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Production of juvenile salmonids in small Norwegian streams is affected by agricultural land use

SUMMARY 1. We estimated the biomass and production of juvenile anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) (parr) in 12 streams in the Skagerrak area of Norway to identify controlling environmental factors, such as land-use and water chemistry. 2. Production estimates correlated positively with fish density in early summer, but not with the size of the catchment. The summer biomass of age-0 brown trout and Atlantic salmon was smaller than that of age-1 and constituted 27.4 and 25.7%, respectively, of the total biomass of the two groups. 3. Mean production of brown trout from July to September varied between streams, but in most cases it was below 2 g 100 m )2 day )1 . Yearly cohort production from age-0 in July to age-1 in July was 10 g m )2 or less, with mean annual production of 1.32 g 100 m )2 day )1 , equivalent to 4.8 g m )2 year )1 . The corresponding annual cohort production of Atlantic salmon was 0.38 g 100 m )2 day )1 or 1.4 g m )2 year )1 . Annual production to biomass ratio (P ⁄B) for brown trout of the same cohort in the various streams was between 1.47 and 4.37; the overall mean (±SD) for all streams was 2.25 ± 0.94. Mean turnover rate of Atlantic salmon was 2.73 ± 0.24. 4. Production of 0+ brown trout during the summer correlated significantly with the percentage of agricultural land and forest ⁄ bogs in the catchment, with maxima at 20 and 75%, respectively. Age-0 brown trout production also correlated with concentration of nitrogen and calcium in the water, with maxima at 2.4 and 14 mg L )1 , respectively. 5. The results support the hypothesis that brown trout parr production reflects the quality of their habitat, as indicated by the dome-shaped relationship between percentage of agricultural land and the concentration of nitrogen and calcium in the water.