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Communities that live in shallow sandy habitats subject to storms or natural traumas such as ice scouring tend to be resilient and resist physical disturbances. Deeper communities that do not often experience natural disturbances are more vulnerable and less equipped to recover quickly from trawling. Watling and Norse's global review of studies covering various habitats and depths found that many of these were done in relatively shallow waters which are generally more resilient than deeper areas. 0 showed general increases in species after bottom trawling, 1 showed that some species increased while others decreased, 4 indicated little significant change, 18 showed serious negative effects. Watling has said that is trawling stopped today, some areas would recover substantially within months but certain bottom communities would need as much as a century. (Safina)
The Grand Banks study Banks study showed trawls making furrows but furrows but also smoothing out the sandy bottomsandy bottom, lowering abundance of sea urchins, snow crabs, soft corals, and other epibenthic creatures. The effects were less than expected and overshadowed by natural variability. Recovery took less than a year and no long terms effects terms effects were observed. The 1997-1999 Western Bank of Nova Scotia study of gravelly seabed typical of trawling areas made some food organisms food organisms such as horse mussels and worms and worms more available as prey items. Physical disruptions were less than on sandy bottom, but lasted longer because gravel bottom is less is less disturbed by storms. On all banks, trawls did less damage than expected. Harm increased with the size of the bottom organisms and sponges and corals were particularly vulnerable. (Does fish-trawling harm the seabed?)
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The removal of macrobenthos has variable effects. In shallow waters where damage is intermittent, recolonisation occurs quickly. Where macrobenthos is substantially removed and recovery is not permitted, the change is permanent (Ex - Sabellaria beds of Wadden Sea and the bryozoan beds of Tasman Bay) (J. B. Jones)
Nonmobile Gear
Purse Seining
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The purse seine is a vertically hanging net with floats on its surface line and lead weights on its bottom edge. Attached to the weighted line are rings strung together by a drawstring wire. Ships encircle entire schools with the outstretched purse seine, then they pull the drawstring wire tight to trap the fish inside (AFMA). This method is extremely effective for catching both surface dwelling and mid-water fish, especially tuna, sardines, mackerel, jack mackerel, and herring (Kuznetsov). The Alaska Department of Fish and Game reports that, in the commercial fisheries of Southeast Alaska, purse seines are responsible for 70-90% of the tuna catch alone. Current regulations in Alaska allow purse seining only in specific districts (ADFG).
While fishing companies have practiced purse seining essentially unchanged for the past 100 years, slight innovations in technology have recently made the net even more efficient and, therefore, lucrative. One such example is the "autonomous distance-controlled hydroacoustic system" which, once attached to strategic points along the net and submersed, emanates a low-frequency pulse which frightens fish into the net. This invention consists of a management block onboard the ship as well as the underwater blocks, attached to the net, which emanate the frequencies (Kuznetsov).
One invention analogous to the hydroacoustic system, but used for trawling, is the "towed remote controlled pneumoacoustic system." This machine, which is dragged between ship and trawl, creates a strategically located acoustic field, imitating the acoustics generated by predatory whales. Seeking to avoid the predator, fish congregate in the trawl's catching zone. This method greatly increases the catch without the necessity of altering boats or nets (Kuznetsov).
Sources
Alaska Department of Fish and Game: Division of Commercial Fisheries. "Commercial Purse Seine Fishery." 26 July 2005. 17 Oct. 2007 <http://www.cf.adfg.state.ak.us/region1/finfish/salmon/netfisheries/ps_info.php
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Australian Fisheries Management Authority, Australian Government. "Seine." Canberra, Australia. 29 June 2005 <http://www.afma.gov.au/information/students/methods/>.
Kuznetsov, Dr. J.A. "Innovational Projects." Intensification of Multispecies Fishery. Vladivostok, Russia. <imf.fish-net.ru/inpr.htm>.
Traps
Traps are one of the most environmentally friendly fishing methods. They are highly selective, since fishermen can release unwanted fish alive when the traps are hauled up. They also do little to no harm to the ocean floor or other oceanic surroundings, as they are nonmobile. However, there is a huge problem associated with traps - ghost fishing.
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