Ontogenetic Diet Shifts in Nassau Grouper: Trophic Linkages and Predatory Impact
Understanding which fauna and flora from seagrass beds serve as primary food for reef-based commuters is critical in defining trophic linkages between shallow-water habitats of tropical oceanic regions. Although numerous studies have documented the relative importance of crustaceans in the diet of reef fishes associated with tropical seagrass meadows, it is unknown if trophic importance corresponds to a significant effect on prey distribution and abundance patterns. We quantified size-specific diet of juvenile Nassau grouper (Epinephelus striatus) inhabiting natural and artificial patch reefs, and manipulated the density of artificial patch reefs (0, 8, and 16 patch reefs per ha) to examine the predatory impact of patch reef associates on nocturnally foraging crabs. Stomach content analysis indicated an ontogenetic shift in diet, where small grouper (<20 cm TL) consumed mainly brachyuran crabs and other small crustaceans, and large grouper (>30 cm TL) consumed primarily fish. Mid-size fish (20.0–29.9 cm TL) were transitional in diet, with crustaceans occurring more frequently than fish. Diet was least diverse for large fish, and nearly identical for mid-size and small fish. Similar numbers of prey taxa were ingested in natural and artificial patch reefs. Before patch reefs were deployed in the field, nighttime band censuses revealed a mean density of 3 crabs per 240 m2 (ranges = 0–8 crabs/transect) in nearby seagrass beds. After patch reefs were deployed, crab densities dropped by one-half to 43-fold in sites with patch reefs compared to control sites (0 patch reefs). There was a negative and significant relationship between grouper density (no. ha−1) and crab density (no. 240 m−2) at all sites and locations combined. These results suggest that patch reef associated predators have a significant predatory impact on nocturnally foraging crabs in adjacent seagrass meadows, and highlight an important trophic link between tropical patch reef and seagrass habitats.
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Document Type: Research Article
Publication date: 01 July 1998
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