52 March 2012 completion of metamorphosis (van Maaren and Daniels 2001). Daniels et al. (2010) recommends that temperature should be maintained at 17 °C for egg incubation and hatching, but then increased by 1 °C per day to 21-22 °C for larval rearing. We examined the effects of different rearing temperatures (12-24 °C) on the growth and survival of flounder larvae spawned by adults collected from waters near Port Aransas, TX in the western Gulf of Mexico. We initially examined the effects of temperature on larval rearing during two feeding stages: (1) from hatching through the end of the rotifer feeding period and (2) from the beginning of Artemia feeding through the onset of metamorphosis. The rate of growth and development in marine fish larvae is more strongly linked to a combination of temperature and age than chronological age alone. Therefore, endpoints for each stage were established by calculating degree-days (DD) as the cumulative product of mean daily temperature (°C) and age (days after hatching). For example, a 10-day old larva reared at 10 °C would accumulate 100 DD. The degree-day ranges that represent the two feeding stages were 0-414 DD for stage 1 and 414612 DD for stage 2. Adult flounder were collected and spawned using the temperature and photoperiod manipulation protocol described previously. For each temperature tested, larvae were reared in three replicate 150-L (18-24 °C) or 265-L (12-18 °C) recirculating tanks at a photoperiod of 10 h light and 14 h dark and mean salinity of 31.5 ± 0.5 ppt. Larvae were fed rotifers (3/mL) from 72-414 DD and Artemia (0.05-0.10/mL) from 414-612 DD. Prior to feeding, live prey were enriched for 12 h with a commercial product consisting of heterotrophic microalgae high in lipids, parTable 1. Spawning parameters and egg production of southern flounder from 2009 through 2011. Tank Spawning period Number of spawns Temperature (°C) Salinity (ppt) Eggs/spawn (x 1,000) Viability (%) Mean Range A Dec. 2009-Mar. 2010 68 18.3 ± 0.9 30.3 ± 0.4 151 13-488 6.9 A Nov. 2010-Mar. 2011 121 19.9 ± 0.8 33.6 ± 0.3 353 20-1,859 21.9 B Jan.-Apr. 2010 56 18.7 ± 0.8 33.7 ± 0.2 230 7-956 12.9 B Dec. 2010-Mar. 2011 72 19.3 ± 0.9 34.6 ± 0.6 156 20-416 31.4 ticularly those containing the fatty acid docosahexaenoic acid (DHA). For stage 1, eggs (10/L) were placed in each tank at 18 °C. On the day after hatching, temperature in the larval rearing tanks was either held constant or gradually changed by 2 °C/day until reaching 12, 15, 21 or 24 °C. For stage 2, eggs (10/L) were placed in six replicate tanks and larvae reared at a constant temperature of 18° C. Two days prior to the start of Artemia feeding, larvae were counted and moved into additional rearing tanks (5/L) at 18 °C. At 414 DD, Artemia feeding commenced and tank temperatures were either held constant (at 18 °C) or gradually changed by 2 °C/day until reaching 12, 15, 21 or 24 °C. Separate experiments were conducted for temperatures ranging from 18-24 °C (trial 1) and 12-18 °C (trial 2). At the end of each study, the standard length of 20 larvae from each tank was measured and all remaining fish counted to calculate survival (Fig. 5). Survival was calculated by dividing the number of larvae remaining by the number of eggs or larvae initially placed in each tank, expressed as a percentage. The effect of temperature was evaluated with a one-way ANOVA followed by a Tukey’s test for multiple comparisons of means using SYSTAT 10.0 (SPSS Inc., 2000, Chicago, IL). Larval growth and survival during the rotifer feeding period (stage 1) was greatest at 18 °C and significantly less at higher or lower temperatures (Table 2). Survival was greatly improved at all tested temperatures when larvae were first held for 414 DD at 18 °C (Table 2, stage 2) and was significantly lower only at the temperature extremes. Further, growth was depressed only at temperatures less than 18 °C. Because survival and growth improved when the temperature change was delayed until the start of Artemia feeding, we conducted an additional study in which larvae were reared for one week (126 DD) at 18 °C and temperature gradually increased to 21 or 24 °C as described above. Upon study termination at the end of the rotifer feeding period (414 DD), survival was significantly greater at 18 °C although there were no significant differences in larval growth (Table 3). Cumulatively these results indicate that growth and survival of newly hatched southern flounder larvae from the western Gulf of Mexico is greatest at 18 °C and highly sensitive to relatively small changes in temperature (±2 °C), especially during the first three weeks after hatching. This temperature is less than that commonly used to (Continued on page 54) (Continued from page 50) Fig. 5. Southern flounder larvae a) 8 and b) 32 days after hatching (Photo by Cynthia K. Faulk).
RkJQdWJsaXNoZXIy MjExNDY=