Protecting the Spawning and Nursery Habitats of Fish: The Use of MPAs to Safeguard Critical Life-History Stages for Marine Life

Marine protected areas are often designated to protect sites of high biodiversity or scenic character, like coral reefs. But many fish species that inhabit such places as adults have rarely spent their entire lifetimes there. “Nursery grounds” such as seagrass beds or mangroves provide protective habitat for juvenile-stage fish before they migrate to their adult habitat. And sites where adult fish gather on a seasonal basis to spawn can be located elsewhere as well. Such habitats encompass critical life-history stages for marine life, and their protection from harvesting or other pressures can help ensure the maintenance of broodstock and sustainability of populations.

As such, these habitats can form a crucial part of MPA networks. This month, MPA News discusses the use of MPAs to protect spawning and nursery habitats with several experts, each working to raise awareness of these habitats’ importance to marine systems.

Spawning aggregation sites

Many commercially important fish species reproduce in spawning aggregations that range in size from tens to tens-of-thousands of individuals. Such periodic gatherings can be lucrative for fishermen: they yield large catches, and are often easy to locate again and again. Overfishing can occur rapidly as a result, as has happened for several reef-based species worldwide.

Biologist Yvonne Sadovy is director of the Society for the Conservation of Reef Fish Aggregations (SCRFA), an international NGO that works to protect such aggregations and raise awareness of the problems of aggregation fishing. “In the context of fisheries management, communities and managers alike typically view spawning aggregations as fishing income opportunities, rather than as a life-history feature that is vulnerable, needs management, and supports the fishery of a particular species,” says Sadovy. SCRFA, conceived in 2000 and funded mainly by the David and Lucile Packard Foundation, has undertaken an array of education and outreach programs to spread its message on spawning aggregations, including pamphlets and posters in multiple languages for local stakeholders, an educational DVD, a handbook for NGOs and conservation/fishery officers, and a detailed manual for managers and researchers. It has also pushed to include spawning aggregation protection on the international agenda through statements of concern at major forums, including the 2003 International Tropical Marine Ecosystems Management Symposium and the 2004 IUCN World Conservation Congress.

SCRFA considers fishery closures – whether seasonal or permanent – to be an appropriate tool for managing aggregations for certain species and circumstances. “From a biological perspective, closures at the spawning site are particularly important when spawning habitat of a species is limited to specific areas, and in cases where a particular species forms very few, but very concentrated, aggregations with very large numbers of fish,” says Sadovy. Closures are also important, she says, in cases where spawning animals may be easily disturbed or their behavior disrupted by fishing activities. For species like grouper and emperor, which mature first as females then can become male later in life, conventional harvest of older, larger individuals may result in imbalance of sex ratios and potential limitation of available sperm at spawning time. (Sex changes in these species are informed by social information, including the ratio of males to females during aggregations. The change by females to males is not fast enough, however, to compensate the loss of large males during an aggregation event – the change occurs between reproductive seasons.)

SCRFA maintains an online, GIS-capable database (http://www.scrfa.org) of known reef-fish aggregations worldwide, based on information from published and unpublished literature, personal communications, and interviews with fishers. From that database, Sadovy estimates there are roughly 20 spawning aggregation sites that receive some protection in the Caribbean and tropical western Atlantic, and a similar number with such protection in the Indo-Pacific; in many places, the protection is seasonal rather than year-round. These figures amount to a small fraction of exploited reef-fish aggregation sites worldwide (the database lists more than 500), but the number of protected sites is increasing, says Sadovy.

“A few years ago, a review of MPAs in the Caribbean and tropical western Atlantic showed that only 5% of MPAs had specifically incorporated spawning aggregations in their design,” she says. “Since that time, there has been much more awareness of the need to consider aggregations. Major successes include 11 aggregation sites protected year-round in Belize recently, following a demonstration that the sites were spawning areas for many different species. Also, the Cayman Islands have closed several sites during spawning season as part of an eight-year pilot program.” In the Indo-Pacific region, the Association of Southeast Asian Nations this year released guidelines for its member states on the protection of spawning aggregations (see news brief below, “Guidelines released for responsible fishing in SE Asia…”). In 2003, Australia’s Great Barrier Reef Marine Park Authority incorporated five spawning aggregation sites in no-take zones as part of its major rezoning initiative.

Sadovy notes that closures are not always the best management option for aggregations. In an interview in June 2004 (MPA News 5:11), she said that low levels of subsistence fishing on spawning aggregations can probably be sustained, and not all aggregating species are equally vulnerable to fishing. In cases where protection is necessary, the appropriate measures depend on the biology of the species, nature of the fishery, and local management and social contexts. Seasonal sales bans on certain species, coinciding with their known aggregation times, can work well in some situations, whereas quotas or other conventional management tools may be more appropriate in others. Also, for species whose aggregation locations shift from year to year, spatially fixed closures – unless very large – would do little good.

Nonetheless, says Sadovy, seasonal and/or spatial closures can be invaluable under the right conditions. For resource managers around the world who are in the process of selecting sites for MPAs – particularly where protection of spawning biomass is a major goal – she encourages them to consider spawning aggregations. “We would like to see this important and vulnerable life-history phase considered, routinely, as a factor in MPA planning,” she says.

Nursery sites: seagrass beds, mangrove forests

Seagrasses are underwater flowering plants that can occur in extensive beds or meadows, generally in shallow coastal waters. A vital part of the marine ecosystem, seagrasses provide food and habitat for numerous vertebrate and invertebrate species, from fish and crustaceans to sea turtles and dugongs. In the case of juvenile fish, the three-dimensional structure provided by seagrass meadows provides an ideal environment for concealment from predators, and serves as a buffer against strong currents.

Seagrasses are declining, and even disappearing, around the world, impacted by human actions. Sensitive to changes in water quality, seagrasses have become an indicator of the overall health of coastal ecosystems. “Stretches of coastline in North America, Europe, and Japan that are highly developed have lost much of their seagrass,” says Fred Short, a biologist at the University of New Hampshire (US) and director of SeagrassNet, a global monitoring program (http://www.seagrassnet.org). “In the developing world, deforestation with concomitant erosion and mangrove removal has also resulted in seagrass declines.” Of the 48 sites being monitored by SeagrassNet and partner institutions around the world, the majority are experiencing some degree of decline, almost always due to direct or indirect human impacts, he says.

In his book World Atlas of Seagrasses (University of California Press, 2003), Short estimates there are at least 250 MPAs worldwide that include seagrass habitat. Yet few of these, he says, were established with protection of seagrass in mind. “In fact, in the majority of MPAs with seagrass habitat, seagrasses are not acknowledged by management or directly protected,” he says. SeagrassNet is working to become established in MPAs and to have seagrass monitoring become an integral part of long-term MPA monitoring programs. Currently SeagrassNet has 20 monitoring sites located in MPAs, ranging from small community-based protected areas to large World Heritage sites.

“The biggest challenge in protecting seagrass is making people aware, first, of its presence, and second, of its importance,” says Short. “Once the role of seagrass habitat in coastal ecology, coral health, and fisheries is made clear, leaders and citizens are easily convinced of the need for monitoring and protection.” Protection can come in the form of restrictions on various human activities, such as aquaculture, fishing, mangrove removal, or nearby coastal development. “In several of our monitoring locations, protection measures or public awareness initiatives have been instituted as a direct result of SeagrassNet involvement and advocacy,” says Short.

Ecologist Peter Mumby at the University of Exeter (UK) says marine conservation strategies need to protect connected corridors of habitat and facilitate the natural migration of species between habitats over time. In a study published in Nature journal in February 2004, Mumby determined that the availability of mangrove habitat – used as a nursery by juveniles of many coral reef fish species – had a pronounced impact on community structure and biomass of reef fish in their adult, coral reef habitat. (The paper is available at http://www.projects.ex.ac.uk/msel/papers/mumbyetal2004.pdf.) Biomass of several reef species in the Caribbean more than doubled when reefs were connected to rich mangrove sources – ones with at least 70 km of fringing mangrove within a region of 200 km². Mumby later used this finding to develop algorithms for use in natural resource planning, including the identification of mangrove sites with unusually large importance to reefs, and priority sites for reforestation projects. These algorithms appear in a follow-up paper he published this year in the journal Biological Conservation (Vol. 128, pp. 215-222), reprints of which are available from the author.

For MPA planners seeking to maximize reef fisheries production, Mumby suggests that protection of nearby sea-fringing mangroves should be a priority. “The mangrove fringe is usually dominated by Rhizophora mangle in the Caribbean, and its prop roots provide excellent nursery habitat,” says Mumby. “Ultimately, it is the perimeter of this habitat that is important for reef fish rather than its area.”

Along the same line, his findings hold implications for the planning of coral reef MPAs. In cases where planners must choose from among several reefs to protect, Mumby recommends that reefs near the largest-perimeter mangrove forests receive priority protection – particularly in cases of heavy fishing pressure. “Protecting a reef near a large-perimeter mangrove would maximize the chance of achieving a persistent reserve population if neighboring areas were intensively exploited,” he says. “This should also offer the greatest chance of achieving a high local population of fish, which could then benefit neighboring areas through spillover.” However, he adds, if the fishery were carefully managed and enforced, there would be a case for fishing the reef near the large-perimeter mangrove and protecting other reefs instead: the reason being that the reef near the large-perimeter mangrove would theoretically be able to sustain higher levels of extraction.

Protecting nursery grounds: a manager’s viewpoint

In the US state of Florida, more than one million people engage in recreational fishing each year, generating an economic impact of US $1.4 billion. Of the species fished, at least 70% of them spend part of their lifecycle in seagrass communities or other coastal nursery habitats.

In St. Lucie County, bordering on the coastal Indian River Lagoon on the Atlantic side of Florida, Jim Oppenborn is county marine resource coordinator. He says an offshore MPA designated nearby by federal managers – to protect the Oculina Banks deepwater coral reefs – has alienated local fishing groups, as it was among the few hard-bottom areas available for them to fish. Any effort by managers to designate a second offshore MPA would likely meet great opposition. But efforts to protect coastal juvenile habitat for fished species are another matter, he says.

“Of the snapper-grouper complex that uses our offshore reefs in St. Lucie County, 21 of the 45 species also use the Indian River Lagoon at some point in their life histories,” says Oppenborn. “Because of the dependence of our offshore fisheries on the Indian River Lagoon, creating special management zones in the lagoon makes sense to anglers.” (Due to the non-regulatory nature of his position, Oppenborn would be required to propose the creation of such zones to state-level government.) These zones, which have been instituted elsewhere in Florida, could include gear restrictions – such as bans on netting for bait, which impacts juvenile sportfish as bycatch. In some cases, the taking of oysters on restored oyster beds would be banned. Restrictions would most likely be instituted in areas that (a) feature newly restored habitat where harvest had not occurred for years, and (b) are close to public areas so that officials, like harbormasters or bridge tenders, could supervise them easily.

These factors contribute to enforceability, a concept that Oppenborn says is appreciated by fishermen. “Many of them agree that some amount of restrictions are needed to protect our fisheries resources, and without enforcement the rules will affect only law-abiding, conservation-minded anglers,” he says.

The zones would have little negative effect on fishermen, since they would be instituted in areas where little fishing has occurred in recent years. Fishing activity, in fact, does not pose a great threat to the fish nursery habitats of St. Lucie County. With a human population that has grown by more than 25% since 2000, the larger impacts on coastal resources are pollution, sedimentation, coastal development, habitat alteration, and other stressors. The effective protection of nursery habitats, even in these special management zones, is a challenge.

Education is key, says Oppenborn. “Most of the local decision makers do not have a background in natural resources, but many understand the importance of preserving natural communities once the concept is presented to them,” he says. “It would be unrealistic to pretend that effective protection could be afforded to the resources overnight. This does not mean that protection should not be attempted.”

For more information:

Yvonne Sadovy, SCRFA, Department of Ecology and Biodiversity, University of Hong Kong, Pokfulam Road, Hong Kong SAR, China. E-mail: yjsadovy@hku.hk

Fred Short, Department of Natural Resources, College of Life Sciences and Agriculture, 56 College Road, 215 James Hall, University of New Hampshire, Durham, NH 03824-3589, USA. Tel: +1 603 862 5134; E-mail: fred.short@unh.edu

Peter Mumby, School of BioSciences, Prince of Wales Road, University of Exeter, Exeter, Devon EX4 4PS, UK. Tel: +44 1392 263798; E-mail: P.J.Mumby@exeter.ac.uk

James Oppenborn, Public Works Department, 2300 Virginia Ave., Fort Pierce, FL 34954, USA. Tel: +1 772 462 1713; E-mail: oppenborj@stlucieco.gov

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