Tracking Long‐Term Trends in Sockeye Salmon (Oncorhynchus nerka) Population Dynamics Using Sterol and Stanol Biomarkers in Lake Sediments

We analyzed biomarkers, including sterols, stanols, and δ 15 N, in sediment cores from lakes with well‐documented sockeye salmon return histories. Our goal was to improve estimates of past changes in salmon escapement, that is, the population that return to their freshwater nursery lakes, inferred using sediment biogeochemical markers. Cholesterol, the predominant sterol in adult sockeye salmon muscle tissue, displayed a strong positive relationship with escapement ( R 2 = 0.8, p = 0.001, and F 1,8 = 28.3). Sediment concentrations of the plant‐derived sitosterol and algal‐derived fucosterol, absent in salmon muscle tissue also related positively with salmon escapement, suggesting that salmon‐derived nutrients from decomposing fish promote the production of these lipids by primary producers in the lakes. We developed a novel salmon sterol index (SSI a ) from values in surface sediments of the nine Alaskan lakes [(cholesterol + coprostanone + epicoprostanol + desmosterol)/(cholesterol + coprostanone + epicoprostanol + desmosterol + fucosterol + sitosterol + stigmastanol)] that was strongly related with salmon return density (pseudo R 2 = 0.86 and RMSE = 0.071). This index also tracked historical sockeye escapement patterns and δ 15 N values in 210 Pb‐dated lake sediment cores from Frazer, Karluk, Red, and Kinaskan lakes that span more than a century of salmon population history, suggesting that the index has potential as a proxy for tracking historical salmon populations, particularly when used in combination with independent biomarkers of salmon‐derived nutrient inputs. SSI a and the other salmon sterol indices we developed show promise for improving and extending long‐term sockeye salmon population estimates using lake sediment records, which will help inform salmon conservation and management efforts.