Records from salmon counters at hydro‐electric dams, and of angling results, in the Tay‐Tummel‐Garry river system are analysed. Spring runs of ascending salmon have decreased since at least 1950, but summer and autumn runs have increased since 1959. Hydro‐electric developments are not responsible for these changes, nor for the decrease in the average weight of salmon in the system. Possible environmental factors are discussed, and a method suggested whereby the ratio of sizes of salmon populations in branches of a river system may be estimated. The numbers of descending kelts have recently increased, but the times of downstream migration have not