Impact of fishing on shipwrecks

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trawl vessels at Woods Hole
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Local trawl fishing vessels (or 'draggers'), tied up at the pier in Woods Hole. (Brendan Foley, WHOI)


Otter trawl doors
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The orange plates on each side of this fishing boat are the steel doors of a trawl net. Cables (warps) connect the doors to the trawler's winch. When fishing, the net is located between the doors, which spread open the mouth of the net. (Brendan Foley, WHOI)


Trawl net hung down on Paul Palmer windlass
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A trawl net hung down on the Paul Palmer's windlass. Shipwrecks act as artificial reefs, attracting fish (and fishermen). (NOAA / SBNMS)


A trawl warp from a fishing net, cutting into the bow of the steamship Portland
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On the wreck of S.S. Portland a cable from a trawl net cuts into the bow. Portland has several nets snagged on it, including at least one wrapped around the iron walking beam for her paddle wheels. (NOAA SBNMS and NURC NA&GL)


The 6th century B.C. shipwreck at Pabuc Burnu, Turkey. This site was heavily damaged perhaps by fishing activity. The ceramic artifacts are smashed and scattered around the site.
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The 6th century B.C. shipwreck at Pabuc Burnu, Turkey. This site, though heavily damaged perhaps by fishing activity, was investigated by archaeologists from the Institute for Nautical Archaeology. (INA, courtesy Elizabeth Greene)


Evidence of trawl fishing at 500+ m off Malta in the Mediterranean
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Evidence of trawl fishing at 500+ m off Malta in the Mediterranean. The furrow in the sediment was plowed by the otter doors on a trawl net. This image was collected with the IFE Argus vehicle during the 2003 season. (IFE, Skerki 2003)


Related Links

» Effects of Fishing Gear on the Sea Floor of New England
Web site of the Conservation Law Foundation, including the report titled "Effects of Fishing Gear on the Sea Floor of New England". Portions of the publication downloadable as pdf.

» Effects of Fishing in the Mediterranean
ECOSYSTEM EFFECTS OF FISHING IN THE MEDITERRANEAN: AN ANALYSIS OF THE MAJOR THREATS OF FISHING GEAR AND PRACTICES TO BIODIVERSITY AND MARINE HABITS

» Quantifying the environmental impact of trawling
Pdf of report by Chris Smith, "Comparison of rapid methodologies for quantifying environmental impacts of otter trawls"



Fishing is one human activity that may directly affect shipwrecks. Wrecks are artificial reefs, with entire ecosystems forming around them. Where there are fish, fishermen are not far behind. Sport fishermen may do some minor damage to shipwrecks, when their lines become snagged on wreck structure. However, it is commercial fishing that tends to have a much greater impact on shipwrecks.




Dynamite

In the past, fishermen in many areas around the globe used dynamite as a fishing tool. Underwater detonations would kill or stun fish, bringing them to the surface where the fishermen could collect them. With repeated targeting of a fish population clustered on a shipwreck, the wreck could be reduced to rubble in short order. It is easy to imagine the outcome of explosions on a site mostly composed of delicate ceramic artifacts. Dynamite fishing is illegal, but is still practiced surreptitiously in some parts of the world.


Trawl fishing

A common fishing method practiced in many waters is trawl fishing, also known as dragging. In the North Atlantic, trawlers generally target groundfish, such as cod, haddock, and hake. A variation of the method can be used to dredge for scallops. In the Mediterranean, trawlers are used to harvest other species of fish and shrimp.

Trawling works by a vessel dragging a roughly cone-shaped net along the sea floor. The mouth of the net is held open by two 'doors' generally made of steel, or sometimes wood with steel shoes. A cable (or 'warp') connects each door to the winch on the fishing boat. Sometimes two vessels combine to drag a net between them, in a method known as 'pair trawling'. Pair trawling allows use of larger nets at greater depths than the standard one-vessel practice.

The easiest place to trawl is a flat sea floor, devoid of rocks or other potential snags for the net. However, rocks and other features attract fish, so fishermen are enticed to work near obstructions. If a trawler captain edges too close to an obstruction, there is a chance that the net will become snagged, or 'hung down'. If the crewmen cannot free the net, then they are forced to cut the trawl warps and sacrifice their gear. Our surveys of the Paul Palmer wreck indicate that at least twice trawlers have hung down on the site (see image).

Substantial modern wrecks like Paul Palmer may be able to withstand a few net snagging events before being torn apart. However, ancient shipwrecks are more delicate. Because the wooden hull is consumed by a variety of animals, an ancient shipwreck typically consists of ceramic or inorganic artifacts lying on the sea floor. If a trawl net is dragged through an ancient wreck, these delicate objects will be smashed and scattered. Repeated trawls may eliminate all traces of the wreck.

Archaeologists have examined ancient wrecks damaged by apparent fishing activity. One example is the 6th century B.C. wreck at PabuÇ Burnu, Turkey. Over the past few years, scholars from the Institute of Nautical Archaeology dived on the wreck, excavated it, and recorded its scatter of broken ceramics (see image to right). However, the rocks directly adjacent to the site would have posed a hazard to a trawl net. Was the destruction of this site caused by some other fishing procedure? 

Trawling can affect large areas of the sea bed even in deep water. In 2003 WHOI personnel participated in a Mediterranean expedition led by Dr. Robert Ballard of the Institute For Exploration. During one part of the project, we optically surveyed the sea floor off the island of Malta, for centuries a center of maritime commerce. At depths of 500+ meters, we expected to encounter marine life and hoped to discover ancient shipwrecks. Instead, we found only furrows in the sediments, indicating intensive trawling (see image to right).

In the Mediterranean Sea trawling is performed routinely to depths of 500 meters, and occasionally we have seen evidence of dragging at depths approaching 1000 meters. It is unlikely that many ancient archaeologically significant sites will survive in areas subjected to trawl fishing. When planning a deep water archaeological survey, this must be acknowledged so research energies are focused in more promising regions.

 



 

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Last updated September 21, 2007
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