Characterization of In-River Plus Growth in Atlantic Salmon Smolt Scales

Wild Atlantic salmon Salmo salar populations in the US have been in decline since the late 19th century and were listed as endangered in the US in 2000. Continued research efforts, including growth studies using scale analyses, contribute to a better understanding of Atlantic salmon’s behavior and survival. Plus-growth occurs in salmon smolt scales after the formation of the last freshwater annulus between April and June, before the smolts emigrate to a marine environment. Smolt plus-growth may be difficult to detect in adult salmon. Characterizing plus-growth in smolt scales may help to characterize run-out (transitional growth between freshwater and marine environment) in adult scales and improve the accuracy of back-calculated lengths-at-age. Little information is available regarding the relationship between plus-growth observed in smolt scales and emigration timing. The extent of plus-growth may vary depending on the state of an individual fish and whether it lingers in freshwater longer, potentially to reach a threshold size before entering the ocean. To characterize the plus-growth formation and consider its relationship to time of migration, scale samples from 68 age-2 wild Atlantic salmon smolts from Narraguagus River and 48 from Sheepscot River in Maine were imaged. The best scale from each fish was selected and measured for distances from the scale focus to each circulus, annulus, and the scale margin. The percentage of age-2 smolts with plus-growth, the amount of plus-growth, and the number of plus-growth circuli were determined per week throughout the migratory season. The results from the two rivers were then compared using regression analyses. The next phase of this research is to characterize the relationship of plus-growth in smolt scales and run-out in returning adult salmon scales.