The nursery idea
Many marine animals begin life as tiny larvae drifting in open water. Some of them eventually settle in coastal habitats where they grow for a time before moving elsewhere. Mangrove forests are one of the places where young fish, shrimp and crabs are often found in large numbers, and for this reason mangroves are commonly described as nurseries.
The basic reasoning is simple. A baby fish is small and has many predators. A place that offers both food and hiding places raises the chance that it will survive long enough to grow. Mangroves can offer both.
What young animals find there
Shelter. The tangle of prop roots and pneumatophores forms a maze of narrow gaps. Small fish can slip between roots that bigger predators cannot easily enter. Shallow water also keeps many large fish out, since they generally prefer deeper channels.
Murky water. Tidal creeks are often cloudy with suspended mud. This makes it harder for sight-hunting predators to see their prey.
Food. Leaves, twigs and other plant material fall into the water and are broken down by bacteria, fungi and small invertebrates. This decaying material feeds worms, snails, small crustaceans and other animals, which in turn feed fish. The role of leaf litter is similar to what happens on a forest floor on land, but here the material is swept and mixed by the tide. Algae growing on roots, and plankton in the water, add to the food supply.
Calm, warm water. Sheltered channels and lagoons are often calmer than the open coast, and warm shallows can speed growth for many species.
Who uses them
A wide range of animals spend part of their lives among mangroves:
- Fish such as snappers, groupers, mullet and many small species that stay for their whole lives.
- Shrimp and prawns, some of which spawn offshore and move into creeks as juveniles.
- Crabs, including mud crabs, which live in burrows and are often caught for food.
- Oysters and other shellfish, which attach to roots and hard surfaces.
- Birds, reptiles and other animals that feed on the fish and invertebrates.
The use is not the same for every species. Some animals live in mangroves from birth to death, others visit only as juveniles and then move to seagrass, reefs or open water, and others visit only occasionally, perhaps at high tide to feed. This is why saying that all the fish in a region depend on mangroves would go too far. In general terms, though, the connection between mangroves and many coastal fisheries is well documented.
Linking habitats
Mangroves seldom work alone. Mangrove channels, seagrass beds and coral reefs often lie close together, and many animals move between them over their lives, a pattern called ontogenetic migration. A fish might hatch on a reef, spend months in a mangrove creek, then return to the reef as an adult. If a link in this chain disappears, the effects can show up in a neighboring habitat.
Tides move the animals, too. Many fish move into the forest on a rising tide and drift out again as the water falls, feeding when the roots are submerged and sheltering in channels at low water.
Connection to people
Coastal communities often rely on mangrove-associated species for food and income. Fishing in or near mangrove channels is a widespread livelihood, using nets, traps, hand-collection of crabs or shellfish and small boats. Some of this supports families with little other income. Such activity can be sustainable when the number of fishers is modest and the habitat is healthy, and local community forest management arrangements sometimes include rules about when and where to fish.
Scientists have tried to measure the economic value of mangroves for fisheries, and reported values vary a great deal. The size of the effect depends on the species, the region, how much the fishery depends on those species and what else is nearby. Rather than relying on one number, it is more accurate to say that the link is real in many places and that the strength of that link is uncertain.
How scientists test the idea
Researchers use several methods to check whether a habitat truly acts as a nursery. They compare catches of juveniles in mangroves with those in other habitats, track marked fish, analyze the chemistry of fish tissues or ear bones to see where individuals grew up, and compare fisheries near intact and cleared coastlines. A habitat is a strong nursery if a large proportion of the adults in the wider population grew up there, not just if many young fish are seen there. This is harder to show, and results differ among species and regions.
When mangroves are lost
Removing mangroves takes away shelter and food, and it can also alter water flow and water quality. Where fish catches have fallen after mangrove clearing, there may be several causes at once, including overfishing and pollution, so it can be difficult to assign blame. Restoring tidal channels and replanting the right species at the right elevation may allow some of these animals to return over time, but recovery depends on the surrounding habitats and may take many years. Protecting the nursery habitat that remains is usually simpler than rebuilding it, and some communities have found that payments for ecosystem services or protected areas can help keep mangrove fisheries going.