The study of maturational progress in the whitespotted rabbitfish (Siganus sutor) under different photoperiod and light intensity conditions

The rule of light on the timing of maturation and spawning in tropical and subtropical regions is not clear well, because the reproductive cycle in these systems is lunar synchronized. In this study, thus, the effects of different light regimes were investigated on maturational progress of whitespotted rabbitfish, Siganus sutor, the commercial species in Persian Gulf and Oman Sea. During prespawning season, 50 adult fish were randomly divided into ten 300-l tanks (n=5). The fish in control tank received indoor light condition and the fish in each other tanks were exposed to nine different combinations of photoperiod (8L: 16D, 12L: 12D, 16L: 8D) and light intensity (1000, 2000, 3000 lux). After 60 days, GSI and HSI values, serum levels of estradiol (E2), testosterone (T), 17-α hydroxyprogestrone (OHP), calcium (Ca2+) and gonad histology were evaluated for females and males. In females the GSI mean values of exposed fish increased in comparison with control except for fish were kept under 8L, 2000 lux (tank 8). These differences were significant only for fishes in tank 7 (8L, 3000 lux). In the cases of HSI, the results were converse, so that, the most of thanks showed significant decreasing than fishes reared under indoor condition. Morphology and histology study of Ovaries showed three developmental phases including 3, 4 and chiefly 5 that were parallel with GSI values. Fortheremore the serum levels of E2 was recorded between 0.54-15.8 ng/ml in different fish and their mean values were lower than control in all treatmants (P> 0.05). In males, the similar results were obtained. The GSI and HSI mean values in experimental regimes compared with control were upper and lower, respectively, except for fishes were reared in tank 1 (16L, 3000 lux). Testes histology of fishes were reared under different regimes showed signs of stage 5, since no blood vessels observed and thick milt exuded on slight pressure. The mean values of testosterone consentration in fishes were kept in tanks 1 and 6 (12L, 1000 lux) were higher and in other ones were lower than control group. It is also noted that the OHP and Ca2+ had diverse results including increasing and decreasing mean values than control. So, these factors contrary to E2 and T were not suitable to evaluation of maturity in both sexes. On the basis of ovarian structure in stage 5, oocyte development pattern in this species was group synchronous. So, increased mean of GSI versus decreased values of HSI, E2 and perhaps Ca2+ were the signs of improved maturation. But in males, reduced levels of T and similarity of testes morphology in all samples caused that GSI mean value was the only indicator for differentiating among treatments. These findings suggest that alternations were used in light regimes have been the reason of improved maturity in all treatments except fishes reared in tank 8. The ١٠٧ rule of light intensity on induction of maturity was cleared by comparision between fishes in tanks 4, 5, 6 and control group. Because day length was the same whereas fishes in tanks 4, 5 and 6 were exposed to increased light intensity compared with control. This fact verified by results was obtained from fishes in tanks 9 and especially 7, since photoperiod in this group was lower than control. So, higher intensity was considered as the reason of alternations. Contrasting with indoor condition, Induced maturity was also cleared for fishes were kept in tanks 1, 2 and 3, where both of light duration and intensity were increased. But, the rule of photoperiod was individually demonstrated when obtained results were compared with similar light intensities in other treatments. In conclusion, with comparison among different light regimes it is declared that siganids were kept under light condition of tank 2 including 16h light duration combined with 2000 lux intensity showed the best signs of sexual maturation in both sexes. On the basis of this study, setting up the spring light condition during prespawning season induces maturation in withspotted rabbitfish. This improvement not only by influence of photoperiod or light intensity, separately, but obtained through interaction between them. Thus, determination of threshold and resistance to light be recommended before exposure, although using proper regime during suitable time are necessary to achieve purposes considerably.