Temporal variability of pelagic fish assemblages around fish aggregation devices : biological and physical influences

Pelagic fishes were counted around four fish aggregation devices (FADs) moored between 3 and 10km offshore on the continental shelf off Sydney, Australia. Visual counts were made at FADs on 81 days periodically from April 1999 to April 2002. Surface water temperature and current speed were also measured at the FADs. Assemblages of fishes at FADs followed a seasonal pattern, however, biological and physical variables influenced seasonal structure greatly. Abundances at FADs were greatest in spring due to the appearance of large schools (100s to 1000s) of juvenile Trachurus sp. In contrast, diversity was far greater in summer and autumn, principally due to the appearance of schools (10s to 100s) of juvenile Coryphaena hippurus (Coryphaenidae), and other warm water species from January to May when water temperatures were >20° C. Short-term variability differed among species; C. hippurus fluctuated greatly among counts separated by 2-3 days, while Seriola lalandi (Carangidae) and Alutera monoceros (Monacanthidae) abundances were more stable, indicating greater residence times at FADs for these two species. Marked differences in fish assemblages occurred between times when predators were present and absent, with few small fishes being observed when piscivorous predators occurred at FADs, regardless of season. Furthermore, abundances of C. hippurus and A. monoceros were correlated with current speed, with greatest abundances observed when currents were strong and weak, respectively. The results indicated that much of the temporal variability in fish assemblages at moored FADs could be explained by biological and physical factors.

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