Scale Growth Analysis of Atlantic salmon (Salmo salar Linnaeus) Unlocking Environmental Histories

Abstract

Atlantic salmon (Salmo salar L.) populations have declined rapidly in recent years across all geographical ranges with populations becoming extinct within certain areas. Direct observation of the salmon’s life is difficult and costly; therefore, scales remain the most widely used material to indirectly assess and monitor the recent changes in growth. Growth marks (circuli) in scales of Atlantic salmon are used to estimate age and to reconstruct growth histories. This thesis investigated mechanisms of circuli formation and the causes of variation in scale growth measurements. Comparison of scales from multiple body locations (Chapter 2) showed that growth, size and shape measurements varied significantly between body locations. Scale measurements taken from the sampling location recommended by ICES were sufficiently correlated with measurements from two adjacent locations in the posterior body region to facilitate conversion; calibration equations are presented for this purpose. Scale measurements from the anterior body region were highly variable and their use is not recommended. Scale size measurements from the recommended sampling location and from the two adjacent locations in the posterior body region were sufficiently correlated with fish fork length. Differences in scale size could potentially be used to determine the body location from which a scale was most likely sampled if this information has not been recorded (e.g. in archived scale collections); regression equations are presented for this purpose. Analysis of scales from experimentally reared Atlantic salmon post-smolts (Chapters 3 and 4), showed that scale growth and circuli number was proportional to fish growth under a range of different water temperatures and feeding conditions, justifying the use of these measurements as a proxy for growth. The rate of circuli deposition varied between temperature and feeding treatments and circuli number was proportional to cumulative degree day. Narrow inter-circuli spacings were observed during periods of slow growth at low temperatures and during periods of fast growth at high temperatures; therefore, circuli spacing should not be used to infer growth rates. In Chapter 5, scales from Atlantic salmon collected from three Irish rivers (Burrishoole, Moy and the Shannon) between 1954 and 2008 were analysed to determine if marine growth has changed during that period and to establish if trends are consistent across populations. Scale growth measurements and their temporal trends varied between populations. Post-smolt scale growth and circuli number were negatively correlated with SST (Burrishoole and Moy), NAO (Burrishoole) and AMO Burrishoole and Shannon). The results indicate that trends observed in one national index river may not be representative of change across all populations. The new knowledge generated in this thesis supports more accurate interpretation of scale growth measurements, furthers our understanding of this important species and ultimately benefits the future management of this species.