Responses of stream fish populations to farming intensity and water abstraction in an agricultural catchment

1. Agriculture affects streams worldwide, and multiple stressors are usually at work. The effects of farming intensification in the catchment are likely to interact with flow reduction due to abstraction for irrigation, but their combined effects on fish communities are unknown. 2. In a low-rainfall, agricultural catchment, we sampled instream physicochemistry and fish populations (non-native brown trout, Salmo trutta, and native upland bullies, Gobiomorphus breviceps, dominated the fish communities) at 36 stream sites chosen to cover wide gradients of % Farming Intensity (%FI) (% catchment in high-producing exotic grassland') and % Water Abstraction (%WA) (estimated from a published hydrological model). These landscape variables were not well correlated, allowing us to unravel their individual and combined effects. 3. Presence of trout was best described by an additive multiple-stressor model consisting of a unimodal response to %FI and a negative response to %WA. Trout density only showed a negative response to %FI. Upland bullies were unrelated to either landscape variable. 4. When populations were modelled using instream variables, trout presence was negatively related to fine sediment depth, while density was negatively related to both sediment depth and total nitrogen (themselves closely related to %FI). Upland bully presence and density showed unimodal responses to just total nitrogen. Ammonium concentration was the only measured instream variable related to %WA. 5. The final models for instream stressors explained more of the variation in fish density, whereas the final models for landscape stressors explained more of the variation in fish presence. 6. Overall, farming intensity showed stronger negative relationships with fish populations than water abstraction, and fish were absent from stream reaches whose upstream catchments contained more than 40% high-producing exotic grassland. Resource managers considering intensifying water abstraction or agriculture in low-rainfall river catchments should be aware of the interplay between these two agricultural stressors.