Responses of cobia Rachycentron canadum larvae to abrupt or gradual changes in salinity

Abstract

Cobia Rachycentron canadum has recently been recognized as a potential candidate for aquaculture because this species exhibits high growth rates during the larval and juvenile stages. A series of salinity tolerance tests were performed on larval cobia in order to identify the salinity requirements of this species during culture. The effect of spawning salinity on larval tolerance is also discussed. The 18-h survival of cobia larvae 3, 5, 7 and 9 days post-hatch (dph) following abrupt transfer to salinities ranging from 4 to 48 ppt was evaluated using logistic regression. The salinity range within which 90% of the larvae would be expected to survive appeared to be age-dependent and was narrowest at 3 dph (20.1–35.6 ppt) and wider at 7 and 9 dph (7.5–32.8 ppt). The 18-h tolerance of larvae to abrupt changes in salinity was unaltered by spawning salinities of 28.0 and 36.5 ppt. In the second part of the study, rearing salinities were dropped by 5 ppt day − 1 from 32–34 ppt (control) to 5, 10, 15 and/or 20 ppt beginning on 1, 4, 7, 10 or 13 dph. Larval survival from hatching through 10 days following the initial drop in salinity was significantly ( P < 0.05) lower (< 2%) in the low salinity treatments than the control (12–15%) when the salinity drop was initiated 1 and 4 dph. No significant differences in larval survival were detected between the control (12.5%) and 20 ppt treatment (8.9%) when the salinity drop began on 7 dph but survival in the 10 ppt treatment (3.2%) was significantly lower than the control. When the salinity drop was initiated on 10 dph, no significant differences in survival (10.7–14.7%) were detected among treatments. Finally, no significant differences in survival (9.6–15.4%) were found when the salinity drop was initiated 13 dph and terminated 22 dph. However, when a similar study was extended to 28 dph survival from 13 to 28 dph was significantly lower in the 5 (49.4%) and 10 (72.5%) ppt treatments than the control (96.5%) due to disease. No significant differences in standard length were observed for larvae within each experiment irrespective of rearing salinity. The results of this study indicate that rearing cobia larvae in salinities as low as 15 ppt may be possible beginning 13 dph.