The combined effects of salt feed, photoperiod, and temperature on Atlantic salmon smolt physiology, migratory behaviour, and sexual maturation

Summary

Salt-enriched feeds (SF) are used to stimulate seawater tolerance in Atlantic salmon aquaculture, and heated water is applied to accelerate smolt development. However, the impact of SF and increased temperatures on smoltification and the occurrence of early male maturation are poorly documented. In this study, 0+ Atlantic salmon parr (50 g) were reared at 12 °C under either a ‘square wave’ photoperiod with 6 weeks of short day (12 h light [L]: 12 h dark [D]) followed by continuous light (hereafter LD), or continuous light throughout (hereafter LL). When continuous light was re-instated in LD, both photoperiod regimes were tested for how diet (5.5%NaCl and 0.5%CaCl2 [SF] diet vs. control, experiment 1) or temperature (8 °C vs. 16 °C, experiment 2) influenced the development of key smoltification traits (reduced condition factor, seawater tolerance, with-current swimming behaviour), the occurrence of sexual maturation in males, and vitamin D receptor (vdr0, gill and intestine) and growth hormone receptor (ghrb1, intestine) expression in two separate experiments. Experiment 1 and 2 ended 750 and 1000 degree-days (d°C), respectively, after LD fish were switched to continuous light. LD caused a reduction in condition factor, however neither SF nor LL triggered this response. Hypo-osmoregulatory ability was developed within 300 d°C after LL was re-instigated in LD, in all regimes, indicated by the ability to regulate plasma Cl− and Na+ -levels after a 24 h seawater challenge. SF diet increased hypo-osmoregulatory ability in a manner that was independent of timepoint and photoperiod regime. Rearing at 16 °C in freshwater lowered the expression of a transcript related to ion regulation (cystic fibrosis transmembrane regulator 1 [cftr1]) in the gill relative to at 8 °C. Intestinal upregulation of vdr0 and ghrb1 in response to a 24 h seawater challenge at 50 d°C coincided with a lack of smolt traits. Ion-regulatory gene transcripts in the intestine (Na+,K+-ATPase α-subunit 1c (nka1c)), were upregulated following a 24 h seawater challenge in a manner that was independent of timepoint. Both 12 °C and 16 °C triggered sexual maturation in males (12 °C < 16 °C and LD > LL). A change in swimming direction from positive to negative rheotaxis occurred at 276 and 288 d°C in LD fish reared at 12 °C and 16 °C, respectively. With-current swimming was positively associated with increases in precipitation levels at the site of the flow-through rearing facility where the experiments were conducted. Non of the SF diets, LD fish at 8 °C, or the LL fish at any temperature, display with-current swimming. In summary, SF diet improved hypo-osmoregulatory ability but did not stimulate other smolt traits such as a lowered K or with-current swimming, and while higher rearing temperature accellerated smoltification development, it negatively influenced hypo-osmoregulatory ability and elevated rates of sexual maturation in males. Welfare was best maintained at 8 °C rearing as low rates of male maturation were detected at this temperature. The present results have important applied implications for the rearing of Atlantic salmon smolts, and opens new questions regarding the regulation of migratory behaviour in Atlantic salmon.

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DOI : doi.org/10.1016/j.aquaculture....
NVA : hdl.handle.net/11250/5562874