Kingfish (Seriola lalandi) adjust to low salinity with only subtle effects to cardiorespiratory and growth performance

At reduced salinities nearing isosmotic conditions, marine fishes experience reduced osmotic gradients between body fluids and the surrounding water, which should lower energetic costs of osmoregulation. Such energy savings on osmoregulation can potentially be directed to other processes such as growth, suggesting the possibility that rearing euryhaline fishes under reduced salinity in aquaculture may be a beneficial practice. The yellowtail kingfish (Seriola lalandi) is one such aquaculture species that appears to benefit from low salinity, as juveniles display faster growth when reared at salinities approaching isosmotic conditions. Here, we tested the hypothesis that, due to reduced osmoregulatory costs, growth is improved in sub-adult kingfish reared at low salinity, and this is associated with a down-regulation of resting cardiorespiratory activity reflecting energetic savings. Furthermore, to gain a more complete picture of how low salinity impacts cardiorespiratory function in kingfish, we also assessed maximal cardiorespiratory responses to enforced exercise. Two experiments were carried out. The first assessed resting and maximum cardiac and respiratory performance of kingfish acclimated to seawater (35 ppt), and after ~20 h (acute) or > 4 weeks (chronic) exposure to brackish water (17 ppt). Acute 17 ppt exposure reduced standard metabolic rate (i.e. resting O2 consumption rate; 179.4 mg O2 center dot h- 1 center dot kg- 1) relative to fish maintained in seawater (198.3 mg O2 center dot h- 1 center dot kg- 1), but this difference was no longer apparent following chronic acclimation to 17 ppt (191.3 mg O2 center dot h- 1 center dot kg- 1). While maximum metabolic rate was not affected by salinity, kingfish both acutely transferred and acclimated to 17 ppt had a reduced excess post-exercise O2 consumption compared to fish in 35 ppt. Acclimation to 17 ppt was also associated with a reduced proportion of ventricle compact myocardium. In a second experiment, we analyzed growth rate and feed conversion efficiency of kingfish acclimated to seawater (35 ppt) or brackish water (24 and 12 ppt) for 8 weeks. Contrary to our initial hypothesis, growth rate and feed conversion efficiency were slightly deteriorated (by ~5 and ~ 6% for growth rate and feed conversion efficiency, respectively) after 4 weeks acclimation to 12 ppt; however, these differences were no longer apparent after 8 weeks acclimation. This study shows neither substantial benefits nor costs of brackish water exposure to cardiorespiratory and growth performance in sub-adult kingfish, suggesting this species can be reared at salinities as low as 12 ppt.