HAYDOCK: GONAD MATURATION OF GULF CROAKER 



mone dosage all exceed certain threshold levels, 

 the fish spawned viable eggs an average of 30 

 hr after the first injection. Hormone dosages 

 above threshold and the type of preparation had 

 no apparent effect on this result, which exhibited 

 only small variability. Subthreshold doses of 

 salmon pituitary did delay ovulation, but this de- 

 lay could not be accurately predicted, and there- 

 fore the spawn obtained was never viable. Sub- 

 threshold doses might theoretically be useful if 

 females were to spawn naturally in captivity, 

 but croakers never exhibited complete spawning 

 behavior in tanks following hormonal injections. 

 Injection of oxytocin in hydrated females and 

 in males appeared to cause heightened pre- 

 spawning behavior, with males following and 

 touching the vent of females, but actual spawn- 

 ing was not observed. Hydrated females event- 

 ually expelled their eggs into the tanks if they 

 were not hand-stripped shortly after ovulation. 

 Actually, the relatively constant latency and the 

 fact that fish must be hand-stripped are highly 

 advantageous to scheduling laboratory oper- 

 ations. 



At Salton Sea, eggs of gulf croakers sampled 

 from the plankton and staged at various times 

 during the day and night showed that there is 

 a diurnal pattern of spawning, most early cleav- 

 age stages appearing in the early evening (Whit- 

 ney, 1961). The same diurnal pattern was 

 found in a closely related species from the East 

 Coast, B. chrysura (Kuntz, 1914). When the 

 effect of this diurnal pattern on laboratory 

 spawning was tested by injecting fish at a time 

 corresponding to the beginning of the labora- 

 tory dark cycle, all five injected fish spawned just 

 at "dusk," 24 hr after injection rather than the 

 usual 30 hr. However, such enhancement was 

 not found in any subsequent spawning attempts 

 carried out at many diff"erent times of day and 

 night. Clemens and Sneed (1962) found no 

 change in latency in goldfish, groups of which 

 were injected at 2-hr intervals over a period of 

 12 hr. Evidently, the injection of hormones 

 usually overrides any effect of diurnal spawning 

 patterns. 



Clemens and Sneed (1962) found that the 

 latent period decreased with increasing temper- 



ature, doubling from 12 hr at 30° C to 25 hr 

 at 10° C. Croakers spawn above 20° C in the 

 Salton Sea, and eggs develop to hatching between 

 20° and 30° C in the laboratory (Robert C. May, 

 Scripps Institution of Oceanography, personal 

 communication). However, all laboratory 

 spawning was accomplished at 20° to 22° C, and 

 no tests were run to determine if higher tem- 

 peratures would decrease the latent period. At 

 temperatures below 17° C, the croaker does not 

 hydrate or ovulate in response to hormone in- 

 jection. 



For 50- to 100-g croakers, with GSI levels 

 above 4 to 5 %, a single injection of 50 lU PMS, 

 50 lU HCG, or 1 mg salmon pituitary proved 

 adequate to induce spawning. A dosage of 100 

 lU PMS and 2-3 mg salmon also produced viable 

 eggs, but 5 mg salmon produced nonviable eggs 

 in the four fish tested. A single injection of 10 

 mg (not 5 mg) carp pituitary or 100 III HCG 

 was adequate for hydration but not for ovulation. 



An injection of 20 lU oxytocin or 5 mg DOCA 

 apparently had little or no effect on hydration, 

 although DOCA may have caused some slight 

 change in the GSI. 



Oxytocin may affect spawning directly. Liley 

 (1969) reviews evidence that the spawning re- 

 flex is controlled by behavioral stimulation of 

 the CNS which releases oxytocin. Oxytocin is 

 used up during the reproductive season of fishes, 

 e.g., Fjindulics and Oncorhynchus (Perks, 1969). 



The possibility that a second hormone, acting 

 (in concert or independently) directly on ovula- 

 tion, was absent from HCG or in too low a con- 

 centration in carp pituitary was evaluated in a 

 preliminary way by injecting 10 mg carp or 100 

 lU HCG fish with 20 lU oxytocin at 30 hr or an 

 otherwise inadequate dose (0.1 mg) of salmon 

 24 hr after the initial injection. Evidence was 

 obtained for ovulation shortly after injection 

 (oxytocin) or at 30 hr (salmon), although the 

 eggs usually were not viable. These experiences 

 indicated that hydration alone was not sufficient 

 to initiate ovulation and that the latter may be 

 a separately controlled process. 



The apparent contrast observed with respect 

 to the different abilities of HCG and PMS to 

 hydrate and ovulate fish may possibly be 



175 



