CONSEQUENCES OF FOOD RESTRICTION ON SHORT-TERM GROWTH VARIATION AND ON PLASMA CIRCULATING HORMONES IN OREOCHROMIS-NILOTICUS IN RELATION TO SEX

In tilapia, there is a sex-related growth difference between males and females. This study tried to detect any correlation between the somat ic growth and the plasma endocrine status. For this, individually mark ed (Floy-tags) male and female tilapia (BW 82 +/- 10 g) were either st arved or fed on different daily food rations (1, 2, or 3% of the bioma ss) during 15 days. We have found that specific growth rates (SGR) wer e positively and significantly related to feeding levels. Growth hormo ne (GH) plasma levels tended to increase with the decrease in food lev els, and thus with the decrease in growth rate, No significant correla tion was found between GH levels and SGR. Triiodothyronine (T-3) level s in well-fed fish were higher than those in restricted fish (0 and 1% ), but no differences in thyroxine (T-4) levels were observed. No sign ificant relationship was found between plasma levels of steroid hormon es and feeding ration, even though 11-ketotestosterone (11-KT) levels tended to increase with the ration in fed males. SGR were not signific antly different between males and females at the same feeding level, b ut taken as a whole, they were significantly different in favor of mal es (P < 0.05). There was no important difference in GH levels between the two sexes. Steroid hormones were, in general, higher in males for 11-KT and in females for 17 beta-estradiol (17 beta-E2). Males and fem ales exhibited significant differences in T-3 levels (respectively 4.2 5 +/- 0.18 and 2.71 +/- 0.09 pmol/ml), whatever the food ration, but n o significant differences in T-4 levels were observed except in the hi gh-ration group. The correlation between T-3 levels and SGR was low bu t stronger in males (r(2) = 0.21; n = 90) than in females (r(2) = 0.10 ; n = 105). The slope of the log-log regression of T-3 levels with bod y weight was much lower in females (b = 0.87) than in males (b = 1.31) . This relationship suggests the involvement of T-3 in tilapia growth and probably in the differential growth between males and females. In both males and females, a significant but low correlation was observed between T-3 and 11-KT levels (respectively r(2) = 0.12; n = 82 and r( 2) = 0.08; n = 89), while no correlation was found between the levels of T-3 and 17 beta-E2. T-3 plasma levels were found to be the most dif ferent parameter between males and females. This hormone seemed to be involved in the control of somatic growth, and could explain the diffe rential growth rate between males and females. (C) 1996 Academic Press , Inc.