|Marianas Trench Marine National Monument: one of the largest MPA in the world, established in 2009 and covering 95,216 square miles. Source: http://ocean.nationalgeographic.com/ocean/photos/us-marine-protected-areas/|
MPAs can have numerous benefits. One of the primary benefits is linked to the fact that a few large female fish generally contribute most of the eggs to the next generation of fish. For example, one large female red snapper in Florida was estimated to produce the same number of eggs as 212 females just 7 inches smaller and 3 years younger.
|One 24in female red snapper (~8yrs old) produces as many eggs as 212 17in females (~5yrs old). Source: http://gulfcouncil.blogspot.com/2013/02/more-fish-less-fishing.html. Artwork Credit: Dianne Rome Peebles.|
Protecting an important area from fishing could, therefore, result in larger fish inside the reserve (that survive to a larger size because they aren’t removed by fishing), increasing the quality and quantity of eggs produced for the whole population. This increased fecundity would ideally lead to an increase in fish abundance over time inside and outside the MPA. As expected, MPAs with pockets of large more productive fish, higher recruitment and increases in fish abundance that also increase catch for anglers have been seen in many cases.
|Dispersal of offspring using fishery reserves. Adult stocks are protected from harvest inside reserve areas while their eggs and larvae are dispersed to reserve and fished areas by natural processes. Source: Bohnsack 1990.|
Despite this success, anglers are often against MPAs as a management strategy. This is likely related to the fact that, even when MPAs are implemented perfectly, a time lag exists between closure of an area and the onset of benefits. This is especially true when fish are long-lived and slow growing, where benefits to the fishing community may not be seen for years after a MPA is established.
Additionally, closing an area to fishing forces some anglers to travel greater distances to fish and open areas to receive more fishing pressure as a result of the displaced anglers. Anglers’ frustrations with MPA’s can be further exasperated when MPAs are not monitored or enforced effectively, and efficacy of the MPA is unknown. This is an unfortunately common occurrence among many MPAs established across the globe.
|Estimated survival of a cohort of 10,000 red snapper under the same conditions listed in the figure above. Source: Bohnsack 1990.|
However, monitoring the effectiveness of MPAs can be expensive and problematic because most estimates of fish abundance and size come from fishing and the primary characteristic of a MPA is to exclude fishing. To circumvent this difficulty some have used visual diver surveys, remotely operated vehicles or camera systems to estimate abundance and fish size. In the research group I am currently part of, we employ a baited camera system that uses two ultra-low light stereo-video paired cameras at very specific angles (set-up similar to a pair of eyes) that with some complicated geometry allows us to estimate not only the number of fish we see but the size of these fish as well.
|Remotely operated vehicle (ROV) used to conduct underwater surveys of marine life. Source: http://www.divetechltd.ca/services.htm|
MPAs are not always the best solution. For instance, species that are highly migratory may not benefit from a MPA that they would likely just swim through. In most cases, only those fish that are relatively site attached or spend a large portion of time inside the protected area will survive and grow to a larger size. In the instances that a MPA is appropriate, evidence suggests that it can be a very useful tool for fishery management, but it is a long term strategy and patience will be needed when applying this method as benefits will likely not be seen for years after the MPA is established.
The information above is not nearly inclusive of all the details, benefits and associated caveats that come with managing with MPAs. If you’d like more information I would suggest checking out some of the references and additional reading material listed below.
References and additional reading material:
Boersma PD and Parrish JK. 1999. Limiting abuse: marine protected areas, a limited solution. Ecological Economics, 31:287-304.
Bohnsack JA. 1990. Proceedings of the 43rd Gulf and Caribbean Fisheries Institute.
Bohnsack JA and Ault JS. 1996. Management strategies to conserve marine biodiversity. Oceanography, 9:73-82.
Friedlander AM and DeMartini EE. 2002. Contrasts in density, size, and biomass of reef fishes between the northwestern and the main Hawaiian islands: the effects of fishing down apex predators. Marine Ecology Progress Series, 230:253-264.
Hilborn R, Stokes K, Maguire J, Smith T, Botsford LW, Mangel M, Orensanz J, Parma A, Rice J, Bell J, Cochrane KL, Garcia S, Hall SJ, Kirkwood GP, Sainsbury K, Stefansson G, Walters C. 2004. When can marine reserves improve fisheries management? Ocean and Coastal Management, 47:197-205.
Roberts CM and Polunin NVC. 1991. Are marine reserves effective in management of reef fisheries? Reviews in Fish Biology and Fisheries, 1:65-91.
Russ GR and Alcala AC. 2010. Decadal-scale rebuilding of predator biomass in Philippine marine reserves. Conservation Ecology, 163:1103-1106.
Willis TJ, Millar RB, Babcock RC. 2000. Detection of spatial variability in relative density of fishes: comparison of visual census, angling, and baited underwater video. Marine Ecology Progress Series, 198:249-260.
Willis TJ, Millar RB, Babcock RC. 2003. Protection of exploited fish in temperate regions: high density and biomass of snapper Pagrus auratus (Sparidae) in northern New Zealand marine reserves. Journal of Applied Ecology, 40:214-227.