River flow as a determinant of salmonid distribution and abundance: a review

River flow regime is believed to have a fundamental effect on riverine biota. It influences key aquatic processes, including levels of dissolved oxygen, sediment transport and deposition, water quality and habitat type and distribution. We review the impact of flow on the abundance and distribution of salmonid fishes in the context of developing approaches to regulating, setting and restoring river flow regimes as a means of conserving and managing populations. Flow can have direct impacts on salmonids, both through peak flow resulting in the washout of juveniles, and stranding of all life stages under low flow conditions. Salmonids can also be adversely affected through indirect effects of flow, from impacts on water temperature, dissolved oxygen condition, sediment deposition, and habitat availability. Early life stages, particularly eggs and larvae, appear particularly susceptible to the adverse impacts of flow, since they have a limited capacity for behavioral responses to altered flow conditions. A constraint to conservation and management efforts for salmonids is in selecting river flow targets at the catchment scale with confidence. Most studies linking flow with salmonid population processes are site specific, and may not be readily transferable to other sites. Despite this uncertainty, the requirement for catchment level flow targets has become critical as pressure on water resources has intensified, at the same time that salmonid populations have declined. Our proposal is that hypothesis-led analyses of broad scale long-term datasets are key to quantifying variability in fish abundance with respect to flow and informing flow modification field experiments. The water industry, conservation organizations, and environmental regulators are charged with collaboratively tackling the question of how to set, manage and restore river flow parameters, within the framework of the emerging science of hydroecology.