March-April 2001 > Features

Notes from the Field

Empty Nets

Molecular genetics may find a cure.

by Stephen Palumbi

On some golden days, you can forget the laptop and take a snorkel to work. For a month of those days, last summer on Bali, I pursued alternative research tools, including a keen eye for shrimp. The only way to push the limits of marine biology is to be there in it, and we were on an expedition sponsored by the Putnam Fund at the Museum of Comparative Zoology to discover the wash of marine-species diversity around the shoals and islands of Indonesia. Indonesia is the cradle of the world’s most impressive marine biodiversity, hosting thousands upon thousands of invertebrates, algae, and fish along its wide-ranging archipelagoes. More than 500 species of reef-building corals live here, the architects of an ecosystem that supports massive productivity and helps feed a quarter of a billion people. But this was no mere dip in a crystal sea, it was an expedition with a grim purpose. The oceans of the world are ailing, and we were on the Bali reefs to take the blood pressure of our declining patient. 



There have been alarming changes in the sea. An unrelenting increase in human population, plus an increase in migration to coastlines, have hit the coastal oceans with a double whammy. Sewage flows unimpeded onto those Balinese reefs, which—like reefs elsewhere—are further damaged by agricultural run-off of mud and pesticides. Smothered by sediment, the reefs are also picked over by thousands of coastal fishers whose livelihood and family protein come directly from the sputtering productivity of the sea.

One of the alarm signals of the declining oceans is the collapse of fisheries around the world. More than half the world’s most productive fishing grounds are overfished or fished to their maximum, threatening the 100 million tons of food we pull from the oceans each year. In Bali these declines hit home all too clearly. Exploring along its northern beaches, watching the sun come up over the rice-covered mountains, I came across a handful of local fishers pulling in their morning’s catch. A hundred meters of badly tattered nylon netting, weighted at the bottom with coral stones and floated at the top by battered styrofoam and empty water bottles, was stretched in an expectant semicircle across the mouth of a small stream flowing down the beach into the ocean. I grabbed one end of the line—everybody likes help pulling a heavy beach net—and hauled. Slowly the net piled onto the beach and more people came to pull or wait. The bottom of the net appeared in the small beach breakers, and boys leaped down to prevent any fish escapes. Finally the deep pocket of the net came up, the payoff wrapped in thin meshwork, blinking in the morning sun.

The entire effort netted no more than a kilogram of useful food. There was a great variety in the net—no two fish or invertebrates were alike—a good showing for biodiversity fans. But all four fish were small, no more than six inches, and the tiny squid and octopus would barely flavor the soup pot they were destined for. Women swooped down to claim the catch in brightly decorated ceramic bowls. The children played with two inedible species, a puffer-fish, forced to prove its name, and a spiny sea urchin more capable of defending itself.

I doubt this was a bad fishing day. No other net I pulled, set by men or children, at night or morning, had a better catch. No fishing boat was full when it returned to the beaches. Pushed by declining water quality and overwhelmed by the unsustainable fishing of a growing coastal population, the sea has become stingy on the north shore of Bali.

 

 

Back in my Cambridge lab, my colleagues and I look at our own catch and wonder about the future of the world’s oceans. There is a growing momentum among marine scientists to throw themselves into battle for the health of the sea, and I have long followed a strategy of homing in on the particular contribution that expertise in molecular genetics of marine species can provide. How does molecular genetics help fill a net? By helping to design a system of marine parks to stabilize over-fished populations.

At the top of the list for new approaches to marine conservation is a move to husband entire marine ecosystems in fully protected reserves that prevent human exploitation and provide seed-beds for continued productivity in fished areas. A partial reserve on the Georges Bank off New England shows some of this potential: large cod-fishing grounds, closed to fish trawling, now yield a wealth of succulent scallops. A new reserve in the turquoise shoals between the main Hawaiian Islands and Midway is designed to protect fished reefs. Such areas have been studied for a decade or more, and show dramatic increases in fish density, biomass, and body size. But a “No Fishing” sign means fishers can’t make a living, and so the long-term success of reserves as a socioeconomic strategy depends on whether these ecosystem-conservation principles also result in increased human prosperity.

That requires that these reserves, scattered across the seascape, do two things. They must export the eggs or larvae of fish and invertebrates into surrounding waters to fuel the growth of populations that can be fished, and they must send each other eggs and larvae so that the (usually) small reserves can be sustained in perpetuity, with new generations flowing from old. This is one of the outstanding problems in marine conservation, and one of the ways marine systems differ startlingly from their terrestrial counterparts. Many marine species, especially those that humans depend on, do not give birth to live young, and do not raise their young in nests or dens. Instead, they release vast clouds of tiny eggs into the sea, and don’t even look back as their millions of offspring drift away on ocean currents. A marine park, then, may host an adult population of groupers, lobsters, or manta rays, but their offspring may live elsewhere, in another park or in the path of the fishers.

Indonesia has been building a set of fully protected marine reserves, scattered across the thousands of miles of coral islands and mangrove shores. Visited by some of the most consistent and strong ocean currents in the world, these shores appear well-suited to keep the connected set of reserves functioning effectively. The Pacific Ocean drains into the Indian Ocean here, cutting the deep Makassar Strait with a current that has flowed for tens of millions of years. Ocean buoys, drifting under the scrutiny of watchful satellites, have traveled 2,000 kilometers—from the Celebes Sea into the Java Sea and through the Makassar Strait—in under three weeks. Surely oceanic eggs and larvae would do the same.

But the genes say differently. My post-doctoral associate Paul Barber and I have been working on small coastal shrimp that live on abundant coral rubble, using the sequence of a small gene segment to map where their larvae go. The eggs hatch in their mother’s burrow, but are soon released to experience the vagaries of the sea and—we thought—helplessly ride the ocean currents for three to four weeks. Much to our surprise, the Celebes Sea and Java populations showed a genetic signature that tells us no larvae journeyed that far. Even more irregular, populations in the same sea, just a few hundred kilometers apart, also showed very different genetics. In fact, a strong but previously hidden genetic barrier runs through the middle of the Java Sea for this species. Other closed pockets in ancient sea basins and exotic bays like the South China Sea, the Flores Sea, and the Bay of Tomini also have static populations that will not drift together in networks of functional marine reserves.

On a return trip to Bali in October, I attended the ninth International Coral Reef Symposium. There were no fishers on the beaches of the huge hotels hosting the conference and I got my morning exercise by swims out to the reef. Paul Barber, Steve Vollmer (a graduate student studying coral genetics), and I gave four talks. Our message came out most clearly in a standing-room-only session on marine reserves and their use in conservation and fisheries. Our maps rolled out of the projector, and the story unfolded: the oceans harbor deep surprises in our efforts to manage them and even cure their ills. These results have begun to have an impact on marine conservation in Indonesia, but our ocean patient needs careful study.

We can design an effective set of reserves in Indonesia, but not if we merely assume we know how the oceans work, not if we build theories in Cambridge and never test them in the real world. By measuring the genetics of shrimp populations as proxies for other species, we have redrawn the lines that connect one marine reserve to another and shown that locally managed sets of reserves within each basin, rather than an array dotted across the entire vast country, will need to be implemented before the people of Bali’s north coast get much relief. Naturally, we need to test other species—those of particular commercial importance call out for study—and I suspect we will be back in Indonesia soon. It may take a while to fill the fishing nets in Bali, and more than marine parks will be needed—a sew-age treatment plant would be nice. But we have firmly made the link between the genetics of the sea and the future of the sea, and look forward to riding the waves of increased commitment to ocean health. 
Professor Stephen Palumbi of the department of organismic and evolutionary biology is also curator of marine invertebrates at the Museum of Comparative Zoology. More details of his research appear in the January 2001 issue of Scientific American .