A simple method of dating marine growth circuli on scales of wild one sea-winter and two sea-winter Atlantic salmon (Salmo salar)

Abstract

A method of dating the marine growth circuli on Atlantic salmon (Salmo salar L.) scales is presented. Data were available for 54 salmon, intercepted as smolts at the completion of their downstream migration in the River North Esk, Scotland.Smolts were tagged, released, and recaptured as return adults after either one (1SW) or two (2SW) winters at sea. Dating of circuli was achieved by fitting a simple quadratic model to the circuli pertaining to fixed points on the scale. For 1SW fish, the latter were the dates of river emigration and completion of marine migration, and the midpoint circulus (equal to the winter solstice) of the winter annulus. For 2SW salmon, these were the date of emigration and the two winter annulus midpoints. A key assumption is that winter annulus formation arises from constraints of light availability on foraging, and hence, reduced growth rate.Comparative analyses indicate the method to be robust and effectively transferable to salmon of known capture date -but of unknown smolt emigration date -by assuming a fixed date of emigration; in such cases, circuli could still be allocated with confidence to a calendar month.Résumé : Une méthode de datation des stries de croissance en mer sur les écailles de saumon atlantique (Salmo salar L.) est décrite. Des données étaient disponibles pour 54 saumons interceptés au stade de saumoneau à la fin de leur avalaison dans la rivière North Esk, en Écosse. Les saumoneaux ont été marqués, relâchés, puis repris en tant qu'adultes à leur retour après un (1SW) ou deux (2SW) hivers en mer. La datation des stries a été effectuée en calant un modèle quadratique simple sur les stries reliées à des points fixes sur l'écaille. Pour les poissons 1SW, ces points étaient les dates de l'émigration de la rivière et de la fin de la migration marine, ainsi que la strie médiane (égal au solstice d'hiver) de l'anneau hivernal. Pour les saumons 2SW, il s'agissait de la date d'émigration de la rivière et des stries médianes des deux anneaux hivernaux. La méthode repose notamment sur l'hypothèse voulant que la formation des anneaux hivernaux soit causée par des restrictions à l'alimentation associées à la lumière disponible et, donc, à une réduction du taux de croissance. L'analyse comparative indique que la méthode est robuste et peut être appliquée avec succès à des saumons dont la date de capture est connue (mais dont la date d'émigration en tant que saumoneaux est inconnue) en postulant une date d'émigration fixe; dans ces cas, les stries peuvent quand même être affectées avec confiance à un mois précis. [Traduit par la Rédaction]

Variation in the post‐smolt growth pattern of wild one sea‐winter salmon (Salmo salar L.), and its linkage to surface warming in the eastern North Atlantic Ocean

“…Example scales showing the presently recognized SF, SFCF and F growth patterns are illustrated in Figure 1a–c, respectively, of Todd et al . (2014). Our recognition of a post‐smolt growth Check is in accordance with the identification of “false checks” as reported by Ottaway and Simkiss (1977).…”

Experimental investigation of the effects of temperature and feeding regime on scale growth in Atlantic salmon Salmo salar post‐smolts

“…The results of this study show that circulus formation was related to growth rather than time, as found with experimental studies of other salmonids (Beakes et al ., ; Fisher & Pearcy, ; Fukuwaka, ). Here, deposition rates varied from 4.8 days circulus −1 at 15°C (constant feeding) to 15.1 days circulus – 1 at 6°C (interrupted feeding), which is close to the range of estimated values from field studies (6–14 days circulus –1 ; Friedland et al ., ; Hubley et al ., ; Jensen et al ., ; Todd et al ., ). Under constant feeding conditions, circuli number varied as a function of cumulative degree day (deposition rates of 0.0103, 0.0125 and 0.0133 circulus ° D C −1 at 6, 10.5 and 15°C, respectively).…”

“…() estimated that one circulus is deposited every 6.3 days during initial post‐smolt growth while Todd et al . () reported deposition rates ranging from 6.6 to 13.6 days for tagged and recaptured S. salar , depending on time of year and fish age. Experimental investigation is needed to validate circuli deposition rates and to determine how this is influenced by environmental conditions.…”

“…based on field evidence, Friedland et al (2005) proposed that during the marine phase, circuli are deposited at a rate of one per week during summer months and two per week during winter months. Jensen et al (2012) estimated that one circulus is deposited every 6.3 days during initial post-smolt growth while Todd et al (2014) reported deposition rates ranging from 6.6 to 13.6 days for tagged and recaptured S. salar, depending on time of year and fish age. Experimental investigation is needed to validate circuli deposition rates and to determine how this is influenced by environmental conditions.…”

Precision and accuracy of Dahl‐Lea back‐calculated smolt lengths from adult scales of Atlantic salmonSalmo salar

“…The relationship between scale growth, somatic growth and time at sea in S. salar is complex (Todd et al ., ). An extension of the work initially undertaken by Lindroth (), exploring in detail the scale– L F relationship and applied to a wide range of size and river‐age and sea‐age classes, would help to validate or refute the hypotheses implicit in various back‐calculation methods (Francis, ).…”