Objective determination of the sea age of Atlantic salmon from the sizes and dates of capture of individual fish

• Published in: ICES Journal of Marine Science
• Main Authors: Bacon, P.J., Gurney, William, Mckenzie, Edward, Whyte, B., Campbell, R., Laughton, R., Smith, G., MacLean, J.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2010
• Subjects: Probabilities. Mathematical statistics; Atlantic salmon
• Online Access: https://strathprints.strath.ac.uk/29098/; https://doi.org/10.1093/icesjms/fsq142

The sea ages of Atlantic salmon indicate crucial differences between oceanic feeding zones that have important implications for conservation and management. Historical fishery-catch records go back more than 100 years, but the reliability with which they discriminate between sea-age classes is uncertain. Research data from some 188 000 scale-aged Scottish salmon that included size (length, weight) and seasonal date of capture on return to the coast were investigated to devise a means of assigning sea age to individual fish objectively. Two simple bivariate probability distributions are described that discriminate between 1SW and 2SW fish with 97% reliability, and between 2SW and 3SW fish with 70% confidence. The same two probability distributions achieve this accuracy across five major east coast Scottish rivers and five decades. They also achieve the same exactitude for a smaller recent dataset from the Scottish west coast, from the River Tweed a century ago (1894/1895), and for salmon caught by rod near the estuary. More surprisingly, they also achieve the same success for rod-caught salmon taken at beats remote from the estuary and including capture dates when some fish could have been in the river for a few months. The implications of these findings for fishery management and conservation are discussed.