The influence of ploidy on saltwater adaptation, acute stress response and immune function following seawater transfer in non-smolting rainbow trout

We investigated the effect of ploidy on osmoregulatory, stress and immune responses in non-smolting rainbow trout during saltwater adaptation. Sibling groups of diploid and triploid trout were acclimated in freshwater (FW) and then subjected to abrupt transfer to full strength (35 ppt) saltwater (SW) or back to FW. Fish were sampled pre-stress, and 1, 3, 6, 12, 24, 48, 72 and 168 h post-stress. Overall mortality in SW was less than 5% in either ploidy, with no mortality in FW. Significant elevations in plasma osmolality and gill ATPase were observed within 1–3 h of SW transfer, but retuned to basal levels within 72 h indicative of rapid saltwater adaptation and did not differ between ploidy. Furthermore, FW–SW transfer also caused significant and sustained elevations in total blood haemoglobin, plasma IGF-I, cortisol, glucose, total white blood cell counts, increased plasma but decreased mucus lysozyme, and enhanced head kidney macrophage respiratory burst activity. Conversely, FW–FW transfer evoked more transient and less elevated responses, more typical of primary and secondary responses to a single stressor. We conclude that the more elevated levels in these parameters are a function of saltwater adaptation as well as the generic stress response, and that this did not differ between ploidy. Strong positive correlations were found between plasma IGF-I and cortisol, and with osmolality, glucose and WBC, while a negative correlation was found with plasma lysozyme irrespective of ploidy. Overall, the current results suggest that triploidy does not affect the ability of non-smolting trout to adapt to full strength seawater under optimum conditions, and that the osmotic and stress response to such transfer is similar to diploids.