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Fish Help Geologists By Gathering Stones From Ocean Floor

November 22, 2007
Informscience Agency
The valuable food fish -- Antarctic toothfish -- can be used as a bottom dredge with a large collection area. It makes no difference to the toothfish as the fish routinely swallows stones.

The valuable food fish – Antarctic toothfish (D. mawsoni) – can be used as a bottom dredge with a large collection area. Such proposal was made by the specialists of the All-Russian Research Institute of Fish Industry and Oceanography and the Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements. It makes no difference to the toothfish as the fish routinely swallows stones.

The Antarctic toothfish is a big fish about 135 centimeters long, its average weight about 35 kilograms (although some specimens are known to have an adult man’s parameters: 190 centimeters and 80 kilograms). The toothfish inhabits antarctic waters of the Atlantic and the Indian Oceans southward of the 58th parallel. It can be also found near the coast, and in the open ocean, at the bottom and in mid-water.

The Antarctic toothfish was named so for its big mouth full of fang-like teeth. It is a predator, which feeds on squids and fish, but the toothfish itself makes part of the menu of sperm-whale and, certainly, of human beings. Such noticeable and useful fish attracted attention of biologists who found stones in the toothfish stomach, the stones being rather heavy, up to half a kilo. Frequency of stomachic stones discovery depends on the fish size. The toothfish shorter than 80 centimeters have no stones in the stomach, stone occurrence with longer fish varies from 0.7% through 1.1%, and with the fish longer than a meter the occurrence increases up to 2.7-3%.

The researchers have not explained yet why the toothfish needs stones. Probably, the fish uses them for grinding food in the stomach, or perhaps as ballast during its migrations up and down the mid-water. The researchers have suggested that the toothfish stomachic stones should be used for the ground composition analysis of the continental slope and the seabed. These areas are hidden from the researchers by ice, and the toothfish is swimming there and swallowing the stones from the bottom.

During two fishing seasons, specialists collected stones from the Antarctic toothfish stomachs in the antarctic Ross Sea and Amundsen Sea. The researchers have assumed that the fish swallows stones without distinction, therefore they can be used for geological characteristic of the oceanic bed.

Even preliminary analysis of stones from the Ross Sea bottom enables to provide qualitative assessment of geological structure of the coastal strip and the continental slope. The toothfish swallowed samples of various rock, at that intrusive rock (diorites, granites and others) noticeably prevails, the second place is taken by metamorphic rock, including quartzites, sandstones, aleurites, limestone and a paleocoral fragment.

From the Amundsen Sea bottom, the toothfish picks up mainly metamorphic and sedimentary rock, and most unexpectedly, sterile coal with fragments of fossilized wood, which testifies to different geological structure of adjacent sections. Thus, the Ross Sea area corresponds to a larger extent to the shield?, and the Amundsen Sea area – complies with folded area at the platform outskirts.

The overwhelming majority of stone fragments extracted from the toothfish stomach are of angular shape, more rarely - of faintly waterworn shape, and only individual fragments have the look of coastal pebble, and one of the fragments from the Ross Sea has glacial hatching traces on its surface. Taking into account discovery of intact coral springs, it can be assumed that ice and sea currents carry stones at short distances.

It is difficult to say yet how far fish can carry them, but the toothfish caught in the same region has similar stone “filling” in the stomachs, i.e. the fish collects stones at the same sections. Actually, the Antarctic toothfish has provided the researchers with a low-capacity bottom dredge, which covers the area much larger than that in case of ordinary dredging. Now, geologists will disembowel fish for the benefit of science, not only for their own good.

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