Phosphorus flux due to Atlantic salmon (Salmo salar) in an oligotrophic upland stream: effects of management and demography
Canadian Journal of Fisheries and Aquatic Sciences
Little is known concerning the role of Atlantic salmon (Salmo salar) in the transport of nutrients to and from river systems. We used demographic data from the River Bran, an oligotrophic river in Scotland, UK, to construct a budget for the transport of phosphorus (P) and applied it to investigate the effects of management strategies and demographic rates on potential transport. At present, because few adults return to their spawning grounds, salmon export 0.2–0.5 kg P\textbulletyear–1. In contrast, increasing passage rates to a level sufficient to maintain a population without stocking would likely result in a gain of up to several kilograms per year. However, this effect depended on the retention of adult-derived P, which varies across systems and is poorly known at present. Egg-derived P exceeded that from adults at low (<25%) retention rates but was insufficient on its own to balance losses. Increased marine survival rates also increased the potential for positive P flux, while reduction in egg–smolt survival reduced the magnitude of transport. These results indicate the importance of considering within-river movements of individuals and nutrients and the need to fill critical data gaps in assessing the role of Atlantic salmon in nutrient transport.