COMPARATIVE-STUDY OF REPRODUCTIVE-BIOLOGY IN SINGLE-SPAWNER AND MULTIPLE-SPAWNER CYPRINID FISH .1. MORPHOLOGICAL AND HISTOLOGICAL FEATURES

To clarify the dynamics and regulation of oogenesis in single- and mul tiple-spawning cyprinid fish with group-synchronous oocyte development , a multidisciplinary approach to their reproduction was undertaken us ing three species from the River Meuse (Belgium): the roach Rutilus ru tilus as a single spawner, and the bleak Alburnus alburnus and the whi te bream Blicca bjoerkna as multiple spawners. The gonadosomatic index (GSI) and histomorphometric changes (distribution of oocyte size, rel ative proportion of the various oocyte stages) in the ovary are compar ed. Different patterns of GSI and oocyte growth were observed both bet ween the single- and multiple-spawner fish and between the two multipl e spawners. Maximum GSIs were higher in roach (21%) than in bleak and white bream (17.7 and 14.5%, respectively); and compared to the rapid decline of GSI in the roach population, the GSI of multiple spawners d ecreased progressively during the spawning season. In roach, a short g onadal quiescent period and an early onset of vitellogenesis was recor ded from late summer onwards whereas, in bleak and white bream, exogen ous vitellogenesis was not systematically observed before winter. A pr otracted spawning season and/or a low water temperature in autumn are hypothesized to explain this long period of gonadal quiescence. In ble ak, during the spawning season, the oocytes recruited arose from the s tock of endogenous vitellogenesis and attained the final maturation st age very rapidly. This recruitment occurred during the whole spawning season. In white bream, the differentiation of vitellogenic oocytes fr om smaller oocytes was completed before the onset of the spawning seas on. During the spawning period, the proportion of vitellogenic oocytes decreased progressively whereas the percentage of oocytes in the fina l maturation stage remained approximately constant. (C) 1996 The Fishe ries Society of the British Isles