What Are Marine Nursery Areas?
Marine nursery areas are habitats that promote the survival of young fish, crustaceans, mollusks, and other marine species. Many of these creatures are important to humans in fisheries7 8 9 and in recreational activities. These habitats are essential for the reproduction of many marine species: take them away or degrade them, and the production of commercially harvested species will decline or cease altogether10 11.
Marine nurseries are also an important part of the interconnected relationships among marine and terrestrial ecosystems and provide food that sustains the high level of biodiversity in the oceans as well as supplying food for numerous birds, mammals and other animals that feed along coasts and coastal wetlands.
Conservation has typically been focused on charismatic habitats – aesthetically pleasing areas with a diversity of species. Recent discussion and debate has highlighted the need for an alterative paradigm that focuses attention on areas that provide high concentrations of ecosystem services12 13. Nursery areas, especially those that are found outside high-profile marshes, mangroves, or reefs, fall into this category of high-priority areas for conservation.
A marine or coastal habitat can be deemed a marine nursery if the density of sub-adult organisms is greater than in other habitats, and if the habitat confers advantages that result in greater survival to the next larger size class14. Estuaries contain particularly important nursery areas, but nursery habitats can also be found in mangrove forests, seagrass meadows, oyster and rock reefs, sea mounts, and even in pelagic environments, such as the oceanographically static areas of the Sargasso Sea, etc., and in ephemeral features such as frontal systems and warm core rings15 16. Some of the many types of nursery areas are described below.
A. Estuaries. Estuaries, areas where the freshwater of rivers meets the saltwater of the oceans, are highly productive and dynamic areas that support many ecosystem services. At one time people considered entire estuaries to be important as marine nurseries 17 18. Recent investigation has shown however, that some portions of estuarine areas are more important than others19 20. Marshes for example are particularly valuable in providing food and hiding places for small fish, larval crustaceans, and juvenile mollusks 21 22 23. Mangrove forests that line some estuaries are also prime nursery areas24.
Some species need specific temperatures and salinities in certain phases of their life -- prawns or shrimp, for instance. Estuaries often have salinity levels that vary with seasonal rain as well as temperature since they are often shallow. Similarly, the physical changes that occur seasonally (e.g. heavy rains) in estuaries assist in dispersing organisms by flushing them out of the system.
In some parts of the world such as temperate Europe, studies have shown that the soft-bottom subtidal areas within and adjacent to estuaries are key nursery areas25. Thus on a large geographic scale, estuaries are key to providing nursery services. Within these ecosystems, different habitat types have different levels of importance depending on the species present and the ecosystem dynamics.
B.Mangroves. Mangroves are trees that are specially adapted to living in seawater, either continually or during high tides, though they also depend on freshwater inputs from terrestrial sources. Mangrove forests and even thin stands of mangrove trees lining coasts and river mouths provide critical habitat for the young of many tropical and subtropical species of fish and invertebrates. The extensive root systems of mangroves stabilize the land, trap sediment and nutrients, and provide vast arrays of nooks and crannies for young organisms to feed, hide from predators, and grow 26 27 28.
C.Seagrass beds. Seagrass is a generic term for the flowering plants that colonize soft-bottom areas of the oceans from the tropics to the temperate zones. In estuarine and other nearshore areas of the northern latitudes, eelgrass forms dense meadows29. Further south, manatee and turtle grass blanket wide areas. Seagrass is thought to be a particularly important nursery area in the tropics, where it provides critical habitat for sea turtles, coral reef fishes, and invertebrates30 31 32.
D. Coral Reefs. Mangrove forests and seagrass adjacent to coral reefs can provide important nursery functions for many marine organisms. However, certain portions of coral reefs themselves provide a nursery function as well. The reef flat or backreef, as well as lagoon areas of coral atolls, are benign environments typically characterized by sponges, soft corals, and small branching stony coral that provide niches for escaping prey and act as crucial nursery habitats for many species33 34 35.
E. Other Nursery Areas: The vast ocean environment contains many other physical features that provide suitable habitat to act as nurseries for marine organisms36 37, and as substrate for microorganisms that are part of the marine food chain. Within estuaries, for instance, oyster reefs are thought to be critically important nursery areas, not just for oysters but also for a wide range of fish species, other mollusks, crabs, and other fauna38. Rock reefs similarly provide nursery habitat for rich fisheries such as occur on the extensive banks of West Africa39. Sea mounts are thought to be crucial for many pelagic fish species, not just as sites for breeding and spawning, but also as safe havens for juvenile fishes seeking refuge from open ocean predators40 41 42 43.
Soft bottom substrates, especially in temperate continental shelf areas, are also key nursery areas44.
Using the broadest interpretation of the term nursery area, one can find examples of nursery habitat not tied to the sea floor in the same way seagrasses and reefs are anchored on the benthos. In open ocean areas (pelagic) where currents converge, macroalgae, wood, and other floating objects form driftlines and vast mats – providing nursery areas for fish, sea turtles, crustaceans, and the like45. The best example of this might be the biologically rich Sargasso Sea, located at the base of the Gulf Stream, east of Florida and the Bahamas46. Even ephemeral oceanographic phenomena such as warm core rings (self-contained eddies that break off of major currents and float shorewards) provide nursery habitat in the open sea47.
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