Fine sediment influence on salmonid spawning habitat in a lowland agricultural stream: a preliminary assessment

Spawning habitat utilized by Atlantic Salmon (Salmon salar) and Sea Trout ( Salmo trutta) was characterized in a 1.6-km reach of the Newmills Burn, a s mall, highly canalized tributary of the River Don in Aberdeenshire. The New mills Burn is typical of the intensively farmed lower sub-catchments of the major salmon rivers on the east coast of Scotland. Such streams have subst antial potential in providing spawning and juvenile habitat for salmonids, with high redd densities resulting in egg deposition rates of >5 m(2). Howe ver, in comparison with upland spawning tributaries draining less intensive ly managed catchments, canalization and intensive cultivation has seriously degraded the physical characteristics of aquatic habitats in many streams. In the Newmills Burn, spawning gravels have a relatively high (> 20% by ma ss) fine sediment (< 2 mm in size) content. The burn is characterized by hy draulic conditions that are suitable for salmonid spawning, with modal velo cities of 0.50-0.65 m s(-1) and depths of 0.20-0.25 m. However, infiltratio n of fine sediments into gravels is rapid during hydrological events in the winter months. Thus, complete siltation of open gravel matrices (simulated redds) can occur within a week, and probably within a single moderate to l arge storm event. Appreciable, but small, deposition of organic and silt/cl ay particles can also affect spawning gravels. Egg mortalities in redds fol lowing spawning are variable, but can be as high as 86% in the Newmills Bur n. This may be related to fine sediment infiltration, reduced permeability of spawning gravels and reduced oxygen supply to ova. It appears that the m ain cause of high influx is sediment loads mobilized from intensively manag ed land. It is suggested that fundamental changes to the management of agri cultural land is required if fish habitats are to he improved and degraded streams are allowed to re-naturalize. The need for closely focused investig ations of the causal relationships between fine sediment infiltration and e gg survival is stressed. (C) 2001 Elsevier Science B.V. All rights reserved .