Abstract
The ovarian fluid properties of the three‐spined stickleback Gasterosteus aculeatus were studied in overripe and non‐overripe ovulated female sticklebacks and in females that were implanted with Silastic capsules containing testosterone (T), oestradiol (E2), 17,20 β ‐dihydroxypregn‐4‐en‐3‐one (17,20 β ‐P) or progesterone (P4) into the abdominal cavity. Overripe females had less ovarian fluid than non‐overripe females, but with higher dry mass, higher protein concentration and lower viscosity. T and 17,20 β ‐P increased the amount of ovarian fluid and the fluid protein concentration was increased by 17,20 β ‐P. 1‐D sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS‐PAGE) showed that ovarian fluid contains several proteins, with high individual variability but with no consistent differences between groups. Some of the ovarian fluid proteins appeared to correspond to proteins from the eggs. The results suggest that secretion of ovarian fluid may be influenced by steroid hormones and that changes in its properties are related to the overripening of ovulated eggs. In at least some respects it appears that the changes in the ovarian fluid is a result, rather than the cause of overripening.