THE AGE STRUCTURE AND GROWTH DYNAMICS OF YOUNG-OF-THE-YEAR BASS, DICENTRARCHUS-LABRAX, POPULATIONS

Back-calculated egg fertilization (spawned) dates and growth rates for young-of-the-year bass, Dicentrarchus labrax (L), captured from Loughor Estuary, Oxwich Bay and Ogmore Estuary, south Wales, UK, during 1988 and 1989 were compared. Spawned date frequency distributions indicated that the bass spawning season began in late February and continued until early June in 1988 and early July in 1989. There were no significant differences in the frequency distributions of back-calculated spawned dates for the bass populations, between either sites or years, which suggested that any variations in growth rates were attributable to characteristics of their respective environments. Growth rates were determined for bass cohorts resulting from early, mid- or late-season spawnings at Oxwich and Loughor in 1988 and Oxwich, Loughor and Ogmore in 1989. There were only slight intra-annual differences between the mean growth rates of bass cohorts at the same site, suggesting that bass from early cohorts would be larger at any given time of year. In 1989, growth was considerably faster at Oxwich and Loughor than at Ogmore, where food supply was considered limiting. However, between-site variations in growth rate were small in comparison with inter-annual differences. The faster growth rates observed at Loughor and Oxwich in 1989 were attributed to higher water temperatures. Relationships between growth rate and subsequent survival of young-of-the-year bass, and the relative importance of estuaries as nursery habitats for juvenile bass, are discussed.