Atlantic salmon return rate increases with smolt length

• Main Authors: Gregory, Stephen, Ibbotson, Anton, Riley, William, Nevoux, Marie, Lauridsen, Rasmus, Russell, Ian, Britton, J Robert, Gillingham, Phillipa, Simmons, Olivia, Rivot, Etienne
• Other Authors: Salmo and Trout Research Centre, The Game and Wildlife Conservation Trust, Centre for Environment, Fisheries and Aquaculture Science Lowestoft (CEFAS), Écologie et santé des écosystèmes (ESE), Institut National de la Recherche Agronomique (INRA)-AGROCAMPUS OUEST, Bournemouth University Poole (BU)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2019
• Subjects: Atlantic salmon; Salmo salar; survival; smolt body size; State-space models; restoration effort; freshwater life-stages; salmonid; Bayesian inference; size effect; marine survival; passive integrated transponder (PIT); probability to return as adult; juvenile; state space model; [SDE.BE]Environmental Sciences/Biodiversity and Ecology; [SDE.MCG]Environmental Sciences/Global Changes; [SDV.SA.STP]Life Sciences [q-bio]/Agricultural sciences/Sciences and technics of fishery
• Online Access: https://institut-agro-rennes-angers.hal.science/hal-02272105; https://institut-agro-rennes-angers.hal.science/hal-02272105/document; https://institut-agro-rennes-angers.hal.science/hal-02272105/file/Rivot_2019.pdf; https://doi.org/10.1093/icesjms/fsz066

International audience Recent declines in Atlantic salmon Salmo salar populations are generally attributed to factors in their marine life-phase. However, it is postulated that factors affecting their freshwater life-phase might impact their marine survival, such as the influence of body size. While larger smolts are widely hypothesized to have higher marine survival rates, empirical support remains scant, in part due to inadequate data and ambiguous statistical analyses. Here, we test the influence of smolt body size on marine return rates, a proxy for marine survival, using a 12-year dataset of 3688 smolts tagged with passive integrated transponders in the River Frome, Southern England. State-space models describe the probability of smolts surviving their marine phase to return as 1 sea-winter (1SW) or multi-sea-winter adults as a function of their length, while accounting for imperfect detection and missing data. Models predicted that larger smolts had higher return rates; the most parsimonious model included the effect of length on 1SW return rate. This prediction is concerning, as freshwater juvenile salmon are decreasing in size on the River Frome, and elsewhere. Thus, to maximize adult returns, restoration efforts should focus on freshwater life-stages, and maximize both the number and the size of emigrating smolts.