of polybrominated diphenyl ethers (PBDE), perfluorooctane sulphonate (PFOS) and mercury compounds, none of which had previously been included in the assessment. As well as barriers to the movement of fish, the river has also been affected by pollution from minewater and by the presence of the invasive species, the New Zealand mud snail. Pollution from minewater has been significantly reduced following the construction of a treatment plant by the Coal Authority. This is located between Langsett and Underbank reservoirs, and the minewater is pumped through it. There is a small amount of minewater which does not get processed by the plant, and a solution for this is being trialled by the Environment Agency, Yorkshire Water, and Highways England. In 2017, Yorkshire Water were assessing the amount of water discharged from the reservoirs into the river, with a view to altering them to more natural levels. Consideration has also been give to removing weirs, or providing fish passes where this is not reasonable. The provision of fish passes around the reservoir dams is not considered to be cost-effective, but installing one on a weir used for industrial water supply on the lower reaches of the river would increase the length of the Little Don accessible to fish from the Don twofold. Another fish pass, just above Langsett Reservoir, enables trout and other fish which were impounded within the reservoir to travel some further upstream. It allows them to negotiate the weir, which was installed to trap gravel before the Little Don entered the reservoir. The fish pass is of the Larinier type, and construction was completed on 19 April 2018. Prior to the pass being constructed, Yorkshire Water funded a research project, which involved implanting passive integrated transponder tags into fish, and then using fixed telemetry equipment to detect the movement of fish both upstream and downstream across the weirs. Fish from the reservoir were tagged, as were fish from the upper Little Don and Thickwoods Brook. Migration in both tributaries was thought to be impeded, as both have a weir just upstream from the reservoir. In order to provide control data, similar monitoring was carried out at Grimwith Reservoir in North Yorkshire where there are two tributaries with no weirs. The study showed that there was no detectable movement from the reservoir into the tributaries, and less than expected in the downstream direction. This research informed the decision to construct