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The Scoop on Spillover: How Blue Parks are Bigger than Their Boundaries

Blog by: Allie Smith, Blue Parks Science Intern

Featured Image: Hammerhead Shark by Shane Gross

What we’ve seen in marine protected areas is promising: evidence that these reserves not only support the ecosystems within their boundaries, but also benefit surrounding marine and human communities. Though hard to quantify, the “spillover effect” has been documented in several Blue Parks. As fully protected areas and strategic networks of MPAs continue to grow, we will continue to see more evidence that what’s good for fish is also good for the local communities that rely on them.

Marine protected areas (MPAs), especially fully and highly protected areas, are safe havens for countless species, but not all animals stay within their protective boundaries—and that can be a good thing! In fully and highly protected areas, where marine life is protected from destructive human activities like fishing, populations of marine animals can potentially survive longer, grow larger, and produce more offspring than unprotected or exploited populations. (In some cases, the result can be a whole order-of-magnitude difference in egg production!) [1] [2] With this vast increase in reproductive capacity, populations of commercially exploited species have the opportunity to grow and rebound within an MPA’s boundaries.

However, both fish and their larvae are dynamic animals and can readily move between an MPA and its adjacent waters, where fishing may occur. As protected fish populations become larger, more of them are likely to spill beyond the invisible boundaries of the MPA. This is known as the spillover effect — the net emigration of organisms across the MPA boundary. [3]

Quantifying spillover effects and confirming that they occur is often difficult and requires generous time and effort. For example, Dr. John Garza – a U.S. National Oceanic and Atmospheric Administration (NOAA) geneticist – and his team studied the movement patterns of kelp rockfish along the coast of California using genetic analysis, which required intensive sampling and a large sample size. Their study demonstrated that kelp rockfish larvae from marine reserves travel as far as 20 km, including to fished areas, where they settle into suitable habitat. [4] Spillover like this can directly benefit local communities and fishers by supporting more abundant and sustainable populations of targeted species. Spillover is also beneficial for tourism – SCUBA divers are attracted to areas with high biodiversity and abundance of organisms, which, thanks to spillover, may be right outside MPA boundaries. [5]

Blue Parks Demonstrating Spillover

Several Blue Parks, the outstanding MPAs that meet science-based standards for conservation effectiveness, have documented spillover effects that benefited local communities. For example, in the Bahamas, the iconic queen conch (Lobatus gigas) supports an incredibly popular and lucrative fishery, but its populations are declining. Exuma Cays Land and Sea Park (ECLSP), a 600km2 no-take MPA and a 2018 Blue Park Award winner, has been protecting the local ecosystem for over 60 years and now hosts an abundant, large, and old population of queen conch. In turn, these older, larger individuals disperse larvae into unprotected and fished adjacent areas. While spillover like this is vital for the health of the local fishery, researchers note that because populations are still vulnerable and the park’s size is limited, protecting larval conch sources upstream of the reserve is crucial for the population to recover. A promising solution could be to strategically place new MPAs in the area to create a well-connected network. [6] [7]

In addition to the queen conch, spiny lobster and Nassau groupers have greater density, size, and potential egg production in the park compared to adjacent areas. The Nassau grouper’s estimated egg production is as high as 7 times greater inside ECLSP than outside, which makes emigration of individuals from the protected area likely. [7] [8]

Queen Conch at Exuma Cays Land and Sea Park, Photo: Shane Gross

In the Channel Islands reserve network (Blue Park Award 2019), there is also strong evidence of spillover among California spiny lobster (Panulirus interruptus) populations, specifically around Santa Cruz and Santa Rosa Islands. There are significant commercial and recreational fisheries focused on California spiny lobster in the Southern California Bight, but in the 6 years following the implementation of two no-take zones, spiny lobsters increased in density and biomass both within the MPA and in fished areas outside the MPA. [9][10] Spillover models tend to predict  a peak in population density near the center of reserves, and a decline as borders and fished areas are approached. [11] A 2012 study in the Channel Islands affirmed this model—fewer lobsters were trapped near the borders of the MPAs than in the centers, suggesting that adult spiny lobsters are emigrating outward from the dense centers of these no-take zones and spilling over into the waters beyond.

California Spiny Lobster, Photo: NPS

What we’ve seen so far in the Blue Parks is promising – evidence that MPAs not only support the ecosystem within protected boundaries, but also benefit surrounding marine ecosystems and local communities that rely on fishing through spillover effects. Strategy is important as the global community begins to take action on protecting 30% of the ocean by 2030. By ensuring that MPAs protect critical habitat (such as breeding and nursery grounds), as well as creating well-connected networks (expanding and creating new MPAs), these benefits will grow.


[1] Roberts, C.M. et al. (2001). Effects of Marine Reserves on Adjacent Fisheries. Science 294:5548, p. 1920-1923

[2] Puget Sound Research ’95 Proceedings (1995). Puget Sound Water Quality Authority Volume 1

[3]Buxton, C.D. et al. (2014). When is spillover from marine reserves likely to benefit fisheries? PLoS One

[4]  Baetscher, D. S. et al. (2019). Dispersal of a nearshore marine fish connects marine reserves and adjacent fished areas along an open coast. Molecular Ecology 29:7, p. 1611-1623

[5] Di Lorenzo, M. et al. (2020). Assessing spillover from marine protected areas and its drivers: A meta-analytical approach. Fish and Fisheries 21:906-915

[6] Kough, A. S. et al. (2019). Ecological spillover from a marine protected area replenishes an over-exploited population across an island chain. Conservation Science and Practice 1:3

[7] Chiappone, M. and Sullivan Sealey, K.M. (2000) Evaluating the Success of the Exuma Cays Land and Sea Park, a Large Marine Reserve in the Central Bahamas. GCFl:51

[8] Sluka, R. et al. (1997) The benefits of a marine fishery reserve for Nassau grouper Epinephelus striatus  in the central Bahamas. Proc 8th Int Coral Reef Sym 2:1961-1964

[9]California Department of Fish and Wildlife (2017). California Spiny Lobster Fishery Management Plan. Retrieved May 2021 from

[10] Lenihan, H.S. et al. (2021) Evidence that spillover from Marine Protected Areas benefits the spiny lobster (Panulirus interruptus) fishery in southern California. Sci Rep 11, 2663

[11]Kay, M.C. et al. (2012). Effects of marine reserves on California spiny lobster are robust and modified by fine-scale habitat features and distance from reserve borders. Mar Ecol Prog Ser 451: 137-150