Reservoir fertilisation and fishery response in a highly managed reservoir with uncertain flows: Ecosystem‐based management using decision analysis

Inland fisheries managers must account for multiple competing uses for aquatic resources; using methods such as ecosystem‐based management allows for different priorities for aquatic ecosystems to be accounted for. Declining abundance of kokanee salmon Oncorhynchus nerka (Walbaum) in Arrow Lakes Reservoir in the 1990s led to the use of large‐scale nutrient addition to improve productivity of kokanee and large piscivores. However, it is unclear what effect these measures had on the system given high discharge and highly variable annual flow regime throughout the watershed. An Ecopath with Ecosim model of the ecosystem was fitted to the available data and used to predict ecosystem structure and reservoir objectives under different nutrient addition strategies and varying annual flow regimes. Results from the model indicate that nutrient addition is an important driver in the system, with lower flows resulting in higher biomass for higher trophic levels. Decision analysis demonstrated the importance of maintaining nutrient additions to achieve management objectives despite losses in some high‐flow years.