The effects of photoperiod on smoltification of Atlantic salmon (Salmo salar L.) reared at a high temperature in recirculating aquaculture systems
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Juvenile Atlantic salmon (Salmo salar L.) undergo a parr-smolt transformation known as smoltification, which involves a series of physiological, morphological and behavioral changes that prepare them for migrating from freshwater rivers out to sea. They rely on changes in seasonal cues such as photoperiod and temperature to induce smoltification. In order to maintain a constant blood plasma osmolality between the hypoosmotic and hyperosmotic environments, the fish must switch their osmoregulatory strategy entirely, from actively taking up salts to actively secreting them. The gills and kidney are two of the main osmoregulatory organs in euryhaline teleosts, and within their epithelia lies the basolateral Na+ K+ - ATPase (NKA) enzyme. Studies performed on the NKA’s α1-isoforms in gills of Atlantic salmon have found that they are differentially regulated during smoltification, where α1a dominates in freshwater, α1b dominates in seawater and α1c is believed to serve more of a constant “housekeeping” role. However, no research has previously been done on the regulation of these isoforms in the kidney of Atlantic salmon during smoltification. Furthermore, the NKA enzyme activity levels and regulation of its α1-isoforms during smoltification in gills and kidney are not well documented under the rearing conditions used in today’s Norwegian aquaculture industry. Farming protocols are becoming more intensive, with higher temperatures and shorter simulated “seasons”. These conditions are ideal for fish growth, though there remains a lack of understanding around their impacts on smoltification. In this thesis, two groups of juvenile Atlantic salmon were reared in RAS with two different photoperiod regimes and a constant temperature of 14 °C. One group received a “winter” signal of 12 hours light and 12 hours dark for four weeks, followed by continuous light, while the other group only received continuous light throughout the experiment. We analyzed gill and kidney NKA enzyme activity levels as well as relative abundance of the NKA α1a and α1b isoforms in gills, and α1b and α1c isoforms in the kidney. In contrast to the existing literature, we found no differences in morphology (growth rate, condition factor) and osmoregulation (NKA enzyme activity and relative abundance of α1- isoforms) between the two groups, nor did we observe the expected preparatory downregulation of α1a and upregulation of α1b. These results implciate that confounding stimuli such as short “winters” and high temperatures, together with a large body size, may override the important photoperiod stimulus and its proper initiation of smoltification. Further research is needed to better comprehend the regulation of NKA α1-isoforms in the kidney of Atlantic salmon during smoltification.
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