Improving fry quality, survival and growth in commercially farmed fish by dietary stimulation of thyroid hormone production in premetamorphosing larvae
Authors: Koven William , Grau Gordon , Ron Benny , Hirano Tetsuya 
Published: March 2022

There is a direct correlation between successful metamorphosis from larvae to post-larvae and the quality of the resultant juveniles or fry. Juvenile quality, in turn, is a major factor influencing fish production level and market price. However, following the profound morphological and physiological changes occurring during metamorphosis, the emerging juveniles in some species characteristically demonstrate heterotrophic growth, poor pigmentation, cannibalism and generally poor survival. The white grouper (Epinephelus aeneus) in Israel and the Pacific threadfin (Polydactylussexfilis) in Hawaii are two promising candidates for mariculture that have high market value but a natural fishery that has sharply declined in recent years. Unfortunately, their potential for culture is severely hampered by variable metamorphic success limiting their production. The main objective was to compare the efficacy and economic viability of dietary or environmental iodine on metamorphic success and juvenile quality in the white grouper and the pink snapper which would lead to improved commercial rearing protocols and increased production of these species both in Israel and the US. The Hawaii Institute of Marine Biology encountered problems with the availability of pink snapper brood stock and larvae and changed to Pacific threadfin or moi which is rapidly becoming a premier aquaculture species in Hawaii and throughout the Indo-Pacific. The white grouper brood stock at the National Center for Mariculture was lost as a result of a viral outbreak following the sudden breakdown of the ozone purification system. In addition, the NCM suffered a devastating fire in the fall of 2007 that completely destroyed the hatchery and laboratory facilities although the BARD project samples were saved. Nevertheless, by studying alternate species a number of valuable findings and conclusions that can contribute to improved metamorphosis in commercially valuable marine species resulted from this collaborative effort. The Israeli group found that exposing white grouper larvae to external TH levels synchronized and increased the rate of metamorphosis. This suggested that sub-optimal synthesis of TH may be a major factor causing size heterogeneity in the larval population and high mortality through cannibalism by their larger more metamorphosed cohorts. Two protocols were developed to enrich the larvae with higher levels of the TH precursor, iodine; feeding iodine enriched Artemia or increasing the level of seawater iodine the larvae are exposed to. Results of accumulated iodine in gilthead seabream larvae indicated that the absorption of iodine from the water is markedly more efficient than feeding iodine enriched Artemia nauplii. Samples for TH, which will be analyzed shortly, will be able to determine if another dietary factor is lacking to effectively utilize surplus tissue iodine for TH synthesis. Moreover, these samples will also clarify which approach to enriching larvae with iodine, through the live food or exposure to iodine enriched seawater is the most efficient and cost effective. The American group found that moi larvae reared in ocean water, which possessed substantially higher iodine levels than those found in seawater well water, grew significantly larger, and showed increased survival compared with well water reared larvae. Larvae reared in ocean water also progressed more rapidly through developmental stages than those in low-iodine well seawater. In collaboration with Israeli counterparts, a highly specific and precise radioimmunoassay procedure for thyroid hormones and cortisol was developed. Taken altogether, the combined Hawaiian and Israeli collaborative research suggests that for teleost species of commercial value, adequate levels of environmental iodine are more determinate in metamorphosis than iodine levels in the live zooplankton food provided to the larvae. Insuring sufficiently high enough iodine in the ambient seawater offers a much more economical solution to improved metamorphosis than enriching the live food with costly liposomes incorporating iodine rich oils.