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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