Deep, Deep Down, Fish Are Booming, Study Says

Nicholas Bakalar
for National Geographic News

March 30, 2006

Although many fish populations are declining, a few species appear to be
increasing their numbers, in particular those beyond the reach of
fishing fleets.

These fish live in the abyssal plains, flat expanses of the ocean floor at depths of 10,000 to 20,000 feet (3,000 to 6,000 meters). Abyssal grenadiers, for example, have more than doubled in number between 1989 and 2004, according to a new study.

The fish— also called rattails or deep-sea grenadiers—grow to about 2 feet (60 centimeters) long and are long-lived. But almost nothing is known about their reproductive habits.

Most experts agree that abyssal grenadiers are slow-growing animals, but estimates of their life spans vary from 6 to 60 years.

The deep-dwelling fish are extremely tough to study.

When they are brought up from the depths, gases in their bladders expand, popping the fish's stomachs and making their eyes bug out. No abyssal grenadier has ever been kept in captivity.

Why the Boom?

David Bailey, a research fellow at the Scripps Institution of Oceanography in La Jolla, California, is the lead author of the new study. He sees two reasons for the species flourishing.

"First," Bailey said, "natural climatic and oceanographic changes have increased food supply to the seafloor, triggering an increase in deep-sea fish abundances."

"Second, the absence of direct fishing pressure on the deep-sea fish has allowed … these changes." The report appears in the March issue of the journal Ecology.

The abyssal plains, covering more than half the Earth's surface, are the world's largest habitat.

The researchers used a camera sled—a camera-equipped rig towed behind and below a ship—to survey the depths.

They swept over an area 135 miles (220 kilometers) west of Point Conception off the California coast (map). The camera recorded 1-mile-long (1.6-kilometer-long) sections, one after another, at depths between 13,000 and 14,000 feet (4,000 and 4,250 meters).

The only fish that were numerous enough for statistical analysis were two different species of abyssal grenadiers.

The grenadiers' numbers were found to be linked to the total abundance of one prey group, in particular: echinoderms—sea stars (or starfish), sea cucumbers, sand dollars, and sea urchins.

Grenadiers also feed on carrion, smaller fish, cephalopods (such as squid and octopuses), and crustaceans (such as shrimps and crabs). But it was the echinoderms that were observed most frequently in the camera-sled photographs, so the researchers used these animals as indicators of overall prey abundance.

Good News?

"It's nice to see this happen," said I.G. Priede, a professor of zoology and director of the Oceanlab at the University of Aberdeen in Scotland.

"But these fish are too slow growing [in terms of reproduction] for this [increase in numbers] to have been the result of a genuine increase in population," Priede said.

"As Bailey suggests, this may be the result of immigration of fish from elsewhere in the Pacific Ocean, aiming to exploit the bonanza [of prey species] that has developed" along the abyssal plains.

Bailey, the lead author of the study, points out that grenadiers are not the only fish population that seems to be increasing.

"If you look at the North Pacific fisheries literature, you'll see that sardine and squid catches have increased over the last few years," he said. Sardine and squid do not inhabit abyssal plains.

"Their abundances, and the abundances of the plankton, which eventually feeds the benthic [bottom-dwelling] invertebrates, are strongly tied to oceanographic changes such as the El Niño/La Niña cycle."

According to Bailey, winds off the California coast push water offshore, drawing cool, nutrient-rich water up to the surface, where it increases plankton productivity. The thriving plankton then sinks to the depths, where it feeds the echinoderms, which in turn feed the fish of the abyssal plains, the thinking goes.

"Long-term studies of this type are extremely rare," Bailey said, "and we were able to examine only a small area of the ocean floor."

"More studies like this would provide useful information about how fish populations work when they're not fished, and how the dynamics of deep-ocean communities differ from those in shallow water."

Studies like these, he concluded, "will be essential in identifying which changes are due to human impact and which are caused by natural cycles."

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