Scientists are trying to fathom why Hawaii’s fish population is declining
Hawaii has a problem. The islands’ fish—one of the state’s most precious resources—are disappearing. The proportion of adults seen in catch reports for one valuable (and delicious) species of bottom fish, ehu, is down to 10 percent of its levels of 50 years ago, and a second bottom fish, onaga, has faced a similarly dramatic decline. Reef fish are also in decline. In 1999, Brian Tissot, a marine ecologist at Washington State University, found that in areas targeted by aquarium collectors, populations of eight of the most popular species had fallen by anywhere from 38 to 57 percent.
Slowly, Hawaii is restricting fishing and the collecting of aquarium fish from certain areas. But it is one thing to draw boundaries and another to know they are doing their job. All over the world, marine biologists are tackling a host of questions: Where should reserves be located? How big should they be? The answers depend, in part, on the fish themselves: How far do they travel? Where do they spawn? What habitats do they prefer? To find answers, scientists in Hawaii are monitoring fish populations, tracking their movements, and using sonar and submersibles to home in on critical habitats.
The first stop of Bill Walsh’s research boat is Keopuka, an area that allows both fishing and aquarium collecting. Of all the fish on the reef, the neon-lemon yellow tang is the flashiest. A favorite among collectors, yellow tang accounted for almost half of the more than 406,098 aquarium fish reported caught in Hawaii in 1999. Walsh calls it an indicator species, one that suggests how other aquarium favorites are impacted. In stores on the mainland, yellow tang retail for anywhere from $15 to $40 per fish. Less common species sell for more. A Hawaiian dragon moray eel, for example, can fetch up to $600.
Walsh, who works for the state Division of Aquatic Resources, is monitoring 23 ocean sites on the west coast of the Big Island of Hawaii, the main source of aquarium fish collected in the state. He and his team visit each site every other month, counting and recording every fish they see in four 100-square-meter areas. For key species, such as the yellow tang, they also note size, and keep a special watch for baby fish, known as recruits.
Unlike freshwater tropical fish, which are cultivated in tanks, most ornamental saltwater fish are caught in tropical reefs. In Hawaii, divers often herd fish into large nets, then use hand nets to scoop them into buckets. Hawaiian collector David Dart says that in good weather, an average diver can pull in 100 to 150 yellow tang a day. On other reefs around the world, collecting is more aggressive: fishermen use cyanide to stun fish, making them easy to scoop up.
The monitored sites include areas protected for a minimum of ten years that prohibit collecting, fishing or both, recently restricted areas closed only to aquarium collecting, and those open to all fishing. Walsh’s numbers are beginning to detect trends. Three years into a five-year study, he has found that yellow tang is no longer declining in the newly protected areas but continues to decline in open areas.
Aquarium collecting is not the only threat to reef fish. Overfishing, warming oceans and pollution from human development all threaten both the reefs themselves and the fish that feed off them. On the day I drove to Hanauma Bay, a Marine Life Conservation District on Oahu and Hawaii’s most famous snorkeling spot, the beach was oddly empty except for police officers. It was closed because a broken pipe was spewing sewage over the reef.
In his surveys, Walsh also counts the baby recruits in protected and unprotected areas. The fact that he has yet to detect much of a difference worries him. "Without an increase in recruitment, the reserves won’t realize their full potential," he says.
On another day, off the Maui coast, the sea is calm and so deeply blue that the sky pales in comparison. Diving just underneath the waves, you can hear the songs of humpback whales. Chris Kelley and his crew are fishing for onaga and ehu, which live at depths of up to 1,000 feet. The researchers record the location of every fish reeled in and correlate that information with seafloor images they get from a sonar fish finder. They note each individual’s sex, length and weight, then throw it back. These fishing trips, combined with observations made in submersible dives, are painting a picture of the lives of these little-known species.
Kelley has found that adult fish prefer craggy outcrops, while the young seem to hover over lower relief outcrops. Ehu live closer to the bottom, while onaga school several meters above it. Kelley has surveyed bottom fish habitat throughout the main Hawaiian Islands, and in several months, he will present his recommendations for Hawaii’s 19 bottom fish reserves to the state government. The areas were set up with scant knowledge of the type and location of critical fish habitat. Now, Kelley’s data shows that at least two of the reserves are misplaced, lacking the jagged topography the fish call home.
In Pupukea on the north shore of Oahu, I follow Carl Meyer, a graduate student at the University of Hawaii, into the water. He shows me how easy it is to catch fish at night, bare-handed. Shine a light into their eyes and they freeze like deer on a road. He grabs a striped convict tang. In one study, Meyer caught dozens of fish, tagged them with ultrasonic transmitters and released them. Then, to learn their movement patterns, he paddled after them in his hydrophone-equipped kayak.
He discovered that the three species he tracked (blue spine unicorn fish, yellowstripe goatfish and blue jack) all had well-defined home ranges. With the regularity of bank employees, they commuted daily between their sleeping areas in the reef and their feeding grounds. While unicorn fish stayed within the reserve, the goatfish and jack had home ranges three times the reserve size, exposing them to daily danger. Meyer concluded that tripling the reserve’s size, which is currently a mere tenth of a square mile, would protect these mobile species.
In a statement issued last autumn, 161 of the nation’s leading marine scientists suggested that reserves strung together like pearls would provide maximum protection, reseeding each other as well as the nearby fishing areas. Designing optimal reserves therefore requires knowledge of current patterns, larval transport routes and fish movement—there is still much left to learn.