FISHERY BULLETIN: VOL. 70, NO. 4 



INHIBITION OF TESTICULAR 

 REGRESSION BY LOW TEMPERATURE 



An experiment was initiated in June 1968 to 

 resolve whether low temperature treatment in 

 combination with a short photoperiod could pre- 

 vent gonadal regression at the "normal" time 

 (early July). The testes of the beginning con- 

 trols were in the prespawning or postspawning 

 conditions (Stages 4 or 5). Fish were exposed 

 to 10° or 20°C at a 10L/14D photoperiod for 21 

 days. The testes of fish collected from nature 

 at the time of sacrifice (July) of the experimen- 

 tals were regressing (Stage 0). Testicular 

 weights in the 10°C group (x ± SE = 60.7 

 ± 4.1) and the 20°C group (61.0 ± 6.2) did not 

 vary significantly from those (52.2 ± 3.9) in the 

 initial June controls, but those of both groups 

 were significantly greater (P < 0.01) than tes- 

 ticular weights of the July sample from nature 

 (32.1 ± 3.1). The testes of 10 fish exposed to 

 10°C and 6 fish to 20°C were in active spermato- 

 genesis (Stage 3), and 4 in each group were in 

 the prespawning condition (Stage 4). Thus, 

 testicular regression does not occur at the nor- 

 mal time when fish are exposed to temperatures 

 between 10° and 20°C. 



COMPARATIVE EFFECTS OF 10° AND 18°C 



TREATMENT ON GONADAL 



RECRUDESCENCE 



To determine whether there is a differential 

 eflect of 10° and 18°C on gonadal recrudescence, 

 a 21-day experiment was initiated in August 

 1970 with fish having regressing or quiescent 

 gonads (Stages and 1). The effects of these 

 treatments on gonadal weight are presented in 

 Table 8. 



Table 8.— Effect of 21-day 10° and 18°C treatments 

 (13L/11D) on ovarian and testicular weights in GUlich- 

 thys mirabilis. 



•Significantly greater [P < 0.05) than initial controls. 



A significant increase (P < 0.05) in testicu- 

 lar weights occurred at both 10° and 18°C; tes- 

 ticular weights in the two groups were not sig- 

 nificantly different. Spermatogenesis was ini- 

 tiated in all fish in both experimental groups. 

 Ovarian weights in fish exposed to 18 °C were 

 significantly greater (P < 0.05) than those of 

 the initial August controls; however, ovarian 

 weights in the 10°C group were not significantly 

 different than those of the initial controls or 

 those of the 18°C group. Nonetheless, the ova- 

 ries of all fish in both experimental groups were 

 in early phases of vitellogenesis (Stage III). 



These data indicate that there is little or no 

 difference in the rate of initiation of gonadal 

 recrudescence at 10° and 18°C. The possibility 

 exists, however, that following the initiation of 

 recrudescence, the rate of testicular and ovarian 

 growth could be different at the two tempera- 

 tures. 



DISCUSSION 



In previous work (Barlow and de Vlaming, 

 1972; de Vlaming, 1972b), the gonadal cycle of 

 Gillichthys was observed to be closely correlated 

 with seasonal changes in several environmental 

 variables. Gonadal regression occurs as day- 

 length begins to shorten and temperature reach- 

 es a seasonal maximum. The initiation of go- 

 nadal recrudescence coincides with the decline 

 in temperature and the continued decrease in 

 daylength. A majority of the spawning in this 

 species occurs when daylength and temperature 

 are increasing. The data presented here sug- 

 gest that temperature may be important with 

 regard to reproductive cycling, photoperiod act- 

 ing only to modify the responses to temperature. 



GONADAL REGRESSION 



In these laboratory experiments, constant tem- 

 peratures of 24°C and above cause testicular and 

 ovarian regression at any time of the year re- 

 gardless of photoperiod (i.e., photoperiod does 

 not seem to have an influence on gonadal regres- 

 sion) . As temperature is increased above 24°C, 

 shorter treatment periods are required to attain 

 the ovarian and testicular quiescent phases. 



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