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D. rhmnbeus migratory lives of associated fishes (Odurn et al. 1982; Ogden and Gladfelter 1983). Difficulty in sampling fringing and overwash mangrove habitats may account for some of this lack of knowledge; in particular, mangrove prop roots hinder movement of both researchers and equipment, while the often turbid waters associated with the prop root environment obstruct visual censuring techniques (Thayer et al. 1987; Rooker and Dennis 1991; Sedberry and Carter 1993).
Fish that migrate between mangroves and coral reefs are typically reef-associated species. There are several reasons for coral reef lish to migrate to the mangroves. The mangrove habitat is generally recognized to act as a nursery for the juvenile stage of many coral reef fish species (Odum et al. 1982; Ogden and Gladfelter 1983; Gilmore and Snedaker 1993; Sedberry and Carter 1993). There are indications that this is not the case for sites in the southwest Pacific (Quinn and Kojis 1985; Blaber and Milton 1990). Mangrove detritus (leaves, branches, etc.) is the base of an extensive food web comprised of dissolved and particulate organic matter, as well as an associated microorganism community (Figure 13.7; Odum and Heald 1972). This detrital-based food source is highly accessible and nutritious for young fish. In addition, the mangrove prop root structure offers physical protection from predatory fishes (Odum et at. 1982; Ogden and Gladfelter 1983), while the turbidity of nearshore waters often hinders visual predators (Thayer et al. 1987; Rooker and Dennis 1991; Sedberry and Carter 1993). Conversely, larger piscivorous fish often find the mangrove habitat an excellent feeding ground, as the mangrove habitat boasts a greater number of fish as well as overall biomass than surrounding habitats in some areas (Thayer et al. 1987; Blaber and Milton 1990; Sedberry and Carter 1993). In the Caribbean region, typical migratory piscivorous fishes are red fish, tarpon and snook (Ogden and Gladfelter 1983).
The fragmentation of mangrove-dominated landscapes is believed to create the same types of problems for migratory organisms that are associated with the fragmentation of upland forests, yet there has been little, if any. research on this topic. There is no documentation concerning diel or seasonal migration patterns of resident species within mangroves, or how such species might be affectcd by the impact of fragmentation. For instance, scagrass and adjacent mangrove habitats arc used by many species of
Figure 13.8 (and opposite) Life histories and habitat utilization of six selected dominant fish species including marine-estuarine spawners, estuarine spawners and fresh-water-estuarine spawners in Terminos Lagoon, Mexico. The fish migrate using SMS and FLS habitats (see bottom of upper panel) for the highest periods of productivity for feeding, spawning or nursery grounds (upper panel). The seasonal abundance of fish species in the SMS habitats (lower panel), (from Yafiez-Arancibia et al. 1988. represented by permission of Academic Press)
nekton and are generally characterized by high fish abundance and diversity. From Yanez-Arancibia et al. (1993) it is clear that the utilization of the two interacting habitats by fishes is spatially distinct, but linked by the life-cycles of organisms (Figure 13.8). They found that there is a strong correlation between the life-history patterns of migratory fish and the pattern of primary production, using the two habitats sequentially in a time period. The fragmentation of mangrove-seagrass landscapes will probably reduce ecosystem complexity and nekton diversity. It has been speculated that one of the consequences in loss of mangrove area and/or increased fragmentation will be reduction in population numbers (and this may be important for commercial species) or outright local extinctions of certain species. However, as recently reviewed by Robertson and Blaber (1992), there is no empirical evidence that such a consequence will occur.
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