10. Evaluation of Nursery function of Mangroves and Seagrass beds for Tropical Decapods and Reef fishes: Patterns and Underlying Mechanisms

Shallow-water tropical coastal habitats, such as mangroves and seagrass beds, have long been associated with high primary and secondary productivity. The ubiquitous presence of juvenile fish and decapods in these systems has led to the hypothesis that they act as nurseries. Earlier studies mainly focused on the faunal community structure of these systems, leaving us with little detailed insight into their potential role as nurseries. Habitats are considered nurseries if their contribution, in terms of production, to the adult population is greater than the average production of all juvenile habitats, measured by the factors density, growth, survival, and/or movement. High food abundance and low predation risk form the most likely factors that contribute to the attractiveness of tropical nursery habitats. Here, the current state of knowledge on nursery function of shallow-water coastal habitats, particularly mangroves and seagrass beds, is reviewed for each of the above-mentioned factors. Most data show that mangroves and/or seagrass beds have high densities of various fish species and some of their food items, and a lower predation risk for fish and decapods due to factors such as low predator abundance, high water turbidity, and complex habitat structure. In contrast, growth rates of fish appear higher on coral reefs. There is increasing evidence that at least part of the fish or decapod population in these putative nurseries eventually moves away to offshore habitats. The current review shows that mangrove and/or seagrass habitats may act as nurseries through higher juvenile densities and survival rates than offshore habitats, but that trade-offs may exist to the detriment of growth rate. With the lack of detailed movement studies, the exact degree to which mangroves and seagrass beds sustain offshore fish and decapod populations remains largely unclear.