Smoltification and growth retardation in New Zealand king salmon Oncorhynchus tshawytscha (Walbaum)

Abstract

Growth retardation in King salmon Oncorhynchus tshawytscha (Walbaum) is a common and significant problem affecting marine farming operations in New Zealand. While the basic marine culture requirements for the King salmon species are well understood, the etiology of seawater adaptation and growth retardation remains understudied. Consequently, this study was established to investigate the physiological state and causative factors of growth retardation in collaboration with a leading New Zealand aquaculture company, New Zealand King Salmon Ltd (NZKS). Hypoosmoregulatory indicators are not currently used by marine farmers in New Zealand due to the belief that King salmon are more adaptable to seawater than their more highly cultured counterparts, Coho and Atlantic, and can be transferred to seawater anytime after a critical weight is achieved. This study sought to investigate changes in hypoosmoregulatory ability and its relation to water temperatures commonly used in the hatchery environment. This was determined by changes in the activity of the predominating seawater-adapting gill enzyme Na+/K+-ATPase, as an indirect measure of its abundance during smoltification. Changes in plasma ion profiles and the ability to regulate ions after abrupt transfer were also measured and compared with enzymatic activity throughout the austral springtime smoltification period in commercial strains of under-yearling King salmon. It was found that King salmon do undergo a distinct austral spring-time temporal increase in hypoosmoregulatory processes. This was characterised by a 2-fold increase Na+/K+-ATPase activity which was concomitant with reduced plasma Na+ in freshwater and following a seawater challenge in fish between fork lengths of 140-160 mm. Despite no consistent reduction in Na±/K+-ATPase activity during desmoltification, it was shown that the percent of ATP dependent activity specific to Na+/K+-ATPase diminished over time. Increased residual ATP dependent activity is hypothesised to be a result of apical H+-VATPase activity as a compensatory mechanism to rapidly normalise plasma Na+ during desmoltification concomitant with elevated basolateral Na+/K+-ATPase. Water temperature has been linked with the advancement and shortening of the smoltification period in several species. Gill Na+/K+-ATPase activity and hypoosmoregulatory ability in King salmon were negatively affected by increasing water temperatures above 12°C in contrast to a constant 12°C. The level of growth retardation was dependent on the time of transfer to seawater and was found to increase during a period of reducing hypoosmoregulatory ability. The transfer of growth retarded King salmon back to freshwater resulted in a complete reversal of the growth retarded state, comparable to that observed in Coho and Atlantic salmon. Growth retarded fish were able to readapt back to freshwater with higher survival and growth rates compared to the transfer of normal growing sub-adult King salmon, strongly demonstrating that growth retarded fish are more adapted to freshwater. Osmoregulatory physiology, and endocrinology during the transfer of growth retarded and normal growing fish were investigated. Overall, these results have fundamental implications for the aquaculture of King salmon that are able to be applied by industry to improve current husbandry practices.