A marine reserve is a type of marine protected area (MPA). An MPA is a section of the ocean where a government has placed limits on human activity. A marine reserve is a marine protected area in which removing or destroying natural or cultural resources is prohibited, marine reserves may also be "no-take MPAs,” which strictly forbid all extractive activities, such as fishing and kelp harvesting. As of 2007 less than 1% of the world's oceans had been set aside in marine reserves. Benefits include increases in the diversity, density, biomass, body size and reproductive potential of fishery and other species within their boundaries. As of 2010, scientists had studied more than 150 marine reserves in at least 61 countries and monitored biological changes inside the reserves. The number of species in each study ranged from 1 to 250 and the reserves ranged in size from 0.006 to 800 square kilometers (0.002 to 310 square miles). In 2014, the World Parks Association adopted a target of establishing no-take zones for 30% of each habitat globally by 2030. Based on early 2026 data, 9.6% of the ocean is protected, but, only 3.2% of it can be considered as highly protected. However, a study conducted in 2024 by a team from MPAtlas that established a baseline for the 30 x 30 target by evaluating over 90% of the global MPA area, found that approximately one quarter of existing reported marine protection is not yet implemented on the water and an additional one-third allows highly impactful activities that are not compatible with conservation.
Design A review of studies of 34 families (210 species) of coral reef fishes demonstrates that the design of a marine reserve has important implications for its ability to protect habitat and focal species.
Size and shape Effective reserves included habitats that support the life history of focal species (e.g. home ranges, nursery grounds, migration corridors and spawning aggregations), and were located to accommodate movement patterns among them. Movement patterns (home ranges, ontogenetic shifts and spawning migrations) vary among and within species, and are influenced by factors such as size, sex, behaviour, density, habitat characteristics, season, tide and time of day. For example, damselfishes, butterflyfishes and angelfishes travel <0.1–0.5 km, while some sharks and tuna migrate over thousands of kilometres. Larval dispersal distances tend to be <5–15 km, and self-recruitment to new habitat is common. The review indicated that effective marine reserves are more than twice the size of the home range of focal/target species (in all directions). The presence of effective marine management outside the reserve may allow smaller reserves. Reserve size recommendations apply to the specific habitats of focal species, not the overall size. For example, coral reef species require coral reef habitats rather than open ocean or seagrass beds. Marine reserve whose boundaries are extensively fished benefit from compact shapes (e.g., squares or circles rather than elongated rectangles). Including whole ecological units (e.g., an offshore reef) can reduce exports where that is desired.
Habitats Minimum sustainable population sizes have not been determined for most marine populations. Instead, fisheries ecologists use the fraction of unfished stock levels as a proxy. Meta-analyses suggest that maintaining populations above ~37% of those levels generally ensures stable populations, although variations in fishing pressure allow fractions as small as 10% or as large as 40% (to protect species such as sharks and some grouper that have lower reproductive output or slower maturation). Higher fractions of habitat protection may protect areas vulnerable to disturbances such as typhoons or climate change. 20–30% protection can achieve fisheries objectives in areas with controlled fishing pressure and is the minimum level of habitat protection recommended by IUCN-WCPA.
Special areas Many fish species congregate to facilitate spawning. Such congregations are spatially and temporally predictable and increase the species' vulnerability to overfishing. Species such as groupers and rabbitfishes travel long distances to congregate for days or weeks. Such gatherings are their only opportunities to reproduce and are crucial to population maintenance. Species such as snappers and parrotfishes congregate in feeding or resting areas. Juveniles may congregate in nursery areas without adults. Such special areas may require only seasonal protections if at other times no vital activities are taking place. Such reserves must be spaced to allow focal species to journey among them. If the location of such special areas is unknown, or is too large to include in a reserve, management approaches such as seasonal capture and sales restrictions may provide some protection. Sea turtle nesting areas, dugong feeding areas, cetacean migratory corridors and calving grounds are examples of other special areas that can be protected seasonally. Other types of special areas include isolated habitats that have unique assemblages and populations, habitats that are important for endemic species and highly diverse areas.
Isolated populations Isolated populations (e.g. those on remote atolls) have high conservation value where they harbor endemic species and/or unique assemblages. A location or population 20–30 km from its nearest neighbor generally qualifies as isolated in the absence of a persistent linking current. Their isolation (low connectivity) requires such areas to be largely self-replenishing. This leaves them less resilient to disturbance. Sustaining their marine species requires a higher fraction of living areas to be protected.
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