Plasma estradiol-17 beta levels and gonadosomatic index in tench (Tinca tinca L.) reared in natural and controlled conditions

The plasma estradiol-17 beta levels (E-2-17 beta) and gonadosomatic index ( GSI) of female tench (Tinca trinca L.) reared outdoors and indoors were stu died from December 1996 to April 1997. Water temperature and daylight were gradually increased (from 10 to 25 degrees C and from 9 to 16 hours per day , respectively) in the indoor group. Animals reared in natural conditions h ad similar plasma levels throughout the experiment but animals under contro lled conditions experienced an increase in these levels in March in relatio n to previous months (4.81 +/- 0.73 ng/ml in March vs. 0.29 +/- 0.03 ng/ml and 0.99 +/- 0.31 ng/ml in January and February, respectively; P < 0.05) an d in relation to animals reared outdoors (4.81 +/- 0.73 ng/ml vs. 0.47 +/- 0.19 ng/ml, respectively; P < 0.05). In April, a sharp decrease in estradio l-17 beta in these animals was observed, probably associated with the end o f vitellogenesis. GSI increased in both groups through the experiment and i ndoor animals showed greater values in March vs outdoor fishes (7.57 +/- 2. 18 us. 4.05 +/- 0.63; P < 0.05). No significant correlation was found betwe en E-2-17 beta and GSI, (r = 0.35). It is concluded that manipulation of th e photoperiod and temperature can modify gonadal development in tench, and this is similar to the situation in other Teleostei species. This research is the first step towards achieving, out of season, spawning in tench.