Global ChangeIntersection of Nature and Culture

Important social and ecological dimensions to conserving and restoring marine environments

Wednesday, November 18th, 2009

Since the industrialization of fishing in the 1970s, the combination of longlining, trawling, dredging, and other forms of seafood harvesting  have decimated marine species populations.

Predatory fish, including tunas, marlin, cod, and sharks, have declined more than 80 percent (here and here) over the past twenty years as a result of overharvest and accidental bycatch.  In the Caribbean alone, green turtle populations may have numbered over 90 million three centuries ago compared with 300,000 today.

That’s so staggering I have to repeat it—80% declines.  This is some of the most visible evidence of global change on the planet.  It’s almost unbelievable.

Because people preferentially remove top predators when harvesting seafood, this leads to what we call a “trophic cascade,” as the abundance of other species lower on the food chain adjust in response to the loss of predators.  In cooler, temperate marine ecosystems, the loss of predatory fish and lobsters often causes an increase in sea urchins and gastropod species (e.g., snails).  Many of these species are herbivores, grazing on algae.  So an increase in their populations leads to a situation of algae overgrazing, sometimes creating what are known as “urchin barrens.”  It’s analogous to a deforested area on land, where both habitat and food are lost.

We often don’t think about these connections—how removing tasty fish from the sea can lead to widespread loss in algae, causing ecological systems to collapse.

Over the past decade, marine protected areas (MPAs) have become a popular tool for slowing the decline in marine populations, especially in coastal areas where  sensitive habitat (like coral and rocky reefs) and fishing grounds often overlap.

The idea of MPAs is simple:  Cordon off an area and eliminate or restrict fishing within the zone.  Over time, the populations of species (like fish) increase and animals get bigger.  These animals can then disperse out of the protected areas into legal fishing zones where they can be harvested.  In an ideal system, it’s a win-win situation—habitats and species are protected and sustainable fishing harvests can be maintained.

There are a few problems, however…

Problem 1: Most of these generalizations are derived from short term studies (< 3 years), that, while useful, may not tell the full story about how marine ecosystems change following protection.

Problem 2: New MPAs may have different histories, from lightly fished to severely depleted, leading to different post-protection legacies (i.e., we may not expect species recovery to be the same).  This could skew our interpretation of how successful MPAs are.  Enter the social dimension… As nations move to develop MPAs, fishers often co-opt good fishing grounds (ones that are often highly depleted) and leave the marginal, lightly fished areas for MPAs. Does this matter?

In the latest issue^1,2 of Ecological Applications, Graham Edgar and colleagues report longer-term changes (up to 16-years) in MPAs located in southern (temperate) Australia.  [Side note: Edgar (in Aussie, it's pronounced "aid-gaaah") also wrote one of the best Australian temperate marine taxonomy texts there is.  So beautiful it makes a great coffee table book].

What did they find?

• Changes in populations can be dramatic and can persist for decades.  In Tasmanian MPAs, predator (fish and lobsters) densities increased 500-2000% (5-20 fold increase) in MPAs compared to fished zones.  Densities are still going up.
• Not surprisingly, populations of urchins and gastropods fell 60-75% as predation recovered. Densities are still going down.
• These trends continued even after 16 years.
• This means that algae species like giant kelp are probably increasing due to the reduced herbivory from urchins and snails.
• In turn, this algae can serve as a food and habitat source for other species, thereby compounding the benefits of MPAs.
• These changes were so large that Edgar and colleagues believe they swamp out any effect of starting conditions.  Put another way, the ecological changes over time within a MPA are more significant than whether that MPA had a history of light fishing vs. heavy harvesting.
• Nevertheless, they note from the dramatic changes in Tasmania, that heavily fished areas have the potential to show very high magnitudes of ecological recovery.  Therefore, these areas (which are currently often co-opted by fishers) should be negotiated for inclusion within MPAs.  Of course, the fishing communities need to be convinced that the benefits of MPA creation outweigh the loss of some fishing grounds because the MPAs will one day become the source of larger and more-sustainable fish stocks.

Bottom line:

This work supports earlier studies suggesting that MPAs can help facilitate large recoveries of marine species.   That’s a good thing, too, because 2012 is the target year for the establishment of MPAs worldwide as part of the UN Convention on Biological Diversity.  However, Edgar warns that because the ecological changes in MPAs are persisting for decades after establishment, the full, long-term impact of fishing and MPAs has yet to be assessed.  That’s where long-term monitoring networks will need to play a big role.  And more work needs to be done to get some of the most heavily fished areas into conservation.  Stay tuned…

¹Edgar, G. et al. (2009) Exploited reefs protected from fishing transform over decades into conservation features otherwise absent from seascapes. Ecological Applications 19(8):1967-1974.

²Bowdoin people can access the article here.

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Photo credit:  http://www.flickr.com/photos/edbierman/ / CC BY 2.0