Recent marine growth declines in wild and ranched Atlantic salmon Salmo salar from a western European catchment discovered using a 62-year time series

To address population declines, many salmon populations are supplemented with hatchery-bred fish. Little is known about how growth performance of hatchery-bred and wild salmon compare during critical periods such as the first summer at sea. This study compares scale growth measurements between 1725 ranched and 1976 wild salmon from the Burrishoole catchment (western Ireland) over 62 years (1956-2017). Post-smolt growth (PSG) was greater in wild than ranched fish; this difference was consistent with compensatory growth rather than size-dependant growth. Temporal trends in marine growth of wild and ranched fish were similar, with a marked decline in PSG and return size after the early 2000s. The decline in PSG was significantly related to freshwater growth and autumn sea surface temperature in the Norwegian Sea. Temporal changes in the pattern of intercirculi spacings along the PSG growth trajectory were also identified; circuli deposited during the first summer at sea were narrower in the 2000s and 2010s compared to earlier decades, while circuli from the first winter were wider. The recent decline in growth affects both populations similarly, is associated with increasing temperature and is manifested as a change in growth pattern as well as overall growth.