CAN DRIFTNET FISHERIES BE MANAGED?


Fishery Management Requirements

Management is accomplished by several techniques that not only limit the total amount of the target species and by-catch species that may be caught, but also the year classes that may be targeted and the take of particular sub-populations that may be in trouble. Restrictions can include: - catch limits and species prohibitions - time (season) and area closures - limiting the number of vessels permitted to fish (limited entry) - limiting the size and load capacity of vessels - limiting the total amount of gear used - gear-type restrictions and gear modification requirements

Driftnet Gear Modification Studies

Both Japan and Taiwan have suggested that lowering the driftnet down in the water column a meter or two will reduce by-catch sufficiently to qualify as a management measure. Between November 1989, and March 1990, a JAMARC driftnet research vessel conducted 42 operations using a standard surface driftnet and one experimental subsurface driftnet set two meters below the surface away from the commercial fishing area. The experimental net was submerged for less than three nights commercial fishing time (SPF 1991). The CPUE of albacore was higher on the subsurface net; the CPUE of skipjack was lower (FAO 1990). Cetacean subsurface by-catch was reportedly one tenth of that of the surface net. Although the sample size was too small to be statistically significant, the results suggest that lowering the net may reduce the by-catch of surface feeding skipjack tuna, some transiting dolphins (but not feeding dolphins) and a few species of very shallow diving seabirds. They also indicate that the general absence of albacore in the lower half of most surface nets as they clear the water may be due to a high dropout rate, not to the distribution of albacore in the water column.

Subsurface and acoustic driftnets have been tested by others (Hayase, Watanabe and Hatanaka 1990; Hembree and Harwood 1987). Hembree and Harwood (1987) set the nets 4.5 meters down and reported a reduction in cetacean catch rate of 50% and a reduction of the target catch by 25%. The Republic of Korea and Taiwan also reportedly conducted research on alternative gear types. Reports of earlier tests by JAMARC in the South Pacific note several problems associated with submerged nets: "the fishing efficiency was extremely poor; there were operational problems such as frequent tangles of the net and the total time for net setting plus net stacking was about twice that for surface nets" (JAMARC 1988, cited by SPF 1991).

Both monofilament and multifilament nylon driftnets are acoustically similar to water in the way they respond to cetacean sonar. Researchers have attempted to make driftnets acoustically visible to dolphins by using chains of metallic beads and air-filled nylon or plastic tubes (Hembree and Harwood 1987; Odate and Ito 1984), hollow strands of monofilament (Snow 1987; Jones et al. 1987), sound generators (Au 1991; Hatakeyama, Ishii, Akamatsu, Soeda, Shimamura and Kojima 1990; Odate and Ito 1984), and synthetic net material with a different density than water (Prado and Smith 1990). In 1987, three types of modified gear were tested by the salmon mothership fishery: (1) three air tube threads in the central portion of the net, (2) three multifilament threads in the central portion of the net, and (3) sound generators (INPFC 1988). There was no statistically significant difference between the mean CPUE of the air tube and the multifilament nets. There was some reduction in CPUE by nets rigged with sound generators, but the authors concluded that additional studies were needed to obtain statistically reliable results. Prado and Smith (1990) reported that increasing mesh size reduces seabird by-catch, but increases cetacean by-catch.

The high frequency sonar of a feeding dolphin is narrow and very directional. A driftnet acts acoustically like a field of very small objects (Martin pers. comm. 1991). Although theoretically a dolphin could detect a driftnet in sufficient time to avoid it (Au and Jones 1991), the dolphin has to know or suspect that the net is there or it won't notice it (Martin pers. comm. 1991). Japan reported that studies of echolocation clicks and the response of captive harbor porpoises indicated that the porpoises did not detect the net until they were close to it, and if they approached at less than a 90 degree angle, the detection distance decreased even more (INPFC 1988). Studies of feeding dolphins caught in the swordfish gillnet fishery in the Gulf of Maine suggest that if the sonar of a feeding dolphin is focused on prey such as squid, it probably won't notice a driftnet until it is too late. When asked if driftnets could be made safe for dolphins, a member of the U.S. Marine Mammal Commission's scientific committee said that there is "not a chance in hell to make them safe" (Smith pers. comm. 1991).

The INPFC and the International Whaling Commission have reviewed a number of reports on alternative gear research. The review indicates that "subsurface driftnets could substantially reduce seabird entanglement". As noted previously, the nets would have to be set several meters down in the water column to significantly reduce the seabird by-catch. Shearwaters, boobies and several other species normally dive several meters down to feed. Several scientists meeting at Sidney to evaluate the driftnet fisheries data found the results of the submerged net studies inconclusive due to sample sizes "too small to be statistically valid" (U.S. Summary Report 1991). They also noted that "the submerged net approach did not address the fundamental, international objection to large-scale driftnet technology, that of massive, indiscriminate, uncontrolled waste of marine life" (U.S. Summary Report 1991).

Time and Area Closures

Limiting driftnet fishing to areas distant from land masses has also been proposed as a management measure. However, non breeding seabirds, non breeding turtles, marine mammals such as pelagic dolphins and several whales remain unconnected to land masses for years. Salmon stocks intermingle on the high seas with squid stocks and with each other, making management impossible if they are taken by indiscriminate driftnet fishing. Time and area closures do not reduce the problem of the massive waste of valuable protein due to damaged, putrefied and half eaten fish, discards of unwanted species and dropouts.


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