Modelling relationships between habitat and dynamics of a wild brown trout (Salmo trutta L.) population in the River Piddle, Dorset, UK

The status of "wild" brown trout (Salmo trutta, L. 1758) populations in the UK is increasingly giving cause for concern (Giles, 1989; Crisp; 1993). Declines in freshwater stocks are often associated with anthropogenic influences destructive to river channel structure and ecosystem function which are contributing to widespread loss of salmonid habitats (Crisp, 1989; White, 2002). Chalk streams are subject to considerable habitat degradation such that rehabilitation requires management actions which better integrate habitat and ecological processes operating to influence fish populations. The influence of local meso-scale habitats upon brown trout population dynamics in two contrasting sectors of the River Piddle, Dorset, UK was quantified using the Physical Habitat Simulation Model (PHABSIM). Sectors examined represented 'typical' semi-natural chalk-stream conditions in the Piddle/Frome catchment area. Spatial availability and temporal variations in habitat quality (WUA) were modelled and tested for correlation against age-specific trout densities obtained from eight years quantitative electro-fishing data. Analyses indicated; (1) availability of marginal habitats associated with abundant bank-side cover was critical to adult over-winter survival and was a key factor determining local carrying capacity
, (2) abundance of juvenile trout was strongly related to critical periods for spawning and rearing habitats, (3) low habitat durations during the first summer acted as a bottleneck at the juvenile life stage and (4) a variety of different meso-habitat types was important to juvenile recruitment dynamics. The implications of these findings are explored in the context of management of chalk rivers with respect to;- (i) habitat factors limiting trout recruitment dynamics (ii) utility of PHABSIM as a management tool for identifying population bottlenecks and (iii) potential for river rehabilitation strategies to effectively manipulate natural mechanisms regulating brown trout populations in chalk streams.

A thesis submitted to Middlesex University in partial fulfilment of the requirements for the degree of Doctor of Philosophy.