The Value of Mangrove Forests for Environment and Climate of Warm-temperature Regions


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Temperate climatic regions (Hogarth 2015). These warm, temperate mangrove stands typically contain fewer species (sometimes only a single species) than stands in the tropics (Hogarth 2015). The complex prop-roots and pneumatophores of mangrove trees form a solid substrate within the intertidal and subtidal zones in estuaries, where they provide suitable habitat for a wide diversity of other floral and faunal life (Ellison and Farnsworth 1992).

Mangrove forests are considered one of the most productive of all marine and coastal ecosystems (Duarte and Cebrian 1996), contributing high levels of nutrients into estuarine ecosystems through detrital food chains starting with the decomposition of leaf litter (Nagelkerken et al. 2008). Kristensen et al. (2008) noted that detritus from mangroves and benthic microalgae are typically the greatest contributors to autochthonous carbon sources in tropical estuarine ecosystems. This high nutrient input, coupled with the refuge and habitat which mangrove forests provide, lends an explanation for the high abundance and diversity of fishes associated with mangrove estuaries (Laegdsgaard and Johnson 2001, Blaber 2007).

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Mangroves form a vital component in the life history of many fish species in both tropical and subtropical regions (Mahesh and Saravanakumar 2015). Up to 30% of commercial fishery species globally have been found to be dependent on mangroves (Naylor et al. 2000), which produced a total annual catch of approximately 30 million t in 2002 (FAO 2004). In the Gulf of California fishery catches have been positively related to the abundance of nearby mangroves, which are utilized by numerous fishery species as a nursery and feeding area (Aburto-Oropeza et al. 2008).

Mumby et al. (2004) also found that the biomass of important commercial fishery species is more than doubled when mangroves are utilized at some point in the fish’s life cycle. Numerous studies show similar findings, asserting that mangroves play a crucial role in sustaining production in fisheries (for example, see Rönnbäck 1999, Manson et al. 2005, Aburto-Oropeza et al. 2008). This typically forms the foundation for any management decisions with regard to the conservation and reestablishment of mangrove stands, as well as other important coastal wetland habitats (Manson et al. 2005).

Commercially important fish species commonly found utilizing mangroves as a nursery habitat include, among others, snappers (Lutjanus spp.), barracuda (Sphyraena barracuda), mullets (Chelon and Mugil spp.), groupers (Epinephelus spp.) and catfish (Arius and Tachysurus spp.) (Rönnbäck 1999, Lugendo et al. 2005). Despite their economic and ecological importance, mangroves are under threat globally. Approximately 90% of mangroves occur in developing countries, where they are critically endangered and on the brink of local extinction in 26 known countries (Kathiresan 2008). Experts suggest that ecosystem services offered by mangroves may be lost within the next century (Kathiresan 2008). Threats to mangroves include habitat clearing for aquaculture and development, harvesting of wood for fuel and timber, hydrological alterations within estuaries, pollution and climate change (Alongi 2002, Gilman et al. 2008).

In South Africa, mangroves are restricted to the eastern coastline and can be found in 37 estuaries covering almost 1700 ha (Adams et al. 2004). Dominant mangrove species in South Africa include the white mangrove (Avicennia marina), the black mangrove (Bruguiera gymnorrhiza) and the red mangrove (Rhizophora mucronata), with the former extending to the southeastern warm temperate coast (Macnae 1963). The southeastern coast forms home to the majority of mangrove estuaries in South Africa, and the lack of infrastructure and scientific knowledge in the region makes conservation and management of this habitat type difficult.

The role of mangroves in warm temperate regions remains relatively unstudied in terms of the advantages they provide (including a refuge/nursery habitat for larval- and juvenile-stage fishes and feeding opportunities). It is therefore important to investigate the role of warm temperate mangroves for fishes utilizing estuaries as nursery areas, especially since these vegetation types are under threat. This knowledge of ecosystem value will help to enable the proper conservation of habitats for fishes. The aim of the study was to investigate catches of juvenile and small adult fishes during the peak summer recruitment period in mangrove and non-mangrove estuaries to determine whether differences in catches exist, and whether mangrove presence lends an advantage to fish survival in warm temperate South Africa. It was hypothesized that mangrove estuaries would have a greater abundance and diversity of young fishes than non-mangrove estuaries.

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