Nicholas Lowry, University of Washington, School of Fisheries,
P.O. Box 357980, Seattle WA 98195-7980. E-mail:
Tel: (206) 860 6782, Fax: (206) 860 3394
Collaborators: University of Washington, Northwest
Fisheries Science Center (NWFSC), and commercial fishers
Participants: Nick Lowry, Prof. Don. Gunderson (UW),
C. W. (Bill) West (NWFSC).
of finfish in the West Coast fisheries for spot prawns
(Pandalus platyceros) causes wastage of fish which
cannot be landed and utilized and causes conflicts when
the species of bycatch are desired by other fishery segments.
Reduction of bycatch is especially critical for several
species of rockfish (Sebastes sp.) whose stocks
are in poor condition or where rebuilding efforts are
where there is no substantial impact on commercially valuable
species, fish bycatch imposes a burden on shrimp trawlers,
increasing the need for sorting and reducing the quality
and value of the catch.
efforts and innovations have shown some promise, but use
of bycatch reduction devices is patchy. Acceptance has
not been wide, partly due to reports of high catch loss
and the lack of adaptation of the devices to local conditions.
objective of this project is to evaluate existing shrimp/fish
separator technology, to determine which methods are the
most effective and practical for reducing bycatch in the
fisheries on this coast without loss of a significant
amount of the target catch.
different types of fish excluders, which have shown potential
in shrimp/prawn fisheries elsewhere in the world, were
tested. These were a Nordmøre grid, a separator panel
and a "fish eye" (Figures 1-3 respectively).
HERE TO SEE THE FIGURES AND TABLES FOR THIS FINAL
system was developed in Norway for separating shrimps
from fish in pink shrimp (Pandalus borealis) trawls. It
consists of a rigid grid, made of stainless steel or plastic,
mounted at a 45-50 degree angle in the aft section of
a shrimp trawl. The grid has a netting funnel in front
of it to ensure that all catch initially comes to the
bottom of the grid to increase the chance of small objects
(i.e. shrimps) passing through the grid and into the trawl
cod-end. Above the grid, the top panel of the net has
an opening to allow large fish to escape. Normal bar spacing
is maximum of 19 mm for shrimp fisheries in Norway. The
performance of this device tends to be good in pink shrimp
fisheries, average shrimp loss is 1-3% by weight and fish
exclusion up to 95% for small fish, 100% for larger species.
The Nordmøre grid is a device that has been extensively
studied and has been shown to be effective. However, it
is not popular in some fisheries due to its cost and perceived
difficulties in handling.
grid used in this project was manufactured from a ½"
thick polyethylene sheet, into which the grid openings
were cut using a milling machine. The openings were 3"
in width. This was determined by estimation of the differences
between the sizes of the pink shrimps targeted in fisheries
using the grids and the size of spot prawns. The overall
grid dimensions were 4' in height and 2' in width. It
was mounted at a 45-degree angle in a pre made 160 mesh
round extension section (2" mesh size), complete
with an 85 mesh long guiding funnel. Four 6" floats
were mounted on the top corners to ensure that it fished
in the correct orientation, and the bottom edge was reinforced
with nylon twine to avoid abrasion damage from contact
with the sea floor.
device consists of one or a pair of elliptical framed
openings, which are inserted in the top panel of the aft
part of the trawl. The frames are made from stainless
steel and have bars attached to ensure that they remain
vertical and that the openings point forwards relative
to the towing direction of the net. For a fish to escape
it has to swim forwards and upwards from the back of the
net and then exit. This device is the simplest of the
three tested, and is already in legislation in some spot
prawn fisheries. There are fewer variables that can be
adjusted with the fish eye, the major question is where
it should be positioned in the cod-end. Workers in the
Gulf of Mexico have researched the position of this in
the net for those fisheries, and the supplier suggested
a similar position. Two fish eyes were mounted in the
top of the net, 88 meshes from the end of the cod-end,
with a single 12" float attached between them to
ensure that they would remain on the top of the extension
devices are intended to separate shrimps from fish in
shrimp trawls. They are adaptations of the Gulf of Mexico
turtle excluder devices and are made entirely of netting
or with only a few rigid parts. The concept is similar
to the Nordmøre grid, shrimps pass through an angled mesh
panel and fish are deflected upwards and out through an
escape hole. Panel mesh sizes are normally 3-8 inches
in USA fisheries. Performance of these devices tends to
be less consistent than rigid grids. Shrimp losses have
been reported between 2 and 30% and fish exclusion 30-80
% for small fish. They have advantages over rigid grids
in being easier to handle on small vessels and cheaper
to produce and repair. The separator panel has the most
potential for performance problems caused by incorrect
separator panel used in this project was made from 5 1/2"
polyethylene mesh and framed with a loop of 1/2"
steel cable, which was wrapped with nylon cord to give
protection from abrasion. A crosspiece of 1/2" cable
was also installed to help the panel retain its shape.
The panel was installed in a section of 2" mesh size
netting, 160 meshes in circumference. Three 4" floats
were installed inside the net at the back of the panel
to help it retain its shape and ensure that it would fish
the right way up.
net makers on the West Coast supplied all of the gear
used. The separator panel was supplied by Astoria net
shop, Astoria (George McMurrick), and the Nordmøre grid
from Seattle Marine and fishing supply. As these are commercial
products, the precise details of the designs are not presented
here. The fish eyes were manufactured by Robert P Driscoll
net services according to a standard design. The elliptical
openings were 10" in height and 15" in width.
experiment was designed to use a commercial fishing vessel
during normal fishing operations. The vessels skipper
was paid a small fee for participation in the experiment,
but it was not a full charter. This limited the scope
of the data collection, as it was necessary to fit in
the experimental tows with a normal program of commercial
tows. The weather conditions that prevailed during the
period of the charter also affected the scope of the data
general approach was of first using trawl instrumentation
characterize large-scale gear configurations and direct
video observation to study small-scale separator geometry
and the conditions that affect the operation of existing
shrimp/fish separator designs. No new design work was
attempted during this experiment. The catch composition
was measured during the experimental tows to assess the
degree of separation and catching performance for target
species using these devices. Data collected from each
net for each haul was the total catch weight, catch of
each species (or species group where precise identification
was difficult), and length frequency of major bycatch
vessel that agreed to participate in the work was set
up to use a double rig, i.e. it normally tows two identical
trawls simultaneously from booms, one on each side of
the vessel. This rig allows direct comparisons to be made
between a net fished normally on one side and a net with
a separator on the other. The instrumentation used on
each net was identical, which enables direct comparison
of fishing performance of each net.
starting the experimental part of the work, a calibration
of the two nets used by the vessel was carried out to
assess their fishing performance. Variables that were
compared included wing end spread and door spread (indicators
of swept width), headline height, and ground gear contact.
Fish catches were compared by weight and species composition.
vessel used was the F/V Nita H, based in Pt. Hueneme,
California. This boat is a large shrimp trawler, approximately
105' in length. Two nets were fished, connected directly
to a pair of wooden trawl doors. Rockhopper ground gear
with approx. 10" discs was used.
the trial period, the weather conditions were generally
dreadful, with mostly strong winds from the north. This
limited the amount of time which we could spend fishing
on the more productive grounds. The 24-hour non-stop fishing
schedule kept by the captain, also made it difficult to
achieve the objectives as little time was available for
altering the gear between hauls. The use of the trawls
hung from extended booms on the sides of the vessel made
it difficult to get access to the gear for making changes
and for positioning cameras and trawl monitoring equipment.
difficulty was found in adjusting the nets to fish similarly.
This was partly because the vessel's nets were not identical.
Due to the nature of the fishery, nets are damaged frequently,
resulting in them needing repair, and whole nets are lost
occasionally. The vessel was using three nets interchangeably.
Two had been manufactured identically but had subsequently
been extensively repaired. The third was similar but not
identical in design, including slightly different ground
gear configuration. We attempted to adjust the nets so
that their catches would be the same, but this aim was
made difficult by the extreme variability in the catches
from haul to haul. This inconsistency seemed to also apply
to the variation between the catches from the two nets.
Side to side variation was big enough to make it impossible
to assess if the catching performance of the two nets
were the same. After a reasonable time, we could still
not be sure that the nets were fishing identically but
we were confident that their performance was at least
was carried out in three general areas, dependent on the
prevailing weather conditions and catches. The first few
hauls were made around San Nicolas Island, but catches
were small, and so we moved to around Tanner Banks. The
rest of the hauls were made further to the north, in the
Santa Barbara channel, which was more sheltered from the
wind (figure 4). Unfortunately, these changes in area
almost coincided with the changes in the gear type tested,
with the result that most of the Nordmøre grid tests were
made at the Tanner banks and most of the fish eye tests
in the Santa Barbara channel. [CLICK
HERE TO SEE THE FIGURES AND TABLES FOR THIS FINAL
catches were generally small, and had a very variable
species composition from area to area. On the Tanner banks,
the catches contained a greater proportion of rockfish,
fewer flatfish, more corals, sponges, and a higher proportion
of prawns to total catch. The Santa Barbara channel area
catches had a greater proportion of bycatch, and this
consisted primarily of small rockfish (stripetail, shortbelly
and splitnose rockfish), flatfish (dover, english, rex
and slender sole), argentines and pacific hake. (Table
HERE TO SEE THE FIGURES AND TABLES FOR THIS FINAL
the comparison of catches, the amount of bycatch of individual
species was too small to be meaningful. The catches are
divided into four species groups by general type. Large
rockfish are; Greenblotched, Blackgill, Bank, chillipepper,
boccacio, cowcod and darkblotched. Small rockfish are;
stripetail, greenstriped, splitnose, shortbelly, halfbanded,
flag, pinkrose rockfish. Flatfish are primarily dover,
english, rex, petrale and slender sole. Roundfish are;
Pacific hake, ratfish, various cottids, grenadiers, eelpouts,
first tests were with the separator panel. The aim was
to see both if the panel was functioning and to test the
potential of the video system to work with natural light.
of these were somewhat unimpressive. The video system
gave excellent pictures on the surface and down to substantial
depth, but there was insufficient light when the net got
to the fishing depth at >200 meters. The surface video
pictures clearly show that the panel does not take up
the anticipated shape but is actually bowed downwards
like a spoon. It also appears to be at too shallow an
angle for anything to easily penetrate the meshes. Some
prawns, which had been caught up in the meshes further
up the trawl, were seen to go out of the exit hole as
the net was hauled back. The mesh size used may also have
been too small to allow spot prawns to penetrate through
to the cod-end in any case.
second haul was attempted with this gear. There was a
substantial quantity of hard invertebrates (corals, sponges,
sea urchins and the like) and inorganic debris caught
during this haul, which stuck on the meshes of the separator
and blocked them. Any prawns that came down the bag would
then be either caught up in this debris or deflected by
water flow out of the exit hole.
this was a commercial fishing trip, the experiments with
the separator panel were discontinued at this time. The
initial performance was seen by the skipper as reducing
the catch too much to be a viable fishing option.
second set of tests was with the Nordmøre grid. Video
observations were made of a number of hauls. These showed
that the angle of the grid during towing was approximately
45 degrees, which was as designed. The few prawns that
were seen passing down the extension went straight through
the grid without seeming to be significantly impeded.
A number of fish were also observed to pass straight through
the grid and into the cod-end. Some fish were seen holding
station in front of the grid. It seemed to be necessary
for these fish to make a definite upward escape attempt
to get away. The majority of them did not do this, and
were eventually exhausted and passed through he grid towards
the cod-end. Some large fish were observed being deflected
by the grid and out of the escape hole. A number of fish
were observed escaping when the net pulsed, presumably
due to being snagged temporarily on some bottom obstruction.
This is further evidence that the water flow is what is
holding the fish in the area in front of the grid and
discouraging escape attempts. As it was necessary to make
all of these observations with the aid of lights, the
behavior of the fish cannot be considered completely typical
of that under normal fishing conditions.
prawns were observed passing down the extension and through
the grid. They all stayed right at the bottom of the net,
where they were directed by the funnel, and passed with
great ease through the grid. These observations, combined
with the fact that only the largest fish were deflected
by the grid, show that the bar spacing chosen was in fact
rather too large. Better results in terms of fish exclusion
would be expected with a smaller bar spacing.
major problem with the Nordmøre grid as with the soft
panel excluder, was that any large sponges or other pieces
of debris were stopped by the grid and remained there,
blocking passage of anything through into the cod-end.
This may cause loss of prawn catch if the blocked grid
creates an upward flowing current that sweeps the prawns
out of the exit hole. It was also somewhat difficult to
clear the grid when it was hauled. This was partly due
to the way it was handled on board the vessel. The second
main problem with the grid was associated with the funnel
used to direct the catch to the bottom of the grid. This
became blocked with sponges and debris, and occasionally
large rockfish were caught in the meshes by their spines,
presumably as they were falling back down the net when
they were exhausted. Using a different taper on the funnel
may reduce some of these problems, but will not avoid
them completely. Removing the funnel altogether risks
a loss in prawn catch.
were made of the catches obtained with the Nordmøre grid
in comparison with the other trawl with no excluder installed.
A total of 10 hauls were available for this comparison.
These were hauls which were made without video camera
installed to avoid fish behaviour differences caused by
the lights, and on which there were no gear problems such
as damage to the nets which would bias the catches on
one side or other. Half of these hauls were made with
the grid installed in the port side net and half with
it in the starboard net, which should compensate any bias
caused by net differences. The results are presented in
table 2. [CLICK
HERE TO SEE THE FIGURES AND TABLES FOR THIS FINAL
comparisons showed that the grid did not appear to cause
any loss in prawn catch. It appeared to be effective at
excluding large rockfish. No large fish were caught in
the cod end of the net with the excluder installed, although
some were caught lodged in the funnel in front of the
grid. Catches of small rockfish were variable. For instance,
on one haul over 500 pounds of shortbelly rockfish were
caught in the side with the excluder, as opposed to less
than 300 in the side without. Overall, the Nordmøre grid
did not seem to have much effect on catches of small rockfish,
flatfish or roundfish.
tests on the catches by side are inconclusive, the result
for large rockfish is of marginal significance, This is
not altogether surprising, as large fish could not get
into the cod-end at all (paired t-test; P (one tailed)
third device tested was the fish eye. Video Observations
of this showed that there were clear openings in the top
panel of the trawl, which were held open by the fish eye
frame. It was not possible to deduce the strength and
direction of the water flow around the fish eyes from
the video. Large numbers of fish (mostly flatfish) were
seen escaping through the fish eyes. The catch during
the hauls observed consisted primarily of flatfish, with
only a few small rockfish, so conclusions on the escape
of rockfish through the fish eyes cannot be drawn from
these observations. Some events where large numbers of
fish escaped were seen when the trawl pulsed due to snags,
as with the grid. Again, this is evidence that the water
flow down through the fish eye is holding the fish back
in the cod-end and preventing escape. It again seems that
the behavior of the fish may be influenced by the presence
of the lights. Some fish were apparently seen to attack
the light before swimming off after escaping from the
comparisons could only be made for four hauls (table 2).
These were rather inconclusive, as would be expected with
such a small sample size. Even though many flatfish were
seen escaping on the video tows, this effect was not pronounced
in the comparison (paired t test: P (one tailed) =0.0761).
The fish eye did not appear to have a great effect of
reducing bycatch of any of the other species groups.
HERE TO SEE THE FIGURES AND TABLES FOR THIS FINAL
fish eyes used in the experiment were manufactured from
1/4" stainless steel rod. This appeared to be very
strong, but nonetheless all the fish eyes became bent
after a few hauls of use, mainly due to their being caught
on the rail as they were brought on board. Use of a stronger
material and greater care during hauling would seem to
be advised in the light of this experience.
the configuration in which the separator panel was installed,
it did not function adequately. This may be an inherent
design flaw in the panel supplied, but it seems more likely
to be due to the aft part of the trawl collapsing. In
either case, it is necessary to re-think the design and
installation. It is likely that mounting the panel in
a pre made extension section as was done here is not appropriate.
For this sort of design to work it needs to be in a part
of the net which is more open than the aft part of the
extension, and the meshes need to be under some tension
to retain the shape of the panel. The most potential for
this sort of separator is if it is designed as an integral
part of the net and mounted in the aft of the belly where
these conditions are met. However, for the spot prawn
fishery, where there is often a substantial catch of debris,
there needs to be a way to avoid the panel becoming blocked.
It may be possible to achieve this objective by using
a flapper on the bottom panel so that any heavy objects
will fall out that way before they reach the panel. Alternatively,
the panel could be installed to slope down from the top
of the net to the bottom, which would direct heavy objects
out of the escape hole. It would probably be necessary
to use a guiding funnel with this arrangement, and that
would have some additional problems.
Nordmøre grid produced more encouraging results than the
separator panel. However, it seems that the 3" bar
spacing chosen (by an educated guess), was quite significantly
too large to be optimum in this fishery. The choice of
bar spacing has to be a compromise between the risks of
losing part of the target catch and blocking the grid
and the benefits of deflecting bycatch. Further work would
need to use much smaller grid spacing. 2" or less
would be an appropriate starting point.
use of a grid with smaller spacing does, however, increase
the chance of the grid becoming blocked by some of the
larger invertebrates (corals, sponges, kelp mats). This
was a relatively serious problem in some of the hauls
made during this experiment. Various solutions exist to
combat this. Workers in Australia have used grids that
are inclined downwards, and let fish out of the bottom
rather than the top (Brewer et al 1998). This is more
likely to be effective at getting rid of debris that reaches
the grid. Alternatives include using a buoyant flapper
over an escape opening in the bottom of the net so that
the weight of heavy debris causes it to drop out, but
lighter prawns will not have this option. The problem
of blocking of the funnel in front of the grid still exists.
This can be mostly eliminated by attaching the funnel
further up the net where it is more open, rather than
in the collapsed extension section. It may be necessary
on some vessels to slightly change shooting and hauling
procedures to accommodate these changes in the rig.
it was necessary to make all of the observations of fish
behavior in relation to the grid and the fish eye with
the aid of lights, the behavior observed cannot be considered
completely typical of that under normal fishing conditions.
Many of the fish observed holding station in front of
the grid may have been entrained to the lights and not
reacting to the grid at all. However, it seems to give
an indication that the fish which are caught in prawn
trawls are not inclined to make escape attempts in an
upward direction in the manner of for instance haddock
and whiting in the Atlantic (Wardle 1993). Some Pacific
hake were observed from within the cod-end to attempt
to escape through the lower panel of the net. This seems
to give further impetus to the use of escape panels in
the lower, rather than the upper panel of the trawl.
fish eye was the simplest of the devices used, and the
video observations seemed to suggest that it worked to
a certain extent. It is clear, even from the small number
of hauls tested, that it is not a particularly efficient
means of getting bycatch species out of the trawl. Improvements
to its performance may be possible by moving it further
back in the cod-end, although this risks loss of prawns.
Its simplicity in use and installation are the main factors
in its favor.
solutions, such as square mesh panels, may be an equally
simple way to exclude small fish from the catch, although
they will not release the large rockfish which are one
of the species groups which may be significantly impacted
by the spot prawn fishery. If the problems associated
with the use of grids and the associated bad press which
has resulted from this can be overcome, then the optimum
solution is probably some sort of combination device.
This could use a grid to let out the large bycatch and
a second device such as a square mesh panel or fish eye
to let out the rest of the small bycatch. The size of
the target prawns is sufficiently large that a panel of
rather large mesh could be used without risking loss of
prawns. The use of a grid/panel combination would reduce
the volume of bycatch and divert some of the water flow
away from the end of the cod-end, potentially reducing
damage to the catch and increasing its value on the dock.
D, Rawlinson N, Eayrs S and Burridge C. 1998. An assessment
of bycatch reduction devices in a tropical Australian
prawn trawl fishery. Fisheries Research 36 (1998) 195-215
D R, Rogers B D, de Silva J A, Wright V L and Watson J
W. 1997. Evaluation of shrimp trawls equipped with bycatch
reduction devices in inshore waters of Louisiana. Fisheries
Research 33 (1997) 55-72
C S. 1993. Fish behaviour and fishing gear. In Behaviour
of teleost fishes 2nd edn. Edited by TJ Pitcher.
Chapman and Hall. ISBN 0 412 42930 6.