Scaling Up Individual‐Based Models for Salmonid Populations in Hydropower‐Regulated Rivers: Key Advances and Challenges

Transformation to a fossil‐free society will require innovative solutions for sustaining biodiversity while allowing for continued hydroelectric production. Individual‐based models (IBMs) have been shown to be valuable tools for assessing complex challenges such as the tradeoffs between hydroelectric operations and instream flow needs for high‐conservation‐value fish populations. However, the application of IBMs at large spatial and temporal scales poses substantial ecological and computational challenges. This study demonstrates a large‐scale application of the IBM “inSALMO 7.3‐SD.” The model was applied to three key spawning and rearing reaches in River Gullspång, Sweden, covering a spatial scale of 0.105 km 2 and 1.1 km in length. Simulation experiments examined the effects of various minimum flow releases on outmigration production of Atlantic salmon ( Salmo salar ) and brown trout ( S. trutta ). Results indicated that flow effects varied among species, age classes, and river reach. No flow scenario produced more trout outmigrants than the baseline scenarios, and higher flows generally reduced trout outmigrant production. For salmon, increased minimum flows had limited or negative effects on age‐1 outmigrants; increased flow in some cases increased age‐2 salmon outmigrants. Increasing flow provided more profitable feeding and hiding areas for larger juveniles in the lowermost reaches but reduced the availability of feeding places in the upper reach. These findings highlight the complex interactions between flow management and salmonid populations. Scaling up IBMs has the potential to produce realistic projections of whole‐river salmonid populations, which should provide valuable insights for conservation and support informed decision‐making in balancing ecological and hydropower objectives.