Abstract
Smoltification in Atlantic salmon is often operationally assessed through growth, size, and seawater tolerance. However, the biological smolt is produced by a tightly timed developmental program in which winter photoperiod history, increasing spring daylength, brain remodeling, endocrine activation, and osmoregulatory remodeling across the gill, intestine and kidney are coordinated. Evidence from neuroanatomical, endocrine, molecular, and physiological studies shows that continuous constant light can uncouple this program: fish may grow well, but key smolt-related changes in retinal–preoptic connectivity, CRF neurogenesis, thyroid hormone, growth hormone, cortisol signaling, together with coordinated development of NKA enzyme activity across the three osmoregulatory organs and subsequent seawater performance may be blunted or incomplete.
This presentation revisits the concept of smolt quality from a developmental perspective. We argue that smoltification is not simply growth plus seawater tolerance, but a seasonal brain-endocrine-osmoregulatory remodeling process shaped by photoperiod history. Thus, the use of continuous light (24:0) without a preceding winter signal may produce "pseudo-smolts" that acquire some smolt characteristics, while key aspects of developmental timing, endocrine activation and osmoregulatory remodeling may remain partly or completely uncoupled, with potential consequences for long-term seawater performance. Such mismatches between photoperiodic signaling and physiological development may also contribute to production disorders including nephrocalcinosis, looser syndrome (stunts), hemorrhagic smolt syndrome (HSS), and broader smolt quality problems. These disorders may therefore, at least partly, reflect mismatches between the biological requirements of salmon and the environmental conditions provided during intensive land-based production.
The objective is to remind producers, researchers, and technology providers that photoperiod balance remains a core welfare and performance factor in modern smolt production, while its wider physiological consequanses are still not fully understood. Optimized lighting protocols should preserve the biological logic of winter preparation and spring progression, rather than treating continuous light as a simple growth accelerator, as they may contribute to several production disorders currently observed in land-based salmon aquaculture
Acknowledgment
This research was supported by research projects funded by Research Council of Norway and FHF- Norwegian Seafood Research Fund.
References
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