Species dispersal through the raw water transfer invasion pathway

Raw water transfers (RWTs) are a globally occurring freshwater invasion pathway. Recently, the need to better understand the risk posed by RWTs and develop effective pathway management strategies has been recognised. However, as a complex and challenging pathway to study, relatively few direct investigations of RWTs have been carried out to date, and an empirical understanding of pathway introduction capacity is currently lacking. We therefore conducted a preliminary investigation of three structurally different RWTs in the northeast of England, using nets to sample the water discharging from RWT outlet infrastructure across a nine-hour sampling period. In total, viable specimens of 16 distinct species were sampled across all three RWTs studied (with an additional 7 distinct species sampled in non-viable form). These included species of fish, mollusc, and arthropods. Two species non-native to Great Britain, Crangonyx sp. and the New Zealand mud snail ( P . antipodarum ), were simultaneously dispersed by two different RWT schemes. Crangonyx sp . was also the most abundant single species transferred overall, with 85 viable specimens captured from a single RWT across the sampling period. The viable propagule size per million litres (VPS) was calculated for each of the three RWTs (57, 4.2, 1.2 retrospectively). Whilst the introduction capacity of the different RWTs varied substantially, even for RWTs which support relatively lower VPS, the transfer of large volumes of water for long periods could enable the dispersal of a substantial number of specimens. This study provides direct evidence of the role of RWTs in the secondary spread of diverse species across various spatial scales, and highlights the pressing need to develop effective pathway management strategies in Great Britain and beyond.