Download the report:
Commercial selective harvest
of coho salmon and Chinook salmon on the Willapa River using tangle nets
and gill nets
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2003 Willapa Salmon Studies
ACKNOWLEDGEMENTS
WDFW biologists Anita
Swanson, Erik White, and Gil Lensegrav. WDFW technicians Shona Bruce and
Keith Sweeney, and Pacific Conservation District technician Pok Vang collected
data during test fishing. Eric Petit, Norris Petit, Bryan O'Shaughnessy,
and Bruce Ogren provided expert test fishing on the Willapa River. Mike
Johnson and his staff at the Pacific Conservation District, and WDFW staff
led by Kurt Holt provided stream surveys. Patrick Verhey directed sport
and commercial fishery sampling for jaw tags. |
Commercial selective harvest
of coho salmon
and Chinook salmon on the Willapa River
using tangle nets and gill nets
Final report - IAC contract 01-1018N July 1, 2002
By: Vander Haegen, G. E., K.W. Yi, J. F. Dixon, C. E. Ashbrook
Abstract
Selective fishing
is the ability of a fishing operation to avoid non-target species or
stocks, or when encountered, to capture and release them in a manner
that minimizes mortality. The tangle net was tested on the Willapa River
to selectively harvest adult Chinook salmon (Oncorhynchus tschawytscha)
and coho salmon (O. kisutch). Experienced gill netters simultaneously
fished tangle nets (3.5" mesh size) and conventional gill nets
(5.75" mesh size) to evaluate their effectiveness for live release
of non-target stocks. Live fish were tagged and released for recovery
in sport fisheries, commercial fisheries, at hatchery racks and traps,
and during spawning ground surveys. The tangle net was as effective
for capturing coho salmon as the conventional gill net, and fish were
generally captured in good condition. The immediate survival (from capture
to release from the boat) of adult coho salmon captured in the gill
net was 89.6%, compared to 86.8% from the tangle net. Coho salmon released
from the tangle nets were recovered at about 22.7%, compared to 25.5%
from the gill net. Significantly more non-target species were captured
in the tangle net than the gill net. These tests showed that using conventional
gill nets with short soaks and careful fish handling results in the
same immediate and post-release survival of coho as using these techniques
with the tangle net.
Purpose
Many salmon populations
in the Pacific Northwest are declining to historically low levels. The
causes of these declines vary by area, but typically include habitat
destruction, mismanagement of harvest, interference by hatchery programs,
and hydroelectric development. In Washington, numerous stocks are listed
as threatened or endangered under the United States federal Endangered
Species Act.
Throughout Washington,
weak stocks of salmon return to their natal rivers to spawn intermingled
with healthy stocks. Selective harvest technologies and practices allow
a continued harvest, while protecting weak stocks. "Selective fishing",
more accurately described as "live capture, selective harvest",
is the ability of a fishing operation to avoid non-target species or
stocks, or when encountered, to capture and release those animals in
a manner that minimizes mortality. Successful selective fishing requires
that two objectives be met. First, a conservation goal must be achieved
for the species or stock of concern, and second, a harvest goal must
be met to make the fishery economically viable. Harvesting salmon with
gill nets in these mixed stock fisheries is a problem because fishers
inadvertently catch weaker species and stocks while targeting salmon
from stronger runs. Because successful live release of salmon from a
gill net is difficult, the only practical way these traditional gears
can be more selective for the target species is by time and area closures.
While these restrictions can be very efficient at reducing by-catch
and meeting the conservation goal for the fishery, they necessarily
reduce fishing opportunity for the target species and do not meet the
harvest goals.
With the current
management strategies, large surpluses of harvestable salmon are returning
to hatcheries without being harvested. If the harvest strategies are
not adjusted, producing fish for commercial fisheries should be reconsidered.
We therefore began working with the commercial fishing industry to develop
acceptable live capture gears that will provide more fishing opportunity
while continuing to protect weak stocks. Simultaneous with the development
of selective fishing methods, large portions of the hatchery production
of coho salmon are being identified by the excision of the adipose fin
before release as juveniles. When these fish return as adults, fishers
can distinguish them from naturally produced fish that do not have the
adipose fin removed.
The tangle net (Figure
1) is a possible substitute for gill nets that may meet the criteria
for selective fishing. Tangle nets look similar to a gill net with a
small mesh size (3.5" compared to 5.75" in a conventional
coho salmon gill net). Tangle nets are made from multifilament web while
gill nets are typically made from monofilament web. Both gears are fished
in the same method and locations, but the similarities stop there. Unlike
a gill net, which captures an adult salmon around the gills or body,
the mesh size of the tangle net prevents adult fish from entering the
net that far. Instead, the fish is caught by the maxillary or teeth,
which allow it to continue respiring in the net so it can be released
live. External and associated internal injuries are also reduced using
this capture method. Modifications in fishing practices, including the
use of fish revival boxes, short soak times, and careful fish handling,
are as important as the gear in ensuring that fish are released live
and unharmed.
The untested premise
of live capture, selective harvest is that the released fish survive
to contribute to rebuilding their stock. It is assumed that fish released
in good condition will survive, but there have been no published studies
looking at the long-term survival of fish that have been captured and
released from commercial gill nets. Studies evaluating the survival
of fish captured in sport fisheries indicate that mortality of released
fish is variable and depends on the species captured, the skill of the
fisher in releasing the fish, the environment, and the fishing method
(Muoneke and Childress 1994). Survival of lake trout captured in gill
nets in Lake Superior and held in tanks for 48 hours varied seasonally
from 68% to 77% (Gallinat et al. 1997) and studies evaluating coho salmon
released from commercial fishing gears in British Columbia have shown
that mortality of fish held in net pens for 24 hours was less than 3%
(Farrell et al. 2001a). However, evaluations of post-release survival
of salmonids held in net pens are unlikely to reflect the post-release
survival of free-swimming fish, because the fish in net pens are not
subject to predation, currents, or encounters with obstacles to migration
(e.g. dams, shallow parts of rivers, etc.) which a severely stressed
fish, such as those captured in gears (Farrell et al. 2000) must contend
with. Many tagging studies evaluating migration and population sizes
suggest that fish can be captured and released with some successful
survival, but these types of studies were not specifically directed
at looking at the effects of the capture gears on survival.
The main goal of
this study was to test the fundamental assumption of selective fishing
- that the released fish we are trying to protect really do survive
at acceptable levels to contribute to rebuilding the weak stocks they
are part of - by comparing the post-release mortality of coho and Chinook
salmon released from tangle nets and conventional gill nets on the Willapa
River. We also estimated and compared the immediate mortality and catch
efficiency of the two gears and evaluated characteristics of fish caught
in each gear. Gear changes may result in encounters with different non-target
species (by-catch), and this is expected with the tangle net because
many small fish species that dwell in the Willapa River can pass through
the large mesh gill nets without incident, but would be captured in
the smaller-meshed tangle net. Because it is undesirable to shift the
impacts from one species to another, we also compared the capture of
non-salmonid species in each gear.
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