Long‐term changes in the sea‐age at maturity and seasonal time of return of salmon,Salmo salarL., to Scottish rivers

AbstractUsing catch data, changes in salmon,Salmo salarL., sea‐age and seasonal return time were compared between several Scottish east coast rivers over the last 150–200 years. Parallel long‐term trends were found, suggesting common influences affecting salmon at sea. Long‐term associations with marine environmental factors were investigated by literature review and comparisons with an index of Icelandic sea‐ice abundance. Apparent associations were shown to exist since about 1900, although the cause was not demonstrated. However, over a longer timescale, the same associations were not apparent. Unique gradual marine changes since 1900 may have resulted in perceptible associations with salmon data which may otherwise be imperceptible when short‐term marine environmental variability is greater than long‐term variability. Marine factors must affect salmon, but the complexity of the marine environment and controls on age at maturity and time of return mean that a retrospective analysis cannot be expected to identify the causal factors.