Temperature and reproduction in tench: effect of a rise in the annual temperature regime on gonadotropin level, gametogenesis and spawning. II. The female.

Female tench were bred for two consecutive years under different thermoperiods; the effect of the thermoperiods on vitellogenesis, spawning, plasma and pituitary gonadotropin (GTH) levels was studied. The temperature did not affect development during the preparatory period. We found no ovarian development below 10 degrees C. The temperature accelerated vitellogenesis only if the mean daily average was more than 10 degrees C, and the rate of increase of that parameter from 10 to 23 degrees C (spawning temperature) determined female fertility. The date of first spawning may be predicted by the summation of degree-days higher than 10 degrees C. GTH secretion increased during vitellogenesis to reach maximal values during spawning. The data indicate a gonadotropin surge at the beginning of each spawning. There was no difference between plasma GTH levels at the various temperatures, but pituitary GTH was higher in the groups bred under the highest temperatures.