Farm practice and trout egg survival
Planting a sedimentation pot with baskets of eggs and a pipe for measuring dissolved oxygen. Š Dominic Stubbing/GWCT
KEY FINDINGS Sediment reduced oxygen and egg survival within the riverbed. Livestock were the main agents for sediment deposition. Fencing livestock away from banks reduced sediment by a third and improved egg survival by a half. Dominic Stubbing
86 | GAME & WILDLIFE REVIEW 08
The deterioration of trout and salmon populations in UK rivers is a long-standing problem. Poor egg survival is believed to be a major contributory factor. It has been suggested that this is caused by the effect of increased sediment on conditions for eggs in the redds (nests) during incubation. Increased sediment load is believed to occur through changes in farm practice. Trout and young salmon are typically residents of clean, fast-flowing streams that are well oxygenated and have areas of gravel substrate, which form the spawning grounds of the adult fish. The female constructs a redd in the gravel using her tail. The incubating eggs are totally dependent on river gravel conditions for their survival. Factors that can reduce survival include floods, which may dislodge the redd, predation within the gravel, and a deterioration in oxygen levels within the gravel caused by sediment load or organic pollution. The amount of sediment in rivers depends on erosion occurring from land surfaces in the catchment and on river bank and riverbed sources. Transport is then by suspension or bed-load movement. Some farming practices have been shown to increase bank-side erosion, leading to sediment deposition. It has been shown that simply fencing river banks can increase riparian vegetation, decrease erosion and even narrow the river. This study investigated the impacts of farm practice on river gravel conditions within streams and the impact of these conditions on trout egg survival. The results will provide guidance on what good farming practices can be used to reduce siltation and increase egg survival. The study was based on eight catchments in North Cornwall, which are tributaries of the rivers Tamar and Neet. The area benefited from farm practice improvement initiatives generated by the Westcountry Rivers Trust and the catchments represented different levels of improved farm practice. The study was divided into three parts: a pilot study in 2002 to 2003 to investigate appropriate sampling methodology, a baseline study also during 2002-2003 to field-test the methods, and a main study in