Improving fish production and welfare requires an understanding of their physiological responses to common maintenance practices. Throughout the world, the longfin yellowtail (Seriola rivoliana) and congeners are increasingly cultured. For seawater (SW) fish, including yellowtail, a freshwater (FW) dip is a common prophylaxis and treatment for ectoparasites. However, the physiological effects of this procedure on SW fish, and whether diet may play any modulatory role on the response remains poorly understood. To address the effects of this practice, we measured plasma osmolality, cortisol, and glucose, in addition to transcripts associated with ion transport and corticosteroid receptors known to bind cortisol and mediate salinity acclimation and stress responses in yellowtail reared on different diets following exposure to FW for 5 minutes (min).
Twenty-four, one-year-old S. rivoliana (1.23 ± 0.08 kg) fed either a commercial pellet diet (pellet, n=12) or a gelatinized fresh seafood diet (gel, n=12), consisting of homogenized squid and herring, were used in this study. Six fish from each diet were dipped in FW for 5 min while six parallel control fish from each diet were dipped in SW for 5 min. Blood and gill filaments from each fish were non-lethally sampled at 1, 6, 12, and 24 hours (h) following the FW or SW dip.
In fish exposed to a FW dip, plasma osmolality and cortisol were elevated in those fed pellet compared with those fed gel by 1 h (p < 0.05). Both parameters remained elevated in fish fed gel by 24 h, regardless of dip treatment. A two-way ANOVA revealed main effects of plasma glucose (elevated in control fish) and diet (elevated in pellet-fed fish) at 1 and 6 h. The expression of branchial ion transporters responsible for ion extrusion, cystic fibrosis transmembrane conductance regulator (cftr) and Na+/K+/2Cl-cotransporter (nkcc), were downregulated at 12 h following a FW dip in both diets; by 24 h, cftr and nkcc returned to baseline only in fish fed pellets and decreased in control fish fed gel. Branchial expression of glucocorticoid receptor (gr) was higher in fish fed gel by 1 h and higher in fish dipped in FW by 6 h. Expression of mineralocorticoid receptor (mr) was affected by the FW dip (higher in control fish) and diet (higher in those fed gel) by 1 h. By 24 h, there were no lasting effects of the FW dip, and the effects of diet, possibly compounded by responses to repeated handling and sampling, were more pronounced. Overall, these data indicate that S. rivoliana can quickly recover from a 5 min FW dip and diet plays a pivotal role in modulating physiological stress responses to routine handling and maintenance practices in aquaculture.