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DOLPHINS AND THE TUNA INDUSTRY 3 Background on Fishing Gear PURSE SEINES Purse seines are used around the world and they are important in the capture of many schooling species of fish (Sainsbury, 1971). They were first used by Southern California tuna fishermen in 1916. The U.S. bait-boat fleet converted to purse-seine gear between 1957 and 1963; the design of the gear and the conversions necessary to change a vessel from bait fishing to seining are well documented by McNeely (1961). The process of purse seining any species of fish involves the encircling of the school with a long net to form a circular wall of netting (see Figure 3-1). The net must be deep enough to discourage escape underneath it and the encircling must be done rapidly enough to prevent escape before the ends are closed. The purse seine is rectangular, typically much longer than it is deep. A seine is approximately 1 mile long and 600 feet deep (IATTC, 1989c). The top edge, or corkline, is kept at the surface by numerous floats attached along its length. The lower edge of the net, or leadline, is weighted by lead or chain to pull it down vertically. The amount of weight used and the flotation power of the net determine the sinking rate and are important in ensuring the capture of a school. The amount of weight also determines the flotation requirements, because the corkline must be kept at the surface to prevent the fish from escaping over the top of the seine. Once the school is located, the skiff is released from the stern of the vessel, with one end of the net (known as the “ortza”) attached. The skiff anchors this end of the net while the seiner encircles the targeted school and rejoins the skiff. The ortza is transferred to the vessel and made fast, thus closing the circle once the towline has been retrieved. At this point the net forms a
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DOLPHINS AND THE TUNA INDUSTRY vertical cylinder around the school of fish. To allow closure of the bottom of the seine, a series of rings are attached to the leadline through which a purse line is run. During the pursing operation, this purse line is pulled in from both ends, choking off the bottom of the seine. When the seine is completely pursed and the rings are alongside the vessel, the process of hauling in, or “drying up,” the net can begin. The remainder of the normal purse-seining operation involves “sacking up” the catch or reducing the volume of water inside the net until it is possible to bring the catch aboard using a large dip net called a “brailer.” This is done through a process of bringing most of the net aboard, leaving only a small sack of reinforced netting in the water to confine the catch for brailing. Once the fish are removed, the remainder of the seine is brought aboard and made ready for the next set. The large size of the seine and fact that the net is only in the water after a school of fish have been sighted all contribute to the efficiency of the purse-seining operation and its high productivity rates compared with other methods of fishing. The particular purse-seine gear and methods that have evolved in the Pacific tuna fishery, in comparison to other purse-seine fisheries, have epitomized the modern, efficient fishing operation. The sacking-up process is complicated considerably during sets on tuna associated with dolphins. To minimize dolphin mortality, a variety of gear modifications and operational procedures have been adopted. The intent of these changes is to remove the dolphins from the purse seine after it has been pursed but before it is dried up for brailing. While relatively effective at its intended purpose, the process can add to the time required to complete a set and adds to the risk of losing tuna. FIGURE 3-1 A purse seine showing the backdown and release of dolphins. Printed courtesy of the National Marine Fisheries Service.
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DOLPHINS AND THE TUNA INDUSTRY The Backdown Process The gear modifications and operational techniques mentioned in Chapter 1 are for the backdown process, in which the seine is pulled out from under the herd of dolphins. The dolphins are quite capable of jumping over the corkline and escaping, but for some reason they will not do so; therefore, the backdown was developed. In this process, the seiner is put into reverse after about one half of the net has been rolled aboard. This has the effect of forming the net into a long, narrow channel and of causing the corkline at the apex of the channel to sink. The dolphins are herded toward the apex where they can then swim away over the sunken corks. The basic techniques of backdown were developed by the industry (the procedure was introduced by Anton Misetich in 1958). Most of the NMFS and industry effort during the 1970s was spent in attempting to perfect the design of the seine and the detailed techniques of its use for improved backdown effectiveness. The modifications that came out of that effort resulted in significant reductions in dolphin mortality (Coe et al., 1984). Modifications include the following: The Medina panel. Normal netting used in the purse seine caused frequent dolphin entanglements when they came in contact with the seine during backdown. Panels of smaller mesh netting in the portion of the net that becomes the backdown channel prevent entanglements. Use of the skiff to prevent collapse of the seine. To keep the net from collapsing while dolphins remain inside, the net skiff is used to pull the seiner to starboard, away from the seine. Use of speedboats to prevent net collapse. The speedboats, whose principal purpose was to prevent the dolphins from getting away from the seiner, are equipped with towing bridles and can be used to tow on the corkline if the net threatens to collapse. Use of rafts and swimmers to effect release. A crewman in an inflatable raft is deployed within the net to herd the dolphins toward the release area near the apex of the backdown channel to prevent them from swimming back toward the seiner and to help in the manual rescue of trapped or entangled animals. Optimized set orientation and backdown maneuvering. Guidelines to aid in determining the best orientation of the set with respect to wind conditions and proper rudder, bow thruster, and skiff controls were developed to minimize the chance of the net billowing, a situation called “canopying.” Pear-shaped snap rings. These devices, which can save up to 15 minutes in beginning the backdown process, are described in Chapter 7. The combined effect of these improvements has yielded a significant reduction in dolphin mortality since the mid-1970s (see Figure 6–1). Appendix 1 includes the gear and operational regulations that U.S. tuna purse seiners
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DOLPHINS AND THE TUNA INDUSTRY must obey when fishing for tuna associated with dolphins. Compliance with these regulations is observed and enforced. Other nations have similar regulations. OTHER FISHING METHODS Although purse seining for yellowfin tuna associated with dolphins in the ETP is the primary focus of this report, other methods, including additional purse-seining modes, are used for catching yellowfin. Some of these methods are known to kill dolphins, as are other techniques of fishing for other fish species (Northridge, 1984, 1991). The following are the most important. Longline Fishing Longline fishing is used to catch yellowfin in many parts of the world, including the ETP. The fishing lines are long and have baited hooks suspended along them, usually at some depth. The ends are buoyed. Usually, the lines are set for periods of a few hours to a day or two. Catch rates for longliners are considerably lower than for purse seiners, but the fish caught are, on average, larger than those caught in purse seines, and their economic value for some specific markets (e.g., Japan) is higher. Without a major expansion of the demand for this type of tuna (mainly used for fresh sashimi), redirecting all effort toward this mode of fishing is not economically viable, because it is considerably more expensive than purse seining. Log Fishing Tuna are attracted to floating objects such as logs and debris. These floating objects are referred to collectively as logs. Sometimes, artificial fish-aggregating devices (FADs) are set in the ocean to attract tuna. The tuna that collect around the logs are then caught by means of purse seines. School Fishing Sometimes, a school of tuna not associated with dolphins or a floating object can be detected from signs on the surface of the water. Schools moving energetically close to the surface disturb the water, which sometimes appears to be boiling or affected by a breeze. Frequently, the presence of birds is a further clue to the presence of a “boiler ” or “breezer.” In other cases, a school swimming close to the surface is detected as a “black spot” from the vessel or helicopter, or the tuna are seen jumping. Setting on all types of unassociated schools is known as school fishing.
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