Using the IFIM (Instream Flow Incremental Methodology) to model salmonid fish habitat in the River Allen, England

The River Allen, situated in southern England, is regarded as a classic example of a chalk stream and historically has a reputation as a high quality trout and salmon fishery. Significant reductions in salmonid populations have been observed, following the start of groundwater abstraction within the Allen catchment in 1946. This has prompted the National Rivers Authority Wessex Region to initiate detailed hydrological, biological and fisheries studies to investigate the perceived problem. Within these studies the Institute of Hydrology was commissioned to investigate the effect of the abstraction regime upon salmonid habitat availability (JOHNSON et a/., 1993). The study used the Instream Flow Incremental Methodology (IFIM) (BOVEE, 1982), implemented using the Physical Habitat Simulation (PHABSIM) model. PHABSIM hydraulic model simulation results were combined with habitat suitability data for life-stages of trout and salmon to develop habitat (weighted usable area) as discharge relationships. Habitat suitability indices were developed by National Rivers Authority (NRA) Wessex Region staff by snorkelling and wading in a number of chalk streams similar in character to the Allen. To assess the impact of groundwater abstraction upon habitat availability, weighted usable area as discharge relationships were combined with a twenty year time series of mean monthly "historical" and "naturalised" flows. The resulting time series of historical and naturalised weighted usable areas were analysed using a standard duration curve program. These habitat duration curves demonstrate the effect of the abstraction upon the availability of habitat for each species life-stage considered. The results provided by this analysis were used to negotiate a proposed 50 % reduction in the levels of abstraction within the Allen catchment.