Assessing the seasonal effect of flow regimes on availability of Atlantic salmon fry habitat in an upland Scottish stream

Physical habitat requirements of fish vary with species and life-stage. Water depth and velocity have been widely suggested as strong abiotic controls on juvenile Atlantic salmon (Salmo salar L.) habitat. The quality of hydraulic habitat is the result of complex interactions between stream flow dynamics and stream bed morphology. In this paper we investigated the seasonal effects of flow regime and bed morphology on the availability of habitat for salmon fry over a 40-year period. The study was based in an upland Scottish river, the Girnock Burn, a long-term salmon monitoring site. We compared six sites with contrasting bed morphologies, representing the range of in-stream habitats used by salmon fry. For each site, detailed topographic surveys and daily specific discharge time series were used in a 2D hydraulic model to obtain spatially distributed water depths and mean column velocities, under different flow conditions. Normalised fish density was used as a metric to characterise potential seasonal habitat quality. This was predicted by combining previously-derived seasonal General Additive Models with 2D hydraulic models. Our results showed that sites can provide a portion of suitable habitat under a wide range of flow conditions. Habitat quality was more stable in autumn than in spring and summer using bench-marked comparisons. Marked inter-site differences occurred in spring and summer, while in autumn differences between sites were relatively small. Habitat quality time series showed consistent temporal patterns between the sites and no clear long-term trends were identified. Overall, dry-weather with low flows interspersed with high streamflow events represented the most unfavourable conditions for salmon fry with summer being the most sensitive period. Although the Girnock provides fry habitat that is generally resilient to flow variability, our results suggest that, under extremely variable and warm summer flows, mitigating strategies might be needed in order to preserve system productivity.