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?

(more…)

Tags: marine protected areas
Posted in biodiversity science, nature and culture, sustainability | 1 Comment »