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Title: Effects of broodstock diet and environmental iodidi concentrations on larval growth, survival, egg and whole body concentrations of thyroid hormones and cortisol in Pacific threadfin, Polydactylus sexfilis 
Author: Witt, Elisha M
Date: 2008
Abstract: Pacific threadfin (Polydactylus sexftlis), known locally by its Polynesian name "moi", is rapidly becoming a premier aquaculture species in Hawai'i and throughout the Indo-Pacific. Nevertheless, threadfin culture is suffering from extraordinary loss of seed stock during the pre-metamorphic and metamorphic periods, and from dramatic size variation among animals after metamorphosis that leads to cannibalism. The objectives of my studies were to: 1) determine the relationship, if any, between diet of threadfin broodstock and thyroid hormone content of fertilized eggs; 2) examine the effects of potassium iodide (KI) supplementation to broodstock rearing tanks on thyroid hormone concentrations in fertilized eggs; and 3) examine effects of iodide concentration of rearing tanks on larval growth, survival, metamorphosis, and whole body concentrations of thyroid hormones and cortisol. Broodstock fed a diet rich in iodine produced fertilized eggs with significantly higher thyroxine (T 4) and triiodothyronine (T3) levels than eggs produced by broodstock fed either a raw diet of squid, lake smelt and shrimp, or a commercial marine broodstock feed. Fertilized eggs from broodstock fed the iodine-rich feed had a T4 concentration of 3.29 ng/g, significantly greater than 0.09 ng/g and 0.59 ng/g found in eggs from broodstock fed either the raw diet or the commercial feed respectively. Fertilized eggs produced from broodstock receiving the iodine-rich feed had T3 concentration of 6.43 ng/g compared with 0.05 ng/g and 0.21 ng/g for eggs from broodstock fed raw and commercial diets respectively. Broodstock held in tanks supplemented with KI (0.08 mg/L) also produced fertilized eggs with elevated concentrations ofT4 and T3. Eggs from KI-treated broodstock had T4 concentration of 1.91 ng/g, significantly greater than 0.09 ng/g found in eggs from untreated broodstock. Eggs from K1-treated broodstock bad T3 concentration of 7.81 ng/g compared with 0.05 ng/g found in eggs from untreated broodstock. Threadfin larvae reared in ocean water grew significantly larger, and showed increased survival compared with larvae reared in water from a seawater-injected well that bad lower iodide concentrations. Larvae reared in ocean water developed more rapidly than in well seawater. At 14 days post-hatch, 50% of the larvae reared in ocean water reached flexion-stage, compared with 15% in well seawater. In ocean water-reared larvae, whole body T4 concentrations increased sharply from 0.4-0.5 ng/g at hatching through day 13 post-hatch to 1.9 ng/g on day 15, and declined gradually to 0.5 ng/g by day 23. Larvae reared in well seawater did no show a peak in T4, as levels increased gradually from 0.2 ng/g on day 1 to 2.4 ng/g by day 25. Profiles OfT3 were similar between the two groups, decreasing from 0.17 ng/g at 8 h before hatching to 0.02-0.04 ng/g by day 7 post-hatch, and then gradually increasing to 0.45 ng/g by day 23. Whole body concentration of cortisol was 0.5 ng/g prior to hatching for both groups, increasing to 27-30 ng/g by day 5. Cortisol levels fluctuated during days 7 to 25 post-hatch between 12 and 26 ng/g for ocean water-reared larvae, and between 14 and 32 ng/g for larvae reared in well seawater. Absence of a peak in the T4 profile in well seawater-reared larvae may indicate incomplete synchronization of development or metamorphosis. Threadfin larvae reared in K1-supplemented well seawater grew significantly larger, and developed more rapidly than larvae reared in well seawater with lower iodide concentration. At 13 days post-hatch 42% of larvae reared in K1-treated seawater reached flexion-stage compared with 26% of untreated larvae. By 15 days post 79"10 of larvae reared in iodide rich seawater had reached flexion-stage compared with 66% of larvae reared in seawater without iodide supplementation. Survival to day 25 however, did not differ statistically between control and treated larval groups. Significant diffirences in whole body T4 and T3 profiles were seen in larval cohorts from broodstock reared in untreated and KI-treated seawater and between larvae reared in untreated and KI -treated rearing water. These results suggest the importance of environmental iodide in maternal deposition of thyroid hormones in eggs, larval metamorphosis, and subsequent survival and growth. possibly through the synthesis of thyroid hormones.
Description: Thesis (M.S.)--University of Hawaii at Manoa, 2008. Includes bibliographical references (leaves 79-82). xii, 82 leaves, bound ill. 29 cm
URI: http://hdl.handle.net/10125/20393
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.

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