Preventing Trophic Cascades

A microcosmic look at a macrocosmic issue

Imagine swimming along a lifeless tropical reef. The concept seems almost oxymoronic. Yet on this reef the largest fish you see is six inches long, and the reef is mostly rubble and sand dappled with the occasional sea fan or hard coral. These sparse corals aren't in very good shape, either; they're becoming overgrown with algae and, even to the untrained eye, they appear to be fighting a losing battle. Throughout the Caribbean Sea, this imagined scene is becoming an unfortunate reality.
Why is this happening?
Why are our favorite dive sites—these vibrant reefs that were teeming with life 30 years ago—becoming drab and lifeless? The answer is a complex combination of factors we can almost recite by heart: Global warming is causing coral bleaching, acidification may be impacting all marine invertebrates, and overfishing is decimating fish populations. Not to mention pollution, invasive species, and a whole host of other factors are impacting the health of coral reef communities.

We know all of these issues play a role in the decline of reef health, but perhaps the most obvious short-term impacts are felt as a result of overfishing. Take, for example, a marine reserve: While a handful of buoys, a park ranger and a bit of legislation can protect fish populations from exploitation, they can't protect against warming water temperatures and the other large-scale threats. Nevertheless, many marine reserves boast healthier coral reefs than neighboring areas open to fishing, mainly a result of natural services provided by certain reef dwellers, such as herbivorous algal grazers (like parrotfish and urchins), which restrict algae growth and promote healthier reefs. Remove these fishy service providers and reef health suffers, especially in already-stressed environments where bleached corals may be struggling against fast-growing algae and other destructive competitors.

The relationship is, of course, a two-way street. Reef health has been shown to significantly affect fish populations, and coral declines have sometimes resulted in fish biodiversity and population loss despite the presence of marine reserves. Passing legislation to protect fish populations is a tangible first step in preservation.
The apex effect
While overfishing in any form can be harmful to reef health, it's typically the extraction of our favorite food fishes that impacts reef ecosystems most significantly. Tuna, snapper, grouper—yes, they're all delicious, but they also tend to fill a similar niche.

These apex predators have intense top-down controlling effects on fish populations as a whole, making their presence critical to the healthy functioning of marine ecosystems. The effects of the removal of apex predators have been evidenced time after time. For example, the eradication of wolves in the Northeastern United States resulted in an explosion in the white-tailed deer population, and in the West, a huge increase in both coyote and elk populations. Similar situations exist in essentially every terrestrial, freshwater and marine habitat in the world where apex predators, such as big cats or sharks, have been removed. These phenomena are known as trophic cascades and they usually have far-flung, unanticipated impacts beyond a simple imbalance in species composition.

The removal of apex predators in marine ecosystems results in trophic cascades that can manifest themselves in obvious explosions of suddenly predator-free fish populations (which in turn heavily impacts their prey species), as well as more subtle changes in ecosystem structure, all of which can severely disrupt the natural functioning of coral reef communities. The breakdown of natural function can make reefs more susceptible to disease, algal competition, invasive species and numerous other impacts that remain poorly understood. Too, the resulting decline in coral health negatively affects reef fish populations, creating a positive-feedback loop and turning our reefs into the dull, brown ecosystems where the highlight of a dive trip might be seeing a parrotfish.

In response to increasing scientific evidence of the negative effects of trophic cascades, many governments have started protecting apex predators and fish populations with a variety of strategies ranging from marine reserves to seasonal fisheries closures.
Examining a microcosm: Nassau grouper

In the Cayman Islands, Nassau grouper have been protected from fishing during spawning season for the last eight years. As one of the most desirable food fishes in the Caribbean, Nassau grouper have been all but wiped out, landing the species on the International Union for Conservation of Nature Red List of Endangered Species.

Nassau grouper reproduce locally in spawning aggregations, assembling on a nearby reef for weeks just once or twice a year before unleashing an absolute frenzy of spawning during one night a few days after the full moon. This mass-spawning behavior increases the chances of larval survival, but it also makes the adult grouper incredibly vulnerable to fishing. Spawning aggregations (SPAGs) with grouper numbering in the tens of thousands have been wiped out all over the Caribbean, and because the Nassau grouper who are in a SPAG all come from local reefs, their populations usually do not recover after such heavy exploitation without inflow from healthy populations.

Using fishing bans during spawning season, the Cayman government has protected what might be the last healthy Nassau grouper SPAG in existence. Since 2002, the REEF Grouper Moon Project and the Cayman Department of Environment have collaborated to study the effects of protection on Cayman SPAGs, and they've found steady increases in the Nassau grouper spawning population since protection began. Every February on Little Cayman, over 3,000 grouper assemble in an area smaller than a football field to spawn, maintaining the healthiest known population of Nassau grouper in the world. Not surprisingly, the reefs on Little Cayman also happen to be some of the most vibrant and lush in the Caribbean.

Unfortunately, a small fishing community in the Cayman Islands is calling for an end to the fishing restrictions. Unless the government renews the current legislation by December 2011, SPAGs throughout the Cayman Islands will be open to fishing, spelling the likely decline of a healthy Nassau grouper population and the health of its marine ecosystem.
What can be done?

Divers are firsthand observers of reef health and appreciate the value of a healthy population of marine life inhabiting reefs and other underwater environments. You can take simple steps to help prevent trophic cascades.

  • Eat lower down the food chain: fishes like Barramundi and Tilapia taste great and have less of an impact on marine ecosystems.

  • Don't eat fish that come from unsustainable fisheries: check how you're doing at Seafood Watch.

  • Support apex predator conservation programs.

  • When diving, be aware of your surroundings to avoid negatively impacting reef health as this can also adversely affect the marine life that depends on the reef.

You can also take some simple steps to help with the Nassau grouper microcosm.

  • If you're visiting the Cayman Islands during spawning season (January to March), don't eat Nassau grouper.

  • Sign the petition to tell the Cayman Government that Nassau grouper are important to you.

  • Send an email to the Cayman or voicing your support of a continued ban on fishing Nassau grouper during spawning season.

  • Watch the video below and learn more about Nassau grouper and REEF's Grouper Moon Project.

Groupers' Last Stand from Josh Stewart on Vimeo.