Why Are Nursery Areas Important?

The world ocean is a big, dangerous place for young marine organisms.  However, not all parts of the oceans are created equally – some places have special value in being able to enhance the survival of small and largely defenseless creatures. Nursery habitats are just such areas -- providing critical living space for eggs, larvae, juveniles, and sub-adults of the vast majority of coastal and pelagic marine species48. The features of nursery areas that make them critical for fish populations and other marine species include providing food, shelter, space, and pathways to and from the site to other adult habitats of the species.

A. Dispersal.  A nursery habitat is valuable to a species only insofar as it is accessible.  Eggs, larvae, and young rely largely on currents to deposit them in nursery areas.  Current flows facilitate dispersal to and from the nursery site while at the same time boundary currents allow for larval retention.  Once there, they need to be able to stay and grow - thus nurseries are relatively static areas able to retain larvae and young until they grow large enough to leave the site on their own49 50.  Fish, sea turtles, marine mammals, and invertebrates that have grown in nursery areas must be able to successfully access adult habitats (or other nursery habitats for other, non-adult life stages) from these sites51 52

B.  Food.  Nursery sites provide food through nutrient loading and prey availability.  In order for a nursery habitat to increase the survival of young organisms, sufficient food must be available on hand.  For primary consumers, this food takes the form of plant life: phytoplankton, algae, and macroalgae.  For carnivorous and omnivorous species, nursery habitat must supply prey.  Estuaries, seagrass beds, mangroves, and other kinds of marine nurseries are notable for their productivity53 54 55.  Nutrients come from outside the site via rivers, run-off, currents, and upwelling. Nursery habitats that are also able to produce food on site retain many nutrients through efficient recycling56.  The wide availability of nutrients in turn fosters blooms of copepods and other prey species.57 58 59 60

C. Refuge from predators.  Nursery habitats are physically complex places, with much   spatial heterogeneity.  Simply stated, nursery areas provide many hiding places that make them suitable as refuges from predators. Survivorship is significantly higher in areas with reduced predation than it would be in the open ocean61 62 63 64 65.  Some nurseries simply provide a different habitat that predators do not generally venture into (e.g. areas with shallow, calm waters or low salinity).

D. Space.  Nurseries provide the space needed for maintaining optimal densities of individuals. Most marine organisms are highly fecund – producing a lot of young to offset the natural mortality caused by predation, including human fishing pressure66.  Vast numbers of eggs and young from many different species find their way to nursery areas, and once there they need space to grow.  Thus, the most valuable nursery areas are those that provide all the functions above with sufficient space to support large numbers of growing organisms67 68.

Thus marine nurseries provide refuge and food to a wide array of species, and in so doing, contribute important ecosystem services. Our conservation attention has not been adequately trained on nurseries, in part because they are often less diverse and less aesthetically pleasing than the conventional foci for conservation like coral reefs.  However, the world is slowly waking up to the importance of nursery habitats, and concern for their preservation is mounting70.

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50 Micheli, F. and C.H. Peterson. 1999. Estuarine vegetated habitats as corridors for predator movements. Conservation Biology 13: 869-881.

51 Beck et al. 2001.

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54 FAOSTAT. The website of the United Nations Food and Agricultural Organization provides statistics on marine fisheries, aquaculture, etc. Online at www.fao.org, accessed on August 6, 2010.

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57 Beck et al. 2001.

58 Bell, J.D., A.S. Steffe, and M. Westoby. 1988. Location of seagrass beds in estuaries: effects on associated fish and decapods. Journal of Experimental Marine Biology 122: 127-146.

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61 Beck et al. 2001.

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64 Heck et al. 1997.

65 Wilson, K., K.W. Able and K.L. Heck, Jr. 1990. Predation rates on juvenile blue crabs in estuarine nursery habitats: evidence for the importance of macroalgae (Ulva lactuca). Marine Ecology Progress Series 58: 243-251.

66 Miller at al. 1998.

67 Minello, T. 1999.  Nekton densities in shallow estuarine habitats of Texas and Louisiana and the identification of essential fish habitat. In: Benaka, L.R. (Ed). Essential Fish Habitat and Rehabilitation. Bethesda, MD: American Fisheries Society. pp. 43-75.

68 Beck et al. 2001.

69 Weinstein, M.P. and D.A. Kreeger (Eds). 2000. Concepts and Controversies in Tidal Marsh Ecology. Dordrecht, The Netherlands: Kluwer Academic Press. 875 pp.

70 Kareiva, P. and M. Marvier. 2007. Conservation for the people? Sci. Am. Oct.2007 32-39.