Long-term decline of barbel Barbus barbus in the original course of the Lower River Lee (England), with particular reference to the survival of tagged fish during a water pollution incident

The status and movements of barbel Barbus barbus was assessed in a by-passed section of the former course of the Lower River Lee (England), which is amongst the most densely populated river catchments in England. Barbel abundance data from historical fisheries surveys and data on the movements of eleven tagged barbel between this section and the adjacent navigable channel using acoustic telemetry between November 2005 and November 2006 were examined. A popular area for anglers, the study stretch suffered a release of pollution (of unknown origin or type) in April 2006, approximately half way through the study. Contrary to unconfirmed reports of mass mortalities, the number of dead fish observed by the Environment Agency was low and none of the tagged fish died during or after the pollution incident. The historical survey data revealed a progressive decline in barbel abundance from the 1970s, through the 1990s, and up to 2009, suggesting that the pollution incident of April 2006 served to exacerbate an on-going, progressive decline in the species' abundance in this sector of the Old River Lee. The tagged barbel demonstrated great fidelity to one part of the study stretch, covered by a single acoustic receiver, including during the pollution event for some tagged barbel. This reluctance of barbel to leave their home area, even under altered conditions, suggests that this stretch of the River Lee satisfies all day-to-day requirements of the fish and provides refuge, at least for some fish, during adverse conditions.