World Aquaculture September 2018

50 SEP TEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG Effects of Sudden Changes in Temperature and Salinity on Juveniles With reliable and controllable natural spawning protocols established, the focus then shifted to determining the environmental tolerances for pigfish juveniles. This is especially important for producers using outdoor ponds for grow-out because water temperature and salinity in ponds along the Texas Gulf coast can be highly variable and unpredictable. Pigfish were reared from six separate spawns until the juvenile stage (4.6 ± 0.2 cm TL and 1.5 ± 0.2 gWW) at a constant temperature (23.8 ± 0.3 C) and salinity (32.6 ± 0.6 g/L). Once fish from a single spawn reached the appropriate size, trials were conducted to assess their tolerance to either a sudden decrease in temperature or salinity. Each study was replicated three times and was conducted in 265-L, dark blue, round polyethylene tanks that were partially submerged in a 4,500-L rectangular fiberglass tank. Water exited each round experimental tank into the large rectangular tank via a central standpipe covered with 400-µmNitex mesh. Water circulated from the large tank, through a biofilter, protein skimmer, sand filter, heat pump and returned to the six experimental tanks. Independent 4,500-L tanks were used for each treatment. Acute temperature tolerance. Immediately prior to the start of the experiment, 50 juvenile pigfish were randomly sampled from each spawn and total length and wet weight measured. Fifty juveniles were transferred directly from 24 C to each temperature treatment (12, 15, 18, 21, and 24 C). Substantial mortalities (> 80 percent) occurred among fish transferred to 12 C, so an additional trial was conducted in which juveniles were transferred to intermediate temperatures of 15 C or 18 C for one day and then transferred to 12 C. After the transfer, all fish were fed a dry commercial pelleted feed four times per day at a daily ration of 5 percent of WW. After one week, total length and wet weight of ten fish from each tank were measured. Measurement of length and weight was repeated after two weeks, at which time the study was completed and the number of fish in each tank was determined. After two weeks, very little mortality occurred in the 15, 18, 21, and 24 C treatments; mean survival at these temperatures was 98 ± 3 percent. Survival was lowest (11 ± 8 percent) when fish were transferred directly from 24 to 12 C (Table 1). However, when fish were transferred to an intermediate temperature (15 or 18 C) for one day, 2-wk survival at 12 C increased to 71 ± 8 percent. Growth was minimal at the lower temperatures. Growth generally increased as temperature increased in the 18, 21 and 24 C treatments (Table 1). FIGURE 4. Photothermal regime for maturation of captive pigfish. Dashed line indicates time between reaching 20 C and the initiation of spawning (triangle). FIGURE 5. Normal and accelerated photothermal regimes employed for maturation of captive pigfish. Dashed lines indicate the time between reaching 20 C and the initiation of spawning (triangle). TABLE 1. Total length (TL), wet weight (WW), and survival (percent) ± S.D. of juvenile pigfish two weeks after acute transfer from 24 C to different temperatures. Mean TL and WW at the start of the study were 4.6 ± 0.2 cm and 1.5 ± 0.2 g, respectively. Within each column, values with different superscript letters are significantly different (P < 0.05). Treatment (C) Total length (cm) Wet weight (g) Survival (percent) 12 4.9 ± 0.2ab 1.6 ± 0.1a 11.0 ± 7.6a 15 to 12 3.6 ± 0.2a 1.5 ± 0.1a 68.7 ± 11.0b 18 to 12 4.8 ± 0.3a 1.5 ± 0.3a 72.7 ± 6.1b 15 4.9 ± 0.2ab 1.7 ± 0.1a 96.0 ± 5.3c 18 5.6 ± 0.2bc 2.8 ± 0.2b 98.0 ± 2.0c 21 5.9 ± 0.2c 3.4 ± 0.2bc 99.3 ± 1.2c 24 6.6 ± 0.4d 4.2 ± 0.6c 100.0 ± 0.0c