Annual reproductive cycle in two free living populations of three-spined stickleback (Gasterosteus aculeatus L.): patterns of ovarian and testicular development

The annual reproductive cycle in two wild populations of three-spined stickleback was studied. Sticklebacks from the Dead Vistula River (Martwa Wisła) (brackish water) and the Oliva Stream (Potok Oliwski) (freshwater) were exposed to annual environmental changes in their natural habitats. Ovaries and livers (females), and testes and kidneys (males) were collected during 1-2 years. The gonadosomatic IG, hepatosomatic IH, nephrosomatic IN indices, kidney epithelium height (KEH) and size of oocytes were calculated.The number of mature oocytes and percentage of ovulating females were determined during the spawning season. Histological changes in the ovaries and testes were described throughout a year. Annual reproductive cycles were similar in both populations of sticklebacks. This is the first histological and morphological study carried out throughout a year, simultaneously in two wild populations of three-spined sticklebacks inhabiting different environments. An improved scale of gonadal development in conjunction with the determined indices and fecundity give a comprehensive description of the reproductive cycle. These new observations, in combination with previously reported features, provide a universal scale that can be successfully used to distinguish all phases of gametogenesis in sticklebacks in different habitats.