Solving bottlenecks in triploid Atlantic salmon production. Temperature, hypoxia and dietary effects on performance, cataracts and metabolism
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In salmon aquaculture, fish occasionally escape from net pens. These domesticated salmon are genetically maladapted for living in natural environments however they still manage to interbreed with wild fish, resulting in severe levels of genetic introgression of farmed salmon in some Norwegian rivers. The use of sterile triploid farmed salmon, with three complete chromosome sets, would avoid further genetic introgression. Initial studies comparing diploids and triploids show reduced performance and higher mortality of triploid salmon that impeded their adoption to commercial farming. With advances in fish husbandry and further knowledge on triploid salmon biology, some of the farming related issues have been mitigated. However, to date, there are still challenges remaining in order to farm triploid Atlantic salmon profitably, sustainably and without jeopardizing fish welfare. Triploids perform poorly at high water temperatures and hypoxic periods, often associated with reduced growth and higher mortality compared to diploids. Further, triploid Atlantic salmon are more prone to develop ocular cataracts that can affect vision, feed intake and welfare. Additional supplementation of the amino acid histidine to the diet successfully mitigated cataract outbreaks and progression in diploid Atlantic salmon. However, the interactive effect between water temperature and dietary histidine level on cataract development in triploid salmon during the risk period of smoltification has not been studied.
In order to investigate the temperature threshold for satisfactory performance and the physiological mechanisms behind reduced or poor performance at suboptimal environmental conditions, diploid and triploid Atlantic salmon post-smolts and adult fish were exposed to water temperatures between 3 and 18 °C (3 °C steps) and to hypoxic periods at cold (6 °C) and warm (18 °C) temperatures. Feed intake, growth and mortality were monitored as well as oxygen consumption, white muscle energy phosphate and carbohydrate storages, blood haematology and plasma parameters. Triploids of both age classes had higher feed intake than diploids at :S 9 °C, similar to diploids between 9 and 12 °C, but lower at higher water temperatures. Triploid post- smolts had higher mortality during reduced oxygen saturations at high water temperature compared to diploids and adult triploids, respectively. Adult triploid salmon had consistently lower white muscle energy phosphates and lower haemoglobin and hematocrit at high temperatures compared to diploids. Over the course of the experiments adult triploids also developed a higher cataract score. The results suggest that triploids have a lower temperature optimum for feed intake and a reduced metabolic scope at higher water temperatures. In the production of triploid post-smolts, periods of low oxygen saturation and particularly during high water temperatures should be avoided.
To assess cataract development in one of the maJor risk periods, around smoltification, both ploidies were reared at medium and high water temperatures (10 vs 16 °C) and fed with two diets differing in the level of histidine (10.4 vs 13.1 g kg- 1) in the weeks before and after seawater transfer. All groups at 16 °C developed significantly more cataracts than groups at 10 °C with additional effects of higher cataract scores due to triploidy and the diet with the lower level of histidine. Furthermore, groups reared at 16 °C experienced severe mortality (triploid > diploid) in the early seawater period due to insufficient hypo-osmoregulatory ability. When reared at 10 °C and fed the diet with more histidine, triploid salmon only developed mild cataracts, well within the level of commercial acceptance. The results suggest that triploids have a higher requirement of dietary histidine compared to diploids in order prevent cataract development and are more sensitive to environmental conditions around seawater transfer.
The main conclusion from this thesis is that water temperature is one of the main drivers in the production of triploid Atlantic salmon. In areas with low or moderate water temperatures, triploids should be a viable option. However, triploid salmon are more sensitive to high water temperatures typically resulting in reduced appetite, increased cataract development, and if combined with low oxygen saturation, increased mortality. Nevertheless, when fed diets with sufficient histidine supplementation and reared at cold or moderate water temperatures, triploid Atlantic salmon can perform better or as well as to diploids.