Threadfin

Abstract: The specific activity of digestive enzymes; aspartic and serine protease, collagenase, lipase, acid and alkaline amylase, acid and alkaline phosphatase, and chitinase was assayed throughout early development in two species of marine fishes: the Pacific threadfin (Polydactylus sexfilis) and bluefin trevally (Caranx melampygus). Specific enzyme activities were determined on whole larval extracts sampled at selected stages of development, from day 0 to day 30 post-hatching. Similar developmental patterns of enzyme specific activity were observed in the two species, although differences in timing, amplitude and effects of first feeding were noted. Amylase activity increased prior to first feeding, peaking at the middle of the larval period, and becoming nearly undetectable by the time of larval-to-juvenile metamorphosis. Serine protease, collagenase, lipase and alkaline (and acid for threadfin) phosphatase activities increased gradually, followed by sharp increases to a plateau during the second half of larval development. Aspartic protease and chitinase activities, in addition to acid phosphatase (for trevally), were low to undetectable in the first half of development, increasing through metamorphosis. In the trevally only, this group of enzymes exhibited high activity levels at the time of hatching, followed by a precipitous drop. Species-dependent differences in enzyme specific activity at first feeding may have been a result of differences in yolk utilization. These results, taken in the context of earlier reports, lead us to conclude that carbohydrate utilization may play a significant role in the earlier phases of development among some marine larvae, followed by a shift to protein and lipid utilization.

Abstract: Release-recapture studies were conducted with Pacific threadfin, Polydactylus sexfilis, the species given highest priority for stock-enhancement research in Hawaii. Their purpose was to evaluate recruitment potential, dispersal, growth, and differential recapture rates of cultured fingerlings released into shoreline juvenile nursery habitats along the windward (eastern) coast of Oahu, Hawaii. We varied fish size at release, release site, and the seasonal timing of releases using a balanced, randomized-block experimental design. After releases of 20,000 tagged Pacific threadfin in 1993 and about 81,000 in 1994, we recaptured 1705 cultured juveniles in net collections made over a 17-mo period. Presence of cultured fish in net samples depended strongly on the interactive effects of release variables. Size at release had an important effect on recapture rates at all release sites, but this effect varied seasonally. At one of the release sites, larger fish apparently had better survival after winter releases and smaller fish had better survival after summer and fall releases. Release site affected dispersal patterns, recruitment, and recapture rates. The percentage of cultured fish in samples of Pacific threadfin taken 8 mo after release varied from 0% to 64%. Cultured fish showed strong site fidelity at some sites, weak at others. What we considered pilot-scale releases clearly were large enough to approach swamping wild recruitment at Kahana Bay. A key question from this study is how many cultured juvenile Pacific threadfin the Kahana Bay site can support without displacement of wild individuals.

Abstract: Ectoparasite infections can cause death or a decline in the general health of farm-raised finfish. This paper reports the findings from two studies conducted to evaluate the efficacy of hydrogen peroxide as a therapeu-tant for the control of infections of Amyloodinium sp. on cultured Pacific threadfin Polydactylus sexfilis (locally called “moi”). Threadfin with amyloodiniasis collected from a commercial farm were used in both trials. Prior to the trials, and following hydrogen peroxide treatment, the extent of infection was determined by a gill biopsy procedure. An initial trial was conducted in the laboratory to assess the response of juvenile threadfin and Amyloodinium sp. trophonts to hydrogen peroxide exposure at four dosages: 0, 75, 150, or 300 mg/L for 30 min. In a trial on a commercial farm, a hydrogen peroxide treatment at 75 mg/L for 30 min was applied to juvenile threadfin in a grow-out tank. In both trials, hydrogen peroxide was immediately flushed from the culture system with sea-water after the 30 min exposure period. In the laboratory trial, treatment with 300 mg/L hydrogen peroxide resulted in 100% mortality of the exposed group of fish. However, single treatments with hydrogen peroxide at concentrations of 75 or 150 mg/L eliminated Amyloodinium sp. trophonts without causing loss of fish. In the field trial, a single treatment with 75 mg/L hydrogen peroxide greatly reduced levels of Amyloodinium infestation, and a second treatment 6 d later reduced Amyloodinium trophonts to a nondetectable level. These findings suggest that hydrogen peroxide is a suitable chemical for the treatment of amyloodiniasis of cultured, juvenile Pacific threadfin.