Scale growth data for analysis in Vollset et al. 2021

Scale samples of adult Atlantic salmon were compiled from 180 rivers in Norway, which comprise almost half of all Atlantic salmon rivers in Norway, during 1989-2016. The scales were collected by anglers when sport fishing in the various rivers. The back-calculation of fish length is based on the linear relationship between fish length and the diameter of the fish scale from the centre of the scale to the edge . We have focused on the average growth across four large geographic regions: South-western, south-western, middle-, and north Norway. In total, 52 371 Atlantic salmon were sampled by fishers. For each individual fish, the following data were used: length at capture (cm), back-calculated freshwater growth (cm), back-calculated early marine growth (cm), and smolt year, which is the year they migrated from the rivers as juveniles to the sea. Freshwater growth was defined as the distance from the centre of the scale to the end of the freshwater zone in the scale. Early marine growth, also termed post-smolt growth, was measured as the distance between the end of the freshwater zone and the end of the first winter at sea.

Growth estimates from scales may be impacted by the size of the fish when they leave the rivers and enter the sea. To correct for this error, a relative growth estimate can be used where the early marine growth is divided by the freshwater growth of the individual, which we refer to as relative growth. Also, the timing of out-migration will vary by up to two months from north to south in Norway, meaning that the time to grow before winter is shorter for northern populations. Putative monthly growth estimates indicate that growth is limited from November, and we therefore calculate growth from day of out-migration to 31 October. Day of out migration was calculated according to using a model that explains out-migration data based on latitude, longitude, air temperature during winter, and river discharge. The estimated growth is called relative growth per day.