WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 43 (CONTINUED ON PAGE 44) One of the goals of this study was to enable hand-stripping of yellowtail kingfish. Eggs can be often collected prior to spawning by applying abdominal pressure to sedated fish. For eggs to be easily released and viable, developing oocytes must be hydrated and ovulated. Gonad biopsies were conducted on 14 females at the end of this trial. Three fish had hydrated oocytes and all were from the hormone-treated group. Despite the presence of hydrated oocytes in the ovary, no eggs could be collected by hand-stripping. Implications While yellowtail kingfish are not prone to reproductive dysfunction in captivity, there is interest in enhancing broodstock management, including synchronization of spawning and developing techniques to hand-strip broodfish. The use of GnRHa implants is a common method to enable this. Slowrelease pellets containing 500 µg GnRHa effectively stimulated ovulation and spawning in female yellowtail kingfish. Despite hormonal manipulation, no female fish were successfully hand-stripped. Furthermore, the viability of spawned eggs was reduced. Based on these findings, the commercial application of this treatment dosage is not recommended for this species. However, further investigation on the dosage and timing of hormone treatment may be justified. Notes S. Muncaster, School of Applied Science, Bay of Plenty Polytechnic, Tauranga 3143, New Zealand. Email: simon. muncaster@boppoly.ac.nz S.M.J. Pether, I. Van de Ven, A. Marchant, G. Irvine and J.E. Symonds, Bream Bay Aquaculture Research Park, NIWA, Ruakaka, New Zealand. Email: jane.symonds@niwa.co.nz S.L. Johnson and P. Rosengrave, Department of Anatomy, University of Otago, Dunedin 9016, New Zealand. P.M. Lokman, Department of Zoology, University of Otago, Dunedin 9016, New Zealand. hormone (‘control’). Fish were implanted at the beginning of the spawning season at Bream Bay, New Zealand. For each fish, the number of spawning events, volume of sinking and floating eggs released, fertilization percentage and volume of viable eggs (floating egg volume × percentage fertilisation) produced were recorded daily over 33 days. At the end of the trial, gonad biopsies were conducted and hand-stripping of any female with hydrated oocytes was attempted. Fish in both groups spawned during the course of the trial. However fish that were administered GnRHa spawned more times, with eggs collected on 30 days, compared to 21 days for control fish. In addition, the total volume of eggs spawned by hormone-treated fish (24.8 L) was greater than that of control fish (13.9 L; Fig. 1). Although GnRHa implants were effective in stimulating gonadal development and spawning, the study did not demonstrate overall treatment success in terms of egg viability. One indicator of the viability of eggs from fish that produce floating or pelagic eggs, such as yellowtail kingfish, is the volume of eggs that remain buoyant after spawning (Kjorsvik et al. 1990). The proportion of eggs that are fertilized in a batch can also be a reliable indicator of quality (Hansen and Puvanendran 2010). The mean fertilization rate of eggs produced by hormone-treated fish was 74 percent, compared to 88 percent from control fish. Similarly, the mean volume of viable eggs spawned by hormonetreated fish was significantly less than that from control fish (Fig. 2). Supplementary treatment with GnRHa induces ovulation and spawning in female yellowtail kingfish, but this has negative implications for the quality of released eggs. Hormonal manipulation reduces egg quality in some species of fish (Papanikos et al. 2003, Avery et al. 2004). In some cases this effect may be related to the dose of hormone administered (Mylonas et al. 1992). It is possible that the 500-µg implants used in this study were inappropriate for the broodfish size. However, broodfish were 8-16 kg, equating to similar doses used in other studies (Mylonas et al. 2004). Nevertheless, a single-dose implant was used in the current study, meaning that physiological dosage varied among individuals of different size. Future studies should test a range of hormone concentrations and assess the effect on egg quality. FIGURE 1. Total volume (L) of eggs spawned during the 33-d experiment. FIGURE 2. Mean and standard error of volume (mL) of viable eggs spawned per day.
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