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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). The Marine Conservation Society summarizes purse seining as, "one of the most aggressive methods of fishing and aims to capture large, dense shoals of mobile fish . . ." (MCI). 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. This yield consists of mainly pink salmon, but it also includes sockeye, coho, chum, and chinook salmon. This Current regulations in Alaska allow purse seining only in specific districts (ADFG).
New Technologies
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>.
Australian Fisheries Management Authority, Australian Government. "Seine." Canberra, Australia. 29 June 2005 <http://www.afma.gov.au/information/students/methods/>.
Pros
From the fisherman's point of view, the most obvious advantage of purse seining is its capability to harvest massive quantities of a species at once. With respect to the geographical environment (i.e. ocean floor, plant life), the purse seine is nearly harmless because, when properly handled, it never touches the sea floor. It doesn't sit in one place for a length of time longer than the boat can set it out and pull it in, thereby hardly altering the long term habitat with its temporary presence. Yet another defense for purse seining is its longevity. Purse seining has been practiced to the same end, and with virtually the same means, for the past one hundred years. Its practice preserves a culture more than one hundred years old of seine net fishing, especially prominent in California and the Northeastern coast of the United States (Skogsberg).
Cons
Two general problems with purse seining are caused by its large-scale capabilities. The first is simply ecosystem disruption. The instantaneous removal of entire schools of a certain species of fish from an area leaves a vacant hole in the food web. This leads, initially, to predator death by starvation and prey overpopulation because their numbers are suddenly unchecked by a predator. In the long run, if the affected species survive, they may adapt their eating/breeding habits to flourish in a new and altered ecosystem.
The second flaw of purse seining is the large quantities of bycatch routinely caught and killed along with market fish. Marine mammals are commonly trapped within the encircling net, and they perish without means to escape. Before regulations forbade it, seining for yellowfin tuna often consisted of setting nets around dolphins (predators of tuna) on purpose to catch the most fish. Congress's Marine Mammals Protection Act of 1972, however, almost entirely eliminated the practice by 1997 (MCI). Additional regulations were set up by the Earth Island Institute and the HJ Heinz corporation in their 1990 "Dolphin Safe" standards (IMMP). Since their introduction in 1990, these guidelines have been accepted by 90% of world canned tuna companies and have reduced dolphin mortality (as bycatch) by 98% (now about 2-3,00 dolphin deaths per year) (MCI).
Reducing Bycatch: The Dolphin Safe Standard
The Earth Island Institute writes:
"In order for tuna to be considered "Dolphin Safe", it must meet the following standards:
- No intentional chasing, netting or encirclement of dolphins during an entire tuna fishing trip;
- No use of drift gill nets to catch tuna;
- No accidental killing or serious injury to any dolphins during net sets;
- No mixing of dolphin-safe and dolphin-deadly tuna in individual boat wells (for accidental kill of dolphins), or in processing or storage facilities; and
- Each trip in the Eastern Tropical Pacific Ocean (ETP) by vessels 400 gross tons and above must have an independent observer on board attesting to the compliance with points (1) through (4) above." (IMMP)
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
>.
Australian Fisheries Management Authority, Australian Government. "Seine." Canberra, Australia. 29 June 2005 <http://www.afma.gov.au/information/students/methods/>.
International Marine Mammal Project, Earth Island Institute. "International Dolphin Safe Monitoring Program." San Francisco, CA. <http://www.earthisland.org/dolphinSafeTuna/>.
Kuznetsov, Dr. J.A. "Innovational Projects." Intensification of Multispecies Fishery. Vladivostok, Russia. <imf.fish-net.ru/inpr.htm>.
Marine Conservation Society. "Fish Online: Fishing Methods." England, Wales, and Scotland, UK. <http://www.fishonline.org/caught_at_sea/methods/>.
Skogsberg, Tage. "Preliminary Investigation of the Purse Seine Industry of Southern California." Fish Bulletin No. 9. State of California Fish and Game Commission, State Fisheries Laboratory. California State Printing, 1925. University of California, 2007 <http://content.cdlib.org/dynaxml/servlet/dynaXML?docId=kt15800140>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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