Extended Incubation Feeding Protocol and Hypersaline Acclimation of Larval Spotted Seatrout (Cynoscion Nebulosus)
Spotted seatrout (Cynoscion nebulosus) is a popular sport fish and highly targeted by anglers on the Texas coast. To offset pressures by anglers and natural events, Texas Parks and Wildlife Department implemented a stock enhancement program in which hatchery-reared fish are released into the wild to augment natural stocks. One of these hatcheries is located on a hypersaline lagoon resulting in fish often being challenged by hypersaline conditions in outdoor rearing ponds. The objectives of this study were to determine: 1) a feeding protocol for use during extended incubation that produces the highest survival and growth of larvae and 2) whether gradual hypersaline acclimation during extended incubation leads to increased growth and survival of larvae when subjected to hypersaline conditions. Two feeding trials (12 days each) were conducted to evaluate rotifer enrichments used in the diet of larval spotted seatrout. Treatments in both feeding trials (100 larvae/60 L tank, 4 replicates/treatment) included Algamac-3050®, Easy Dry Selco®, Ori-one®, and RotiGrow Plus®. After the feeding trials, a salinity experiment (t=17 days, 100 larvae/60 L tank, 4 replicates/treatment) was conducted in which larvae were subjected to hypersaline (50 ppt) conditions at 3, 6, 9, and 12 days post hatch (dph). Initial salinity was 35 ppt. Starting at day 3, salinity was raised by 1 ppt per day until larvae were subjected to 50 ppt at their assigned treatment day. In both feeding trials, survival was significantly (P<0.05) affected by treatment. In the first trial, Algamac-3050® had significantly higher survival (75.2±6.3%) than the other three treatments, whereas in the second trial, Algamac-3050® (83.2±2.8%) and Easy Dry Selco® (79.8±6.0%) had significantly higher survival than the other two treatments. In both feeding trials, larvae in the Algamac-3050® treatment exhibited significantly higher growth (i.e., length, weight, specific growth rate, and percent weight gain) than any of the other treatments. Based on these results, Algamac-3050® was selected for use in the salinity trial. In the salinity trial, survival was affected by treatment, with larvae exposed to hypersaline (50 ppt) conditions at 3 dph having lower survival (8.2±5.8%) than the other three groups. Survival improved significantly when fish were acclimated until 6 (52.5±7.0%) or 9 (65.0±7.7%) dph, although there was no improvement in survival between 9 and 12 (65.3±7.1%) dph. Growth parameters followed the same trend as survival, with the 3 dph group experiencing the least growth. Acclimation until 6 or 9 dph improved growth, but further acclimation to 12 dph did not yield any further improvement. The results of this study suggest that larval rearing of spotted seatrout in hypersaline conditions may be improved by increasing the incubation time beyond three days, and by gradually acclimating larvae to a higher salinity during this extended incubation.