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interesting read about tilefish: published 1986

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#1 Angelo Ruvio

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Posted 02 November 2011 - 10:28 AM


Published: July 22, 1986

A single species of tilefish, burrowing and excavating in the ocean floor, has played a surprisingly central role in shaping the continental shelf off the East Coast of the United States, researchers have found.

In 41 submarine dives, scientists have discovered that tilefish protecting themselves from sharks dig shelters from Cape Cod to North Carolina. Far from being a smooth accumulation of sediment, as long postulated, much of the ocean floor along the edge of the continental shelf is dominated by the resulting grottos and deep burrows, especially where the shelf is cut by canyons formed when sea level was far lower in the ice ages.

The surprising extent to which one species of fish has altered the sea floor was recorded from the four-man submersible Johnson-Sea-Link, operating from the research vessel Johnson of the Harbor Branch Foundation of Fort Pierce, Fla.

Tilefish, which reach three feet and 60 pounds, are shaped somewhat like cod but carry a fleshy appendage behind their heads. They are found along the Atlantic seaboard from Nova Scotia to South America and appear unusually sensitive to cold.

They are found chiefly in the region where the continental shelf begins to drop off - at depths of 400 to 900 feet. This habitat, Dr. Churchill B. Grimes of the National Marine Fisheries Service, who is a participant in the study, explained recently, is a warm layer of water sandwiched between layers too cold for the fish's survival. Below is the frigid water of the deep ocean, and above is the shallow coastal water that becomes deeply chilled every winter. The temperature of the intermediate layer where the tilefish live hovers between 48 and 57 degrees Fahrenheit.

According to Dr. Kenneth W. Able of Rutgers University, another participant, tilefish were first described scientifically in 1879 and the Bureau of Commercial Fisheries, the forerunner of the National Marine Fisheries, hoped the fish would become a major food source. But in 1882 billions of tilefish suddenly died. Ships approaching New York crossed a band of floating fish 70 miles wide.

Scientists suspect that cold water invaded their habitat, possibly after a succession of cold winters, and for a time the species was thought to be extinct. In the 1890's, however, the fish reappeared. Ten million pounds were caught in 1915, and since 1972 they have been an important fishery, sold in large quantities to restaurants, largely through the Fulton Fish Market in New York and featured on many restaurant menus.

But submarine explorers over the last few years have seen abandoned burrows with increasing frequency, indicating a decline in tilefish population. Overfishing is believed to have reduced the population by a half to a third from 1979 to 1982.

Dr. Able, in an interview, said he believed tilefish excavated the burrows as shelters from predators, including hammerhead sharks sometimes seen in the area.

The Johnson-Sea-Link dives were conducted off the East Coast from 1980 to 1984 and last summer in the Gulf of Mexico. Three types of burrows were observed: cavities under boulders, vertically dug burrows, and horizontal ones in canyon walls.

Alongside Lydonia Canyon and Veatch Canyon southeast and south of Cape Cod the fish burrow under large boulders that were dropped there by melting icebergs in the last ice age.

Farther south, on the gently sloping outer flanks of the Hudson Canyon, an extension of the Hudson River Valley through the continental shelf, the fish dig straight down into the ocean floor. Their vertical burrows were found to dominate 300 square miles on both sides of the canyon. The canyon was formed 10,000 to 13,000 years ago by intense discharge of water when the last ice sheet was melting and low sea level exposed the continental shelf. Because the canyon is so deep, its steep lower walls are too cold for a tilefish habitat.

The scientists estimated the density of the Hudson Canyon burrows to be 1,234 per square kilometer. They are conical, and some, enlarged by crab burrows and caveins, are more than seven feet deep with top diameters of as much as 10 to 15 feet. The crabs are believed to be a primary food source for the fish.

To monitor one Hudson Canyon burrow for 24 hours, explorers mounted a 35-millimeter camera and a strobe on a tripod, for photographs every two minutes. The burrow appeared to be the home of a male and female, who foraged for food half the time and remained in the burrow the rest of the time.

The scientists believe vertical burrows are the primary tilefish habitat throughout the southern New England and Middle Atlantic regions.

Other canyons are a favored tilefish habitat because exposed layers of clay in their walls provide ideal places to burrow. Pueblos, homes carved in canyon walls, were first described in 1977 by Dr. John E. Warme of the Colorado School of Mines. More recent surveys have confirmed that they are all dug horizontally into exposed layers of stiff gray clay. They range in size from holes just large enough for entry by one fish to gashes 10 feet long, three feet high and three feet deep.

In their observations, scientists found that multiple openings sometimes lead to a single large grotto, recorded by the researchers when they observed a conger eel enter one burrow and emerge from another or when dye injected into one hole reappeared from others.

Clouds of fine sediment were sometimes seen coming from the holes, presumably washed out by swimming activity within and the currents it generates. Scientists believe the fish excavate their dens this way.

The researchers also suspect that the fish excavate with their mouths, because the explorers have seen what appear to be mouthfuls of clay near the burrows.

Tilefish burrows were often weakened around the rims through additional burrowing by crabs, which are very common at these depths. The resulting honeycombs and linked tunnels make the sea floor vulnerable to collapse, producing deep pits.

Such collapses over the thousands of years since the last ice age ''may have played an important part in shaping bottom topography around Hudson Canyon,'' Dr. Able and the other researchers report in the current Environmental Biology of Fishes.

The fish seem strongly attached to their burrows. Although those using cavities under glacial boulders made no effort to return when chased away by the submarine, those farther south, on sides of the Baltimore Canyon, refused to move even when prodded with the submarine's manipulator arm.

Some fish proved so loyal to their pueblos in Lydonia Canyon that the same ones were recognized when researchers returned a year later.

Authors of the Environmental Biology of Fishes report, in addition to Dr. Able, were Churchill B. Grimes of the National Marine Fisheries Service, and Robert S. Jones of that agency and the Harbor Branch Foundation. Joining them in a report on fish erosion of the sea floor in the Journal of Sedimentary Petrology last September was David C. Twichell of the United States Geological Survey.

photo of a tilefish (Environmental Biology of Fishes)

#2 fishnh

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Posted 02 November 2011 - 06:18 PM

Angelo, thank you for the article. It's a very interesting read.

#3 team.Fast Forward

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Posted 02 November 2011 - 06:35 PM

Good read. Thanks Ang :)

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