Consequences of Food Restriction on Short-Term Growth Variation and on Plasma Circulating Hormones inOreochromis niloticusin Relation to Sex

Abstract

In tilapia, there is a sex-related growth difference between males and females. This study tried to detect any correlation between the somatic growth and the plasma endocrine status. For this, individually marked (Floy-tags) male and female tilapia (BW 82±10 g) were either starved or fed on different daily food rations (1, 2, or 3% of the biomass) during 15 days. We have found that specific growth rates (SGR) were positively and significantly related to feeding levels. Growth hormone (GH) plasma levels tended to increase with the decrease in food levels, and thus with the decrease in growth rate. No significant correlation was found between GH levels and SGR. Triiodothyronine (T3) levels in well-fed fish were higher than those in restricted fish (0 and 1%), but no differences in thyroxine (T4) levels were observed. No significant relationship was found between plasma levels of steroid hormones and feeding ration, even though 11-ketotestosterone (11-KT) levels tended to increase with the ration in fed males. SGR were not significantly different between males and females at the same feeding level, but taken as a whole, they were significantly different in favor of males (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 17β-estradiol (17β-E2). Males and females exhibited significant differences in T3levels (respectively 4.25±0.18 and 2.71±0.09 pmol/ml), whatever the food ration, but no significant differences in T4levels were observed except in the high-ration group. The correlation between T3levels and SGR was low but stronger in males (r2=0.21;n=90) than in females (r2=0.10;n=105). The slope of the log–log regression of T3levels with body weight was much lower in females (b=0.87) than in males (b=1.31). This relationship suggests the involvement of T3in 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 T3and 11-KT levels (respectivelyr2=0.12;n=82 andr2=0.08;n=89), while no correlation was found between the levels of T3and 17β-E2. T3plasma levels were found to be the most different parameter between males and females. This hormone seemed to be involved in the control of somatic growth, and could explain the differential growth rate between males and females.