleviated.