HAVDOCK: GONAD MATURATION OF GULF CROAKER 



by a single layer of vascularized ovarian tissue. 

 Overripe eggs have no covering but are spotted 

 with areas of coagulated yolk. Usually, eggs 

 taken within 1 to 2 hr of ovulation are perfectly 

 round, 0.7 to 0.8 mm in diameter, and have the 

 single oil drop characteristic of this species. 

 These eggs float in normal Salton Sea water 

 (37 %, ) and are perfectly transparent and viable. 

 Fertilization of viable eggs and development 

 to hatching was followed for random samples 

 of eggs obtained from hormone treatments. 

 Eggs which did not cleave were considered un- 

 fertilized. In most cases, test fertilizations were 

 conducted in charcoal-filtered Salton Sea water, 

 which has an ionic composition different from 

 that of normal seawater (Carpelan, 1961). 

 Fertilization and hatching served as final criteria 

 of the usefulness of various preparations for 

 spawning. 



EXPERIMENTAL RESULTS 



The reproductive process in fishes can be ex- 

 perimentally divided into four phases: (1) go- 

 nad maturation — measured as a slow increase in 

 GSI over several weeks; (2) gonad hydration 

 — the final preparation of reproductive products 

 for spawning; (3) the actual spawning act — re- 

 leasing eggs and sperm; and (4) fertilization 

 and development of the eggs. These phases 

 are interdependent and proceed in a timed se- 

 quence as part of a cycle keyed to external and 

 internal cues which are integrated by the fish. 

 Thus, spawning is the culmination of a whole 

 series of events, each of which has particular 

 physical and biological requirements for success- 

 ful completion. 



Although each process in the spawning cycle 

 can be separately evaluated with appropriate ex- 

 periments, it is important to keep in mind that 

 the results must be judged against the general 

 reproductive status of the organism. For ex- 

 ample, the effects of hormone injections differ 

 among croakers with different GSI's. For rea- 

 sons explained in the following section, most of 

 the experiments were focused on the repro- 

 ductive processes of female croakers. 



GONAD MATURATION (MALES) 



Effects of Photoperiod and Temperature 



Male croakers became running ripe in the 

 laboratory under all combinations of experi- 

 mental conditions. Fish of 20 to 30 g, captured 

 in October, well beyond the normal breeding 

 season of the species in Salton Sea, became run- 

 ning ripe within 3 weeks after being placed in 

 tanks with various combinations of 14° C and 

 22° C water and 16L:8D or 8L: 16D photoperiod 

 schedules. Adult fish, captured at the height 

 of the breeding season in May and June, were 

 still ripe under all laboratory conditions in No- 

 vember and remained in this state until the end 

 of the experiments in July of the following year. 

 In the field, males become running ripe at least 

 1 month prior to the natural spawning season 

 and can thus serve as indicators of the ap- 

 proaching breeding season in Salton Sea. They 

 are not ripe prior to this time or after about 1 

 month following the spawning season, although 

 water temperatures and day lengths are similar 

 to those maintained in some laboratory situa- 

 tions in which the males did remain running 

 ripe. It may be that either the absence of the 

 normal Salton Sea cycle of light and temperature 

 conditions (in which the fish experience very 

 warm water — 25°-30° C — followed by a period 

 of winter dormancy), or the omission of the 

 normal spawning act in the laboratory, helped 

 to maintain the fish in running ripe condition. 

 The threshold at which environmental factors 

 induce spermiation may also be quite low, and in 

 the laboratory, food, light intensity, photoperiod, 

 and temperature acting in concert may have ex- 

 ceeded this level. 



Effects of Hormones 



Hormone injection of 1 mg salmon pituitary 

 caused a seminal thinning response similar to 

 that discussed by Clemens and Grant (1964). 

 In comparison, the milt taken from uninjected 

 fish was quite viscous and formed clots which 

 had to be mechanically broken up to disperse 

 the sperm. However, quantitative injections of 

 smaller doses of salmon pituitary caused no ap- 

 parent increase in the percent water of the testes 

 (Table 1). In males, the reaction to hormones 



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