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Serum gonadotropin (GtH) levels were determined by radioimmunoassay (RIA) in adult regressed milkfish injected or implanted with various doses of salmon pituitary homogenate (SPH). Gonadotropin profiles of injected or implanted milkfish show similar trends. Elevation of serum GtH occurred a day after treatment in all groups irrespective of the administered dose. The level dropped after 2 or 3 days and in the implanted fish, was at the initial level 10 days after treatment. The expected slow and sustained release of hormone from the SPH pellet was not obtained even at the high dose given. A preliminary investigation of the milkfish pituitary homogenate using the salmon-GtH-RIA system shows no cross reaction of milkfish GtH with salmon GtH antibody, indicating that the milkfish and salmon gonadotropins are immunologically distinct molecules.

Two gonadotropin releasing hormone analogues (GnRHa), [D-Ala6, Pro9-ethylamide] mammalian GnRH and [D-Arg6, Pro9-ethylamide] salmon GnRH, were shown to induce spawning in see bass. A single injection of GnRHa induced one spawning, but multiple (two to four) injections spaced 24 h apart produced one to four spawnings in individual females. More striking were the sea bass that spawned up to five times after an osmotic pump, which releases GnRHa for 14 days, was implanted. A less expensive, but equally effective method was implantation of GnRHa in pellets with a cholesterol-cellulose matrix. Multiple spawnings in a single female were produced by only two pellets implanted at one time; individual fish released up to 7 million eggs with good fertility and hatching rates. Individual sea bass were shown to remature in the same season and produce multiple spawnings in both June and September if stimulated with GnRHa in pellets. Stimulation of multiple spawnings was not clearly different if the mammalian or salmon GnRH analogues were used or if the stimulation occurred at the new or first quarter moon. Finally, an individual male was shown to be capable of fertilizing the eggs of one female for at least four sequential spawnings, although the fertility and hatching rates were higher if two males were placed with a female. In conclusion, pellets, pumps and repeated injections produced multiple spawnings in sea bass, but the pellets were more reliable, cheaper, and less stressful to the fish.

Nine experiments were conducted to investigate the influence of chronic administration of testosterone (T) and gonadotropin-releasing hormone (GnRH) analogues on first maturation of 4- to 6-year-old fish and rematuration of 6- to over 9-year-old spent/regressed fish. Implantation of T or T in combination with luteinizing hormone-releasing hormone analogue (LHRH-A) had no marked effect on maturation rate of 4-year-old milkfish. The percentage of maturing fish was low and similar to controls in Experiment 1 (T, 31–35%; control, 35%) and Experiment 3 (T, 13%; T plus LHRH-A, 28%; control, 22.2%). Most of the 4-year-old maturing fish were males; maturing females were obtained only from the T-implanted groups in Experiment 1. In Experiment 3, T-implanted maturing females were able to retain yolky eggs whereas maturing control females did not, indicating that testosterone may have enhanced vitellogenesis and maintained the integrity of vitellogenic oocytes. Tank-reared maturing 4-year-old females, about half the size of older first maturing females, were induced to spawn. This is the first case of maturation and spawning of 4-year-old milkfish reared in tanks. As in Experiments 1 and 3, the percentage maturation of spent fish in Experiments 7 and 8 was similar for T-implanted and controls. On the other hand, the 4-year-old immature fish in Experiments 2 and 4, the 5- and 6-year-old immature fish in Experiments 5 and 6, and the spent 6-year-old fish in Experiment 9 were immature or regressed throughout. The factors which may have influenced the results of these experiments include age and reproductive history of the fish, timing of hormone implantation, experimental and holding conditions, and stress.