HAYDOCK: GONAD MATURATION OF GULF CROAKER 



DISCUSSION 



MATURATION 



Many experiments have shown that gonad 

 maturation can result from hormone therapy 

 (for reviews see Pickford and Atz, 1957; Ahsan 

 and Hoar, 1963; Atz and Pickford, 1964; and 

 Hoar, 1969), but in most cases this is a long 

 and tedious approach and has proved of prac- 

 tical use only on a short-term basis for eluci- 

 dating mechanisms of hormone action. A re- 

 cently described catheter implant technique 

 (Frogner and Hendrickson, 1970) , allowing fre- 

 quent or continuous administration of hormones, 

 has been used with partial success to mature 

 mullet, Mugil cephalus, with a minimum of dam- 

 age from excessive handling (Shehadeh, person- 

 al communication ) . A mass of tangled catheters 

 is envisioned if this technique were applied to 

 commercial fish production, but the ease of this 

 method may have considerable merit for exper- 

 imental situations. Implanted pituitary glands 

 might also be used to enhance maturation, and 

 this could easily be tested in croakers. 



In the present study, a slight increase in GSI, 

 possibly reflecting enhanced gonad "growth," 

 followed 1 to 2 weeks of hormone injections giv- 

 en every other day to fish held in 22° C water. 

 Even greater "growth" enhancement was ob- 

 served in 14° C water, and these fish could have 

 been spawned using techniques which were fully 

 developed later in the study. However, for prac- 

 tical purposes, it proved simpler to mature 

 croakers in large groups using appropriate 

 schedules of long days, warm water, and abun- 

 dant feeding. 



The fact that fish kept in cold water respond 

 to hormones by gonad enlargement without sub- 

 sequent hydration or ovulation indicates that 

 different temperature thresholds exist for these 

 various processes. It is possible that the rate 

 of absorption of hormone is considerably slowed 

 in cold water, as some fish do develop a slight 

 reaction following several days of injection at 

 14° C. 



The general relationship of light and temper- 

 ature to gonad maturation is well known (see 

 Harrington, 1959; Henderson, 1963; Wiebe, 

 1968; and Hoar, 1969, for reviews) and requires 



no lengthy discussion here. It is sufficient to 

 note that long-day photoperiod (16L:8D) and 

 high temperature water (22° C) induce gonad 

 maturation in croakers several months prior to 

 the normal breeding season observed in the 

 Salton Sea. Also, a combination of long days 

 and low temperature (14° C) will retard the 

 normal GSI decline when the fish have been cap- 

 tured at the peak of breeding. This technique 

 may prove useful for maintaining fish in a ma- 

 ture state for prolonged periods; such fish may 

 be subsequently spawned following transfer to 

 warm water (22° C) for a period of 1 day. 



Studies on the relationship between maturity 

 and spawning indicate the existence of a GSI 

 threshold value of about 5 %, below which hor- 

 mone injections are ineffective. Also, fish 

 brought to maturity with photoperiod and tem- 

 perature control eventually resorb gonadal tis- 

 sues if they are not subsequently spawned. This 

 resorption process requires several weeks, and 

 the gonad will not grow in response to photo- 

 period and temperature during this time. Fish 

 which are spawned with hormones do not show 

 a refractory state and can be respawned within 

 a few weeks. The practical implication of these 

 findings is that the GSI of maturing fish should 

 be frequently checked so that spawning can be- 

 gin soon after the 5 'r GSI threshold is reached 

 and the fish should be spawned before they reach 

 the maximum GSI value and begin gonad re- 

 sorption. A useful approach would be to hold 

 stock supplies of fish on short days at low tem- 

 perature and mature separate groups as needed 

 for experiments. 



Samples of croakers taken throughout the 

 year from Salton Sea showed that maturation 

 is quite rapid, the GSI increasing from 2 to 10 % 

 in a little over 1 month. It is probable that the 

 increased light, temperature, and food stimu- 

 lation available in the laboratory could bring 

 about even more rapid maturation, but the pro- 

 per fish (early spring) to test this were not ob- 

 tained during this study. 



HYDRATION 



Hormone-induced gonadal hydration is a rel- 

 atively rapid phenomenon which is completed 



173 



