Land drainage infrastructure such as flood pumps, tide/flood gates and levees are used to mitigate flood hazards and maintain productive land. However, these structures fragment aquatic ecosystems, altering the hydrological, geomorphic, and physico-chemical characteristics of waterways. The impact of drainage infrastructure on aquatic biota has received little attention, but likely has significant impacts on the migration behaviour and long-term persistence of important species.
A particular concern that has arisen is the impact of flood pump entrainment on freshwater eels. Eels migrate from freshwater habitats to the ocean to spawn. When eels encounter flood pumps during this downstream migration, there is often no alternative but to pass through the pumps resulting in injury and mortality.
To better understand the risks to eels in the vicinity of flood pumps, we undertook a PIT tag study evaluating the movements of 597 shortfin eels (Anguilla australis) across 2 years at a flood pump with a gravity bypass in New Zealand. We observed clear diel and seasonal patterns in eel movements, with eels most active at dusk and during summer. Eels used the bypass channel in both an upstream and downstream direction, undertaking short-range movements likely in search of food. We also observed a unimodal relationship between daily movements and water temperature.
Few eels were entrained in the flood pumps during the autumn downstream migration, but this was largely a result of low flows during this period that maintained access through the bypass and precluded the need for active pump operation. In contrast, we observed pump entrainment of non-migratory eels when the pumps were operating (and the bypass closed) in winter and spring. 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.