A Study of Brown Trout, Salmo trutta L., Egg Survival and Intra-gravel Ecology in Livestock Catchments with Farm Management Plans to Mitigate Against Diffuse Pollution.

Abstract

This thesis investigates the impact of farm practice on intra-gravel conditions within streams and the impact of these conditions on trout egg survival. The study was based on 8 catchments in North Cornwall, UK which were tributaries of the rivers Tamar and Neet. The area was benefitting from farm practice improvement initiatives generated by the Westcountry Rivers Trust and the catchments represented different levels of implementation of improved farm practice. The study comprised three sections; a pilot study in 2002-2003 to investigate appropriate sampling methodology, a baseline study also during 2002-2003 to field test the methods and a main study 2004-2006 to test relationships between trout egg survival, sedimentation and land-use practice. In the main study, eyed trout eggs were placed in Harris Baskets in sedimentation pots at sampling sites at the base of each of the catchments. Survival, oxygen, temperature and sediment load were measured. Data on the extent of farm plan improvements was collected. Physical measurements were taken of the river at each site and river flow data were obtained. Invertebrate collections were made at all sites and a habitat bank survey was conducted at four sites. The Null hypothesis of the thesis, that farm practice had no impact on trout egg survival was disproved. Major influences on egg survival were oxygen reductions within the redds due to increased sediment load. It was found that trampling and grazing by livestock was the primary cause of the collapse of banks and that local fencing brought about a 34% reduction in sedimentation with consequent improvements of 54% in trout egg survival. Suggestions for improvement in methodology and more detailed research have been made but this study has begun to address the complex relationship between land use in a catchment and the impact this is having on UK streams.