Could bigger be better? Longer Atlantic salmon smolts seem more likely to return as adults

Abstract

Atlantic salmon stock declines are thought to be due to climate-driven changes in the marine environment that have rendered it hostile to migrating smolts. Recently, there has been a growing sense that factors affecting smolts during their development, i.e., in the freshwater environment, might play a larger role than previously judged. It has been hypothesised that smolt size is related to their marine survival, i.e., that larger smolts are better able to survive at sea than their smaller counterparts. With a large database of individual smolt capture histories and characteristics, I test the "bigger is better" hypothesis for Atlantic salmon smolts inhabiting the river Frome in Dorset, UK. By fitting and comparing multi-state mark-recapture state-space models with different covariates, I show support for the "bigger is better" hypothesis in these data, i.e., that longer smolts are more likely to return as adults compared to smaller smolts. This suggests that freshwater environments could be managed to maximise smolt quality (e.g., length) and thereby the numbers of returning adults.