Salmonid Biomass in Streams Around the World: A Quantitative Synthesis

Salmonid fishes are one of the best studied fish taxa, but little is known about their biomass distribution. We created a dataset using published material for over 1000 rivers with estimated salmonid biomass, covering 27 countries, and 11 species. The distribution of salmonid biomass and production across streams was skewed to the right with a mean biomass and production of 5.2 g/m2 (range = 0–70.3 g/m2) and 6.3 g/m2/year (range = 0.03–50.2 g/m2/year), respectively. The top 10% and 1% of salmonid streams in the world had a biomass > 11.9 and 36.5 g/m2, respectively, and a production > 13.9 and 25.6 g/m2/year, respectively. Salmonid production was positively correlated with biomass (r = 0.82, n = 194), with a mean production to biomass (P/B) ratio of 1.08, which differed among species. Mean biomass declined 38% over time, from 8.6 g/m2 before 1980 to 5.4 g/m2 in 2000–2020. Biomass was also higher in small streams (< 10 m wide) and in streams where smaller areas were sampled. Brown trout (Salmo trutta) streams represented a higher proportion of those with biomass > 10 g/m2 than many other species. In addition to the variables mentioned above, salmonid biomass in streams was affected by species, season, method of sampling, elevation, latitude, and migratory strategy. Expanding the list of variables would be useful for developing models to predict salmonid biomass and the conditions for an outstanding salmonid stream, defined as a stream which has a biomass estimate in the top 1% worldwide.