42 DECEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG As global aquaculture continues to develop cultivation of fast-growing fish species with highly desirable flesh quality, increasing effort is being directed toward the domestication of Seriola species such as Japanese yellowtail (S. quinqueradiata), greater amberjack (S. dumerili), almaco jack (S. rivoliana) and yellowtail kingfish (S. lalandi). These fish are found in subtropical seas and may reach 4 kg in 12 months, producing a firm white-fleshed fillet when processed. Research efforts have led to the successful commercial rearing of several species. However, one of the major hurdles to achieving efficient hatchery production is controlling captive reproduction. Furthermore, in some species, such as the greater amberjack, captive broodstock often experience reproductive failure (Mylonas et al. 2004). In New Zealand and Australia, there is growing interest in farming the yellowtail kingfish. One of the notable features of this species is that it will reproduce spontaneously in captivity. Although this trait is a great advantage for aquaculture, it is more desirable to synchronize spawning to further improve broodstock management protocols in the hatchery. In addition, there is an interest in developing techniques for efficient hand-stripping. This would enable application of biotechnological procedures, such as induced sterility and gamete cryopreservation, and facilitate selective breeding. To achieve these outcomes, the timing of broodstock reproduction must be consistent and predictable. Hormonal Control of Fish Reproduction Fish reproduction is controlled physiologically by a cascade of hormones that are produced in and released from the brain, pituitary gland and gonads. Gonadotropin-releasing hormone (GnRH) is a key regulatory hormone in this process (Bentley 1998). It is produced in the hypothalamus and acts on the pituitary, a small gland attached to the base of the brain that synthesizes a number of different hormones. Gonadotropinreleasing hormone specifically targets populations of pituitary cells that produce gonadotropic (GtHs) hormones. Following stimulation by GnRH, GtHs are released into the circulatory system where they ultimately will target specific gonadal cells, stimulating the production of sex steroids. These gonadal steroid hormones drive the development and maturation of male and female gametes. Reproductive failure in fish is often the result of a block at a specific level of the hormonal cascade (Dufour et al. 1983). Therefore, ensuring that the reproductive endocrine system remains in a physiological balance in broodstock is essential for maintaining a productive hatchery. Reproduction in fish can be manipulated through application Gonadotropin-releasing Hormone Implants Induce Spawning in Female Yellowtail Kingfish Seriola lalandi S. Muncaster, S.M.J. Pether, I. Van de Ven, A. Marchant, G. Irvine, S.L. Johnson, P. Rosengrave, P.M. Lokman and J.E. Symonds of various reproductive hormones. The preparation of hormone used may range from a crude extract, such as a pituitary homogenate that contains a mixture of hormones, including GtHs, to synthetic hormone analogues that may be much more potent than the endogenous hormone. One of the most common hormonal treatments used in modern aquaculture involves application of GnRH analogues (GnRHa), which are synthetic mimics of GnRH with intentionally modified amino acid sequences. Analogues are more resistant to enzymatic degradation and have a high potency, up to several hundredfold greater than the endogenous neuropeptide (Lovejoy et al. 1995). Moreover, as a small molecule, GnRHa does not elicit an immune response within treated fish (Donaldson 1996, Mañanós et al. 2009). Although GnRHa can be administered as a simple injection, incorporating the hormone into an implant that allows slow release is more effective (Mylonas et al. 2010). An injection often requires a second treatment within days to be effective, while implants tend to produce sustained concentrations of GnRHa at or above effective physiological thresholds (Zohar and Mylonas 2001). This kind of GnRHa treatment stimulates the pituitary to produce and release gonadotropins and, therefore, activate the cascading effects of the reproductive endocrine system. Studies with other marine broodstock have shown GnRHa implantation to be a highly effective treatment for reproductive failure and synchronization of spawning (Garber et al. 2009). The Effect of GnRHa Implants on Yellowtail Kingfish Reproduction At present, the commercial production of yellowtail kingfish in New Zealand and Australia is relatively small. The majority of research in New Zealand has been conducted at the Bream Bay Aquaculture Park, operated by the National Institute of Water and Atmospheric Research (NIWA). Reliable protocols for hatchery production of this species have been developed over the last ten years at this site and research efforts are currently focused on biotechnology and improved broodstock management (Symonds et al. 2012). A study was conducted to assess the efficacy of a slow-release GnRHa implant on spawning in female yellowtail kingfish, with the goal of improving spawning synchronization and the potential for hand-stripping. Twenty eight captive reared F1-generation yellowtail kingfish females were anaesthetized and implanted with a pellet containing 500 µg of a synthetic GnRHa (D-Ala6, des-Gly10 LHRH ethyl amide) or a sham implant that did not contain
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