DNA and scale reading to identify repeat spawning in Atlantic salmon: Unique insights into patterns of iteroparity

Iteroparity represents an important but often overlooked component of life history in anadromous Atlantic salmon. Here, we combined individual DNA profiling and scale reading to identify repeat spawners among ~8000 adult salmon captured in a fish trap in the river Etne, Norway, in the period 2015-2019. Additionally, 171 outward migrating kelts were captured in the spring of 2018-2020 and identified using molecular methods to estimate weight loss since ascending the river to spawn. The overall frequency of repeat spawners identified using molecular methods and scale reading combined was 7% in females and 3% in males (5% in total). Most of these (83%) spent one full year reconditioning at sea before returning for their second spawning, with a larger body size compared with their size at first spawning, gaining on average 15.9 cm. A single female migrating back into the river for a fifth breeding season was also identified. On average, kelts lost 40% bodyweight in the river, and more female than male kelts were captured during outward migration. The date of arrival in the upstream fish trap was significantly but moderately correlated between maiden and second entry to the river for alternate and consecutive spawners. The estimated contribution from repeat spawners to the total number of eggs deposited in the river each year varied between 2% and 17% (average 12%). Molecular-based methods marginally underestimated the number of repeat spawners compared with scale reading (5% vs 7%) likely due to a small number of returning spawners not being trapped and sampled. Differences between the methods were most evident when classifying the spawning strategy (alternate or consecutive-year repeat spawners), where the scale method identified proportionally more consecutive-year repeat spawners than the molecular method. This unique data set reveals previously unstudied components of this life history strategy and demonstrates the importance of repeat spawners in population recruitment.