HAYDOCK: GONAD MATURATION OK OLLF CROAKER 



case of a standard 1 mg dose of salmon pitui- 

 tary, which caused hydration, but produced eggs 

 from only one fish in the last test (late June 

 1970) carried out with the same stock of fish 

 which had been spawned regularly with the same 

 dose over the prior 2 months. 



FERTILIZATION 



Several early attempts to fertilize gulf croaker 

 eggs all ended in failure. These eggs were ob- 

 tained from hormone-induced spawning, and 

 they appeared normal in all respects; however, 

 the sperm mass was dispersed in the water some 

 time prior to adding the eggs. Later studies 

 showed that no fertilization resulted when the 

 sperm and eggs were mixed more than 30 sec 

 after sperm had been placed in water, while eggs 

 retained their ability to be fei-tilized for several 

 minutes when kept in water and for several 

 hours when stored in moist chambers. The early 

 failures to fertilize eggs thus resulted from not 

 utilizing diluted sperm quickly enough. It is 

 well known that sperm may be stored for long 

 periods of time if it is maintained in concen- 

 trated form or is not activated by the diluent. 

 The rapid decrease in the viability of sperm in 

 water is probably important for maintaining the 

 genetic integrity of the spawners; its signifi- 

 cance for practical laboratory work is that 

 sperm should be added to the eggs and not vice 

 versa. 



Sperm tended to be more active and to re- 

 main motile longer in water from the Salton Sea 

 than in ordinary seawater from La Jolla, Calif. 

 Moreover, developing eggs always floated in 

 Salton Sea water (salinity in 1970, about ;57 /i. 

 on the basis of total dissolved solids) , while they 

 sank in La Jolla seawater (33.5 ',,,). These ob- 

 servations may have important implications for 

 the salinity tolerance and adaptability of Salton 

 Sea fishes (transplanted originally from the Gulf 

 of California), matters of crucial interest in the 

 initiation of this study of fish reproductive physi- 

 ology. 



Several batches of eggs obtained from hor- 

 mone-induced spawning were allowed to develop 

 to hatching, and a few of the resulting larvae 

 were reared to metamorphosis in the laboratory 



on a diet of rotifiers, Bianchionus plicatilis,io\- 

 lowed by brine shrimp, Artemia sali)ia, nauplii. 

 Thus the entire life history of the gulf croaker 

 can probably be completed under controlled lab- 

 oratory conditions. This opens up the jjossibility 

 of using this species for many other biological 

 studies where large numbers (50,000-100,000) 

 of pelagic eggs are desired from a marine spe- 

 cies of known genetic history. Some of these 

 studies are now in progress (Robert C. May, 

 Scripps Institution of Oceanography, personal 

 communication) . It is hoped that future studies 

 will include comparative work on this species, 

 especially with respect to the possible adapta- 

 tions of Salton Sea croakers since their sepa- 

 ration from the Gulf of California population. 



SUMMARY 



1. Adult and immature gulf croakers captured 

 by beach seining in the Salton Sea were trans- 

 ported to the Fishery-Oceanography Center lab- 

 oratory in La Jolla, Calif., and used in labora- 

 tory studies on gonad maturation and hormone- 

 induced spawning. 



2. A bacterial disease which invariably devel- 

 oped on recently captured or frequently handled 

 fish was successfully treated with Fui'acin anti- 

 liiotic. 



3. Long photoperiods (16 hr of light ijer 24 

 hr) and warm water (22° C) , along with optimal 

 feeding, accelerated the gonadal maturation of 

 females captured prior to their natural cycle of 

 gonadal maturation. These fish were ready to 

 spawn in the laboratory 1 to 3 months prior to 

 the spawning season observed in the Salton Sea. 

 Male fish became ripe under all combinations of 

 laboratory conditions and remained ripe 

 throughout the study. 



4. Concomitant field studies confirmed earlier 

 work showing that female croakers ripened in 

 April, while day length was increasing, and 

 spawned when the water temperature reached 

 about 20° C; peak spawning occurred in May 

 of 1969 and 1970. 



5. Mature fish, captured during the spawning 

 season at the Salton Sea, quickly resorbed their 

 gonads when held under short photoperiods (10 

 hr of light) in the laboratory, but similar fish 



177 



