HAYDOCK: GONAD MATURATION OF GULF CROAKER 



Table 2. — GSI values of female Bairdiella icistia cap- 

 tured in Salton Sea compared with captured fish matured 

 in the laboratory. 



Group 



Dots of 

 sample 



Weight 

 (mean ± SD) 



GSI 

 (mean ± SD) 



Group 1- Laboratory stock fish sampled irregularly. All fish kept on 

 16L:8D photoperiod at 22° C. 



Group 2. Laboratory stock seporated from group 1 and maintained on 

 8L:160 photoperiod at 14° C until 16.11-70, when they were 

 switchea step-wise (15 min/day) to 16L:8D and 22° C. 



Group 3: Young fish captured during their first breeding season in 

 Salton Sea. 



Group A: Fish more than 1 year old, captured during the breeding 

 season in Salton Sea. 



Group 5: Fish more than I year old, captured following the breeding 

 season in Salton Sea ond matured early under laboratory 

 conditions of 15L:9D and 14° to 16° C. 



1 At capture. 



before their normal breeding season, reached 

 maturity in the laboratory sometime prior to 

 being sampled in mid-March 1969 (Table 2, 

 group 5). These fish experienced 15L:9D and 

 ambient La Jolla seawater temperature (14°- 

 16° C) during 4 months in the laboratory. Cold 

 water may slow down the maturation process, 

 but it is evidently not as important as the stim- 

 ulation of long days. 



A second sample of adult fish was captured in 

 mid-May (at the peak of the breeding season), 

 subjected to various experimental laboratory 

 conditions and sampled every 2 weeks to deter- 

 mine the status of their GSI (Fig. 2). Fish 

 maintained on long days (16L:8D) at 14° C and 

 22° C showed a slow decline in GSI from a high 

 of 12 9^ at capture to below 5 % by late-August 

 and September. The GSI values at 22° C were 

 more variable and, in general, showed a more 

 rapid decline than those at 14° C. Similar fish 

 given 10L:14D at high and low temperatures 

 showed a similar but much more rapid decline in 

 GSI, and their GSI also declined to a lower 



overall level (1-2 '/c by mid-August) than that 

 of fish which never experienced short-day con- 

 ditions. 



Both groups of short-day fish subsequently 

 showed a slow but steady increase in their GSI 

 in response to having the photoperiod increased 

 15 min/day from lOL to 16L. Although this 

 increase was not followed through a complete 

 cycle, it was evident that exposure to long days 

 for 2 to 3 months would have been required to 

 bring the fish up to the GSI level necessary for 

 spawning (about 5 % — see below). 



It is likely that adult fish brought into the 

 laboratory just prior to the normal increase in 

 GSI observed in the Salton Sea would respond 

 rather quickly, I estimate within 1 month, if 

 they were given adequate light, temperature, and 

 food. It may also be possible to mature fish 

 rapidly after they have gone through their na- 

 tural GSI decrease (Fig. 1), but this was not 

 tested. 



Effects of Hormones on Maturation of Fish 

 Maintained in the Laboratory 



Groups of fish maintained on various light and 

 temperature regimes were subjected to hormonal 

 treatments to enhance gonad maturation. Adult 

 fish captured prior to the breeding season were 

 not available for these experiments, which were 

 conducted after the spawning season on fish 

 undergoing a decline in GSI, as in Fig. 2. None- 

 theless, the results obtained probably indicate 

 the extent to which maturation can be influenced 

 by hormone treatment. In croakers, the tech- 

 nique of hormone-induced maturation is of rel- 

 atively little practical importance, since fish can 

 be matured by appropriate manipulation of 

 photoperiod, temperature, and feeding sched- 

 ules. The results are, therefore, reported for 

 their possible application to other species in 

 which maturation proves more intractable. 



Since the treatments were carried out on fish 

 being used for photoperiod and temperature ex- 

 periments, the results can only be evaluated in 

 relation to the GSI value of the population under 

 each set of conditions. In some cases, sham-in- 

 jected controls were used while in others unin- 

 jected fish sampled for the light- temperature 



165 



