Annual Cycle of Serum Gonadal Steroids and Serum Lipids in Striped Mullet

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Abstract
Monthly and seasonal changes in serum concentrations of testosterone in males, estrogens (estradiol and estrone) in females, and total lipids and cholesterol in both sexes of striped mullet Mugil cephalus were measured by radioimmunoassay (testosterone and estrogens) or colorimetry (lipids and cholesterol). Concentrations of testosterone or estrogens were significantly correlated with gonadosomatic indexes of the respective sex. Resting concentrations were 130 pg/ml serum testosterone and 155 pg/ml serum estrogens from March to September. Peak concentrations of testosterone (10.10 ng/ml) and estrogen (4.50 ng/ml) occurred in November. Gonadal steroid concentrations were at prespawn levels in spent fish caught in December. Both sexes showed a seasonal cycle of serum total lipids and cholesterol. A rapid decline in serum total lipids that occurred just prior to offshore migration and reproductive development (September) may reflect a period of body fat deposition. A subsequent rise in total lipids (September, 418 mg/100 ml, to November, 1,352 mg/100 ml) occurred along with that of gonadal steroids. Cholesterol followed a similar seasonal pattern. These results suggest that the seasonal changes in both serum lipids and gonadal steroids were associated with reproductive development. The data also support the hypothesis that the shortening photoperiod is a major factor in stimulating reproductive activity in striped mullet.