Witkowski A., Błachuta J., Kokurewicz B., Kowalewski M., 1989. Changes of gonadosomatic index (GSI) and egg diameter in the grayling Thymallus thymallus (L.) in annual cycle. Acta Ichthyol. Piscat. 19 (2): 21-28.
The most intense growth of grayling ovaries takes place in autumn and winter (September-March), of testes-in summer (July-September). Older females and males all seasons have a higher GSI, and the diameter of developing eggs is higher.