The fisheries value of mangroves in Honduras
Mangroves may increase the abundance of coral-reef fish, according to a new study at Utila, Honduras. Fished species such as snappers, grunts, and barracudas use mangroves as juveniles before migrating to coral reefs as adults.
Mangroves are the predominant vegetative habitat found at the interface between sea and land of tropical and subtropical zones. Since 1980, 20% of the world’s mangroves have been destroyed for land reclamation, shrimp aquaculture, and wood for fire and building materials (World Mangrove Atlas).
Red mangrove roots provide fish with spatially complex habitat that attracts food and creates refuge from predatorsTwo-thirds of the island of Utila (Bay Islands, Honduras, Central America) is covered by mangroves, but, as elsewhere in the world, these mangroves are currently being removed.
The role of mangroves
Previous scientific studies have found that these trees are essential to reef health and coastal protection. In addition, mangroves are thought to act as nurseries for many species of tropical fish.
Nursery reef fish species require habitats other than coral reefs while in the juvenile stage.
Red mangroves grow at the water’s edge and their roots, which are used for support and oxygen transport, provide these fish with a complex spatial habitat that both attracts food and creates a refuge from predators.
Surveying fish abundance
Sampling and surveying were conducted on the surrounding reefs and mangroves of Utila, Bay Islands and the Cayos Cochinos Islands, Honduras in 2007 and 2008. Utila, which has 13 cays, is the southernmost island in the Bay Island archipelago and is located 29 km off the coast of Honduras. Utila is dominated by mangroves (nearly 66% coverage). The mangroves, Rhizophora mangle and Avicennia germinans, dominate two large lagoons on the south side and mangrove stands on the north side of the island.
Cayos Cochinos consists of Cayos Menor, Cayos Mejor and 13 smaller coral cays. The islands are located southeast of the Bay Islands, 18.5 km from the mainland. In contrast to Utila, Cayos Cochinos has a distinct absence of mangrove lagoons and supports only two very small mangrove stands with lengths of 100 and 150 metres.
Coral reef surveys consisted of eight randomly selected 50m transects at each site (six sites per island) where fish abundances were recorded. To establish which coral-reef species use mangroves as a juvenile nursery (aka nursery species), nine mangrove sites were surveyed using 30m transects.
Of the 13 nursery species whose juveniles were found primarily in mangroves, eight had significantly higher abundances (denoted by asterisk on Figure 1) on mangrove-rich Utila’s coral reefs. The remaining five species were also more abundant on Utila, but were not significantly so.
Some non-nursery adult fish species had higher abundances on Utila as well, but a larger percentage of nursery species than non-nursery species had higher abundances on Utila (see Figure 2).
Overall, coral-reef fish communities of the two islands were significantly different from each other. Therefore, this comparison between coral reefs off a mangrove-rich island and a mangrove-poor one indicates that mangroves may increase the abundance of nursery coral-reef fish, and adds one data point (island comparisons of fish nursery species) to a large scale graph of comparable matched-pairs from previous studies.
Conflicts of tourism
In 1993, Honduras recognised the importance of mangroves by signing the Ramsar Convention, an international treaty promoting the conservation of wetlands. Yet, many mangroves have been destroyed or are threatened with destruction on Utila as development stimulated by eco-tourism encroaches. The irony here is excruciating. Tourists are drawn to the Bay Islands to explore coral reefs, and yet, in decimating mangroves, tourism directly threatens the health of those reefs.