CANADIAN TRANSLATION OF FISHERIES AND AQUATIC SCIENCES
No. 4901
Comparative study of scallop drags used in France
by
H. Dupouy
Original Title: Etude comparée des dragues à coquilles saint-jacques
utilisées en France
From: La Peche Maritime p. 213-218, 1982
Translated by the Translation Bureau
Multilingual Services Division
Department of the Secretary of State of Canada
Department of Fisheries and Oceans
Northwest Atlantic Fisheries Centre
St. John's, NFLD
1982
11
pages typescript
erFes /-/ÎÔ/
LA
PECHE MARITIME
20 April,
1982 (pp.
213-218)
COMPARATIVE STUDY OF SCALLOP DRAGS USED IN FRANCE
by Hervé DUPOUY
of the ISTPM
All the scallops landed by French fishermen are captured with metal drags.
According to the type of bed fished, we can distinguish between two different
types of drags:
— the most common is the regular or inshore* drag, which is made of a
rectangular frame used to keep open a bag, whose lower side is made
of metal rings, while the top consists of a trawl net (Fig. 1);
— the second type (offshore* drag) is of more recent origin (1970) and
can be distinguished from the first by the addition of a sloping metal
flap that is installed over the frame and serves as a pressure plate
(Fig. 2).
Fig. 1 - Inshore Drag Used in the
East Side of the English Channel.
(A: Bail; B: Movable Rake; C; Bag Bracket;
D: Back Bar).
Fig. 2 - Saint Brieuc Offshore Drag.
(A: Bail; B: Left Pressure Plate; C: Movable
Rake; D: Rake Support).
Specifications for the use of these two models consist largely of
regulations concerning their area of utilization (the offshore drag is authorized for use in North Britanny only) and maximum characteristics:
— width: 2 m
— weight: 200 kg
— internal diameter of metal rings: 72 mm
— number of teeth: 1 tooth every 10 cm, or 20 teeth in each 2 m of rake.
Translator's Note: Although not exact equivalents of the terms "drague sans volet" and "drague
à volet", we decided to use "inshore" and "offshore drags" because of their widespread use
in Canada to describe the same type of gear. (See Boume, N., Scallops and the Offshore Fishery
of the Maritimes, Fisheries Research Board of Canada, Bulletin No. 145, Ottawa, 1964; and
Moussette, M., Fishing Methods Used in the St. Lawrence River and Gulf, Parks Canada, 1979.)
The choice of these terms was made in consultation with the DFO Translation Unit.
- 2 Objectives
In order to determine which of the two drag models used in our shores
is better adapted to the rational management of scallop banks, we formulated
the following hypothesis: the best yield of marketable scallops would be
produced by a drag that would need to "work" the bottom for a shorter period
of time in order to reach the desired profit margin; and which would consequently
cause less damage to the beds.
To begin with, we wanted to estimate the efficiency of the two types
of drag and then, using the most efficient model, to determine whether or
not the introduction of certain gear modifications would lead to better results
in the management of the stocks.
Comparative Efficiency and Selectivity of Inshore and Offshore Drags
Experimental Protocol
Our methods followed closely those used by Dickie (1955). The experiments
were carried out during the course of expeditions to estimate the extent of
the scallop banks in the Bay of Saint Brieuc. These were carried out on board
the Roselys-II during September 1975, August 1976 and June 1977.
During each summer expedition («to take advantage of the end of the
fishing season), we released about 10,000 healthy scallops that had been
measured and marked 9with industrial chalk, into a well-defined area
measuring 122,000 m` (745 m x 160 m).
The maximum period elapsed between capture and reintroduction into the
water, which took place at slack tide, was six hours, during which the scallops
were kept in a basin with running water.
Once enough scallops had been released, we wa,ited 4 or 5 days before
proceeding with the rest of the experiment, in order to allow the scallops
to re-establish their normal behaviour.
Then, the Roselys carried out a series of tows, at an average speed of
3 knots, over the longest side of the rectangle (745 m), while alternating
the use of inshore and offshore drags. Aside from the use of the pressure
plate, the two drags were perfectly identical and had characteristics
corresponding to those of the standard models (length: 2 m; weight: 200
kg; internal diameter of bag rings: 72 mm; rake with 20 teeth, 10 cm long).
Each catch was carefully sorted into groups of marked and unmarked scallops;
and each category was subdivided into intact, broken and dead scallops (with
the two valves open). All the individual scallops were also measured.
It is important to point out that the success of the experiment depends
entirely in the proper positioning of the vessel on the ground during release
and recapturing operations. For this reason, we rejected the notion of marking
out the area with anchored buoys; because they are difficult to locate, and
especially because they tend to drift a lot. We also abandoned the idea of
using a Decca navigator, because of its large margin of error (+100 m in the
Bay of Saint Brieuc). Our remaining choice was the use of radar, which is
accurate to between 10 and 20 m, within a radius of 0.5 miles from a pin-point
— 3 —
reference point at sea. Our navigation method consisted of manoeuvering the
vessel in such a way that an echo blip received from a landing turret, was
made to traverse a target traced on the screen, representing the release zone
drawn to the desired scale (Fig. 3).
Fig. 3 - The release zone consists of a rectangle measuring 745 m x 160 m
(side A of the figure). The area was represented on the radar screen (6)
at a scale of 0.5 miles, and subdivided into 5 channels. The manueovres of
the Roselys consisted of moving in such a way that the echo blip received
from the landing tower followed along one of the 5 channels. Thus, by
following the trajectory shown on the screen by large dots, the vessel would
be actually following the course shown on side A of the figure by a dotted
line. Point R on the screen (8) represents the location of the echo blip
corresponding to the position of the Roselys shown on side A of the figure.
The arrow at the top of the radar screen points toward the bow of the ship.
Results
An examination of the catch showed that the ratio of marked (x) and unmarked
(n) scallops was the same as that between the number of released (X) scallops
and those unmarked (N) scallops that were already in the area. Thus, it was
possible to estimate the abundance of scallops in the area (N), for each size
category, by using the formula N = nX/x. The efficiency of each of the tows
can be determined by using the following formula:
Efficiency = n
S
_. _
N
s
where n = the number of unmarked scallops in the catch for each drag tested
N = estimate of the number of unmrked scallops in the area
S = total surface area (122,000 m )
s = area covered by the drag under consideration (number of drags x
length of the area x width of the drag).
Tables 1 and 2 show the results obtained during the expeditions of August
1976 and June 1977. Data from the 1975 expedition was not included, not only
because the experiments that year were used to test the method itself, but
because our radar equipment was not functioning properly.
4
ee
11
Abundance and Density of Scallops in the Area
The number of scallops in the commercial size category >.90 mm)(*)
obtained in the area before the introduction of any marked scallops was 17,900
individuals in August 1976 and 6,200 in June 1977. This represents a density
of 0.15 and 0.14 scallops per m respectively. This figure is relatively
19w for the bay of Saint Brieuc, which has an average of 0.5 individuals per
m in the sectors under exploitation. However, this is not surprising if
we consider that the size distribution of the catch indicates that there is
a predominance of mature individuals, which shows that this sector, which
is considered to be little profitable, has not been exploited very thoroughly.
2.
Efficiency and Selectivity
The two drags were very selective in that practically no scallops under
6 cm were captured. The offshore drag was most successful in capturing
scallops over 87 mm, and it was able to catch about 35% of all the individuals
in its track (Fig. 4). The figure representing the results obtained with
the inshore drag is essentially the same, but displaced towards the right,
which indicates the capture of larger sizes. Full efficiency (30 7.) was only
reached with sizes larger than 97 mm, and it was slightly below the figure
attained by the offshore drag (-177.).
The 507. point on the curve corresponds to a size of 76 mm for the offshore drag and to 82 mm for the inshore drag.
fficience.'
40
offshore
• .
30
t- '
inshore
20
-------
10
.
1
II
3
Il
,•
88
IGO
Ill
U■
size(..)
Ita d v
11$ .P
Fig. 4 - Efficiency and Selectivity of Offshore and Inshore
Drags rigged with standard gear (tooth length: 10 cm;
internal diameter of bag rings: 72 mm; weight: 200 kg).
If we compare the efficiency of drags used in other countries with our
results (Table 3) we find that the gear used by French fishermen is particularly
successful.
(*) Unless otherwise indicated,
between the middle of the hinge
Professionals generally use the
easily be obtained one from the
expressed in millimetres. (See
the sizes mentioned in this report refer to the distance measured
to the opposite edge of the shell (dorso-ventral measure).
largest dimension (antero-posterior measure). The figures can
other by using the following formula: L
1.1 8L
(d .v.J\-6 3
(a.P.)
Fig. 4).
-5 -.
of Marking and Recapture Experiments
Table on the Release Area (745 m x 160 m) Carried
Out by the Vessel Roselys in August 1976.
The characteristics of the offshore and inshore drags were as follows: weight: '200_kg;
width: 2 m; 20 teeth, 10 cm long; internal diameter of rings on the belly side: 72 mm;
upper netting: 35 mm per side. Average drag speed: 3 knots.
1 - Results
Size
Total
Recaptures
to the number
)ffshor e drag
Inshore drag
nearest of
;
(23 trials)
(16 trials)
i cm.
arked
-71
Marked Unmarke Marked Unmarke
Estimate of number o
scaop
llp in_mgc
ffshore
drag
In shore Mean
drag
Efficiency (%)
Offshoreknshore
drag
drag
callops
eleased
'
3,5
4,0
1,5
SA
5,5
6,0
6,5
7.0
7.5
8,0
8.5
9,0
9,5
MO
10.3
11,11
11.5
12,0
--TOTU.
1
2
4
D
16
9
3
16
43
101
350
1 614
3 Oel
2 953
8 293
508
156
106
--80 257
.
1
0
0
0
6
41
157
3 79
321
153
65
.
15
13
3
5
8
3
0
0
2
8
28
118
335
505
.182
283
93
36
1 182
1 991
.
2:
•
2
14
78
162
155
61
29
9
5
4
1
0
0
0
4
11
52
185
306
230
149
58
20
1 018
2 706
5 393
4 073
2 178
938
291
513
1 018
17 016
.
135
239
202
350
1 076
3 196
5 831
4 617 •
2 610
I 005
124
19 611
168
295
1 017
3 101
16.9
33.7
10,0
5609
37.0
39.2
11.9
..33,8
35.6
4 350
2 391
997
359
Ma
12.1
24.2
25,3
30,1
27.8
26,9
31.7
29.7
28,4
18 330
Table II - Results of Marking and Recapture Experiments on the Release Area (745 m x 160 m) Carried
Out by the Vessel Roselys in June 1977.
The characteristics of the offshore and inshore drags were as follows: weight: 200 kg;
width: 2 m; 20 teeth, 10 cm long; internal diameter of rings on the belly side: 72 mm;
upper netting: 35 mm per side. Average drag speed: 3 knots.
,
Estimate of number of
Size
Total
Recaptures
number
o-f
to the
scallops in area
Offshore drag
Inshore drag
neares. marked
Offshore
Inshore Mean
10 trials
10 trials)
drag
i cm. scallop- Marked Unmarked Marked Unmarked drag
release.
.
2„s
n
3.0
M
2.16
3.5
4A
4.3
3.0
5
4
1
na
6.11
6.1
20
' 4
2
20
91
1
7,0
nu
s
7.5
8.0
8.5
387
312
291
556
831
9
15
12
22
36
67
5.S,
.
9,5
811,0
10.1
I St 1
I 311
In
nm
:u9
39
in
11,1
12.0
2.29
le
lee
{I
6
21
21
21
33
41
14
8
3
7 373
263
931
193
1...1u.
..
Offshore Inshore
drag
drag
.
2
32
116
113
40
36
10
122
M3
9, 0
Efficiency (%)
2
190
IS
1 19!
.1 934
2
4
8
69
109
3 911
974
100
2 104
6 676
1
600
1 916
S 832
612
7911
42.1
I 211
3 799
55M
5 829
I 221
1 302
830
510
I 093
3 278
17M
1 191
918
sn
uu
Md
23113$
32 802
29 112
7
W
16
33
133
1116
112
11
M
657
981
2 757
6012
780
2311.11
8.6
13.1
16.2
32.8
37.1
39.3
39.7
3n,1
nm
21.9
36.2
ma
84
6.3
9,6
20,7
17,6
21,2
2.1 .5
33.1
nc
27.6
36.1
27.2
Table 3. — Comparative Table of the Efficiency of Various Drags Used in the North Atlantic.
Type of Drag
Weight
Country
Species
Efficiency
Reference
I
Rake drag
Spring drag
90 kg
Scotland
Pecten maximus
20%
Baird (1955, 1959)
Chapman et al. (1977)
110 kg
Scotland
Pecten maximus
13%
Chapman et al. (1977)
Scotland
Pecten maximus
30 7
Rolfe (1969)
Baird offshore drag
Digby bar drag
300 kg
Canada
Placopecten magellanicus
5 to 12%
Dickie (1955)
!ew Bedford chain drag
350 kg
Canada
Placopecten magellanicus
8 to 10%
Caddy (1968)
oothed drag
200 kg
France
Pecten maximus
30% 30%
Dupouy (1978)
aint Brieuc offshore drag
200 kg
France
Pecten maximus
35%
Dupouy (1978)
,
.•
Variations in Efficiency and Selectivity as a Function of the Nature of the
Bottom
Once the offshore drag had been found to be more efficient in the release
area, we wanted to determine whether or not these results could be extrapolated
to the whole fishery. Thus, the Roselys carried out a series of random drags,
over the Saint Brieuc beds, while simultaneously pulling the two drags rigged
in the same way (10 cm teeth and rings with an internal diameter of 72 mm).
A comparison of the catch obtained in each of 25 trials (Fig. 5) shows
that the offshore drag caught 4,690 scallops under the commercial size limit,
while the inshore drag caught only 1,260 scallops in this category.
In the
marketable sizes, the proportion was 2,790 to 2,140 scallops respectively.
While these values tend to vary from year to year, due to fluctuations in
the distribution of the various age categories, from a relative point of view,
it is still true that, on the average, the offshore drag landed:
- 4 times more undersize scallops (under 90 mm)
- and 30 7e more marketable scallops.
These figures are very close to those obtained on the release area.
However, depending on the area dragged, we observed some variations:
- over very loose bottoms (muddy bottoms which tend to be characterized
by the presence of anomia mollusks), the offshore drag fills up very
quickly (in a few minutes), thus, it is very efficient for catching
the smaller scallops but less so for the larger sizes.
fficiency%)
3 cm teeth
.cm rings
40
s.andatd
30
20
10
•
•
18
size
It
/11
41
118
M
Fig. 5 - Variations in the Efficiency and Selectivity of the
Offshore Drag after Gear Modifications: 1 - length of teeth
to 13 cm; 2 - diameter of rings to 9 cm.
The inshorU drag does not fill up as quickly, but its efficiency for
catching commercial-size scallops is as good or better than that of the offshore drag:
- over medium bottoms (firm sand or gravel, where scallops are usually
found, the results obtained with the two drags were very similar to
those obtained in the release area.
- over hard bottoms, the offshore drag captured small scallops and its
efficiency tended to approach that of the inshore drag.
However, it is possible to change the rakes quickly in order to adapt
them better to the conditions of the bottom. Thus, in order to avoid overfilling the bag, one may use longer teeth (up to 13 cm); and one may use shorter
-8
-
ones (up to 7 cm) over hard bottoms.
Results Obtained with a Modified Offshore Gear
Since the offshore drag tended to capture too many scallops under the
commercial size, we undertook a new series of tows with the Roselys in the
bay of Saint Brieuc, in order to determine what effects a change of gear would
have on the catch.
Our previous studies on the population dynamics of scallop beds on the
bay of Saint Brieuc have shown that the optimal size of individuals in the
catch is •bout 117 mm (along the largest dimension), while the regulation
size is only 102 mm. An increase of 15 mm over the commercial size would
allow an increase of 30% in the exploitable portion of the stock in one year
(that is, an increase of 2,000 to 3,000 t per year). The best way to achieve
this would be to propose certain modifications in the characteristics of the
drags used, in order to avoid catching individuals under the optimal size
(105 mm from the hihge to the opposite edge of the shell).
The offshore drag was rigged with two different types of gear for testing.
One of the modifications affected the length of the rake teeth (20 teeth,
13 cm long, instead of 10 cm long), while the mesh size of the bag was unchanged
(rings with an internal diameter of 72 mm). On the other hand, the second
model kept the length of the teeth the same (10 cm) while changing the internal
diameter of the rings to 90 mm. The inshore drag with standard gear was used
as the basis for comparison. Once the selectivity and efficiency were known,
we were able to deduce the values corresponding to each of the two types of
gear used with the offshore drag (Fig. 5).
•
We found that increasing the length of the teeth in the offshore drag
caused a decrease in the number of undersized scallops caught (<90 mm). This
type of gear was also clearly superior to the standard offshore drag (72 mm
inside diameter for the rings and 10 cm length for the teeth) for catching
scallops over 10 cm.
An increase in the mesh size of the bag to 90 mm produced an even greater
tendency along the same direction, since hardly any undersize scallops were
caught. For scallops over 10 cm, efficiency was also superior to that of
the standard offshore dreg (+7 7.).
Thus, the offshore drag with 90 mm rings seemed to be better adapted
to the rational management of the scallop beds in Saint Brieuc. Furthermore,
this type of modification would be easier to implement than new regulations
concerning the length of the teeth.
Effective
Comparison of the size frequency (dorso-ventral measurement
to the nearest 1/2 cm) obtained with 2 drag models over 25 tows
Effective -
11140
• 400
1200
. 00
• 02
see
.co
400
•
_ -
_, . .
. _ _ bdshore
P00
1 ![
'___. 1
1
1 1 I tl:L
-,..„____ size class
1
8
-
.
4C0
—
000
11-
9 10 11 12 13 14
î :
i. 6
-
. , rti, , r,
•
1 .._. 1
H
.
I
9
10
1fill— 7, -1 7,-..si ze class
11 12
--
Inshore Drag
Drag
Fig. 6 - Comparative representation of the size of scallops caught with a standard offshore drag
(left) and a standard inshore drag (right) during an expedition to the bay of Saint Brieuc
(Roselys, February, 1975).
3 4
9
Scallop Mortality Due to Drag Action
It is important to find out the extent of the losses caused by the fishing
activities themselves. Apart from the scallops that are caught, there are
other mortality figures that are not included in the statistics. These include
commercial size scallops that are captured by the drags but which have suffered
such extensive damages as to render them unfit for marketing. In the catch
produced by the standard offshore drag, about 10.5% of the scallops were in
this category, against 14.8% in the catch of the inshore drag.
Throwing back undersized scallops, which is usually done in the drag
area, also results in a significant rate of mortality which we could estimate
during our experiments in the release zone. Thus, in August 1976, there were
468 dead scallops in a total of 1,700 recaptured scallops bearing our mark,
for a proportion of 27.5 7. . In June 1977, the percentage obtained was 26.7 7e .
Here, it would be important to remember that 10 days had elapsed between the
time the marked scililops were released back into the water and the time when
they were recaptured. In most of the dead scallops the two valves were still
attached, but all the soft flesh had disappeared.
Chapman et al. (1977) conducted the same type of experiment in Scotland,
where they found a rate of mortality of between 0 and 25% (- with a mean of
8 7. ), depending on the length of the tow (5 to 80 minutes) required to capture
the scallops. Our trials lasted about 15 minutes each, so that our results
(around 27 7. ) appear to be overestimated. There is no question that this is
related to the.fact that some of the dead scallops included in our data were
killed by the repeated activity of the passage of our drags over the beds.
In order to find out what happened to scallops found under the path of
the drags but which were not captured, we carried out a further experiment
which was carried out by a team of 7 divers of the ISTPM and the COB — CNEXO.
Very soon after the drag had gone by, the divers examined the debris left
on the bottom over a distance of 200 m. They examined carefully the scallops
left in the area, as well as those that could be found on each side of the
dragged area over a distance of about 1 m.
The examination of the mollusks showed that very few of the small scallops
had been damaged. However, almost 30% of the adult scallops (>8 cm) had been
affected. Most of the damaged scallops were found in the dragged area. On
each side of the track, the scallops were generally in good condition. Some
scallops were found in small depressions in the sand, with their upper shell
covered by a light layer of sedimentary deposits, these were considered to
have already been in place before the passage of the drag. A second group
of scallops was made up of individuals that were not buried in the ground
and seemed to correspond to scallops that had swum sideways in order to escape
the drag.
Several authors (Baird and Gibson, 1956; Hartnoll, 1967; Chapman et al.,
1977) have shown that Pecten maximus tends to remain in the same place for
very long periods of time. It is rare to observe scallops in the process
of swimming spontaneously. On the other hand, at the approach of a predator
or a visual or tactile stimulus, it is possible to observe scallops swimming
over a distance of several metres. However, the swimming response is more
difficult to produce when the individuals are older. This may explain why
the smaller scallops had been relatively spared.
- 1 0-
,
Nevertheless, if the swimming movement is made in the same direction
that the drag is travelling, the scallop is easily tired and can be recaptured
by the drag; because, unless it passes under the frame, it will be caught
by the bag (Caddy, 1968).
Some of the scallops found under the track of the drag had passed under
the frame, but most had gone through the bag or had slipped out through the
spaces between the teeth when the rack had not gone very deeply into the sediment. This latter group was obviously the most severely damaged, because
the scallops had passed under the drag bag. The divers also noticed the arrival
of great numbers of predators (especially whelk and crustaceans), which were
quick to move towards the damaged scallops. At the end of a few hours, all
the flesh parts had been removed.
Our observations were based on an examination of the first 200 metres
of the dragged area; thus, it is possible that our figure of 30% mortality
In fact,
for adult scallops remaining on the bottom has been overestimated.
normal dragging speed is only achieved after some 20 or 30 metres and, under
these conditions, the teeth have not yet had time to penetrate into the sediment
very deeply, so that a larger number of scallops can pass under the bag.
Finally, • we can nevertheless estimate that for each 100 commercial size
scallops landed by an offshore drag, we should add 10% mortality rate for
scallops that have been broken on board the vessel and another 10% for scallops
that have died on the bottom. These figures are a little higher for the inshore
drag: 15% broken on board ship and 15% dead on the bottom, due to the lack
of penetration of the teeth.
For smaller scallops under the commercial size which are thrown back
into the water, mortality is proportional to the length of the tow: between
5 and 10% for a 15-minutes trial, and up to 25% for a tow lasting from 1 hour
to 1 hour and 20 minutes.
Conclusions and Future Outlook
The addition of a pressure plate on the rake drag makes it possible to
improve its efficiency, because this helps to keep the drag on the bottoin.
Efficiency is further improved by the fact that the pressure plate makes it
possible to drag at higher speeds. Also, it is not necessary to pay out so
much warp in order to obtain the proper angle of the teeth over the bottom
(for example, for a depth of 20 to 30 m, it is enough to pay out a length
of about 3 times the lead for an offshore drag, against 5 times for an inshore
drag). This is important, not only because it prevents wear on the ropes,
but also because it saves time during the tow. For the same towing time,
it is possible to expect an increase in yield of about 50% with the use of
the pressure plate.
Another advantage of this type of drag is that it produces fewer broken
scallops and causes less bottom damage, due to the better penetration of the
teeth into the sediment. Baird (1955) has shown that the fishing gear in
the inshore drags is very difficult to control: when the speed is too high,
the frame of the drag tends to come apart, and when the speed is too low,
the frame tips forward, so that the teeth do not form a proper angle with
the bottom, and the scallops, instead of being caught in the bag, pass under
the metal bag and suffer significant damage.
— 11 —
e
4
Nevertheless, the offshore drag does have some inconveniences, such as„
the fact that it tends to capture too many undersized scallops. However;
we have shown that the use of larger diameter rings makes it possible to
decrease this number significantly. As far as the bay of Saint Brieuc is
concerned, better yield can be obtained with the use of rings with an internal
diameter of 9 cm.
Finally, this study has served to remind us of the problems of coexistence
that exist between trawlers and scallopers. It is very true that drags, and
particularly offshore drags, bring to the surface sediments containing pebbles
and other debris that may cause wear and tear to the trawlers working in the
same areas. The drags can also accidentally catch other desirable species
such as sole, turbot, brill, eels and crustaceans. In some sectors, scallopers
have been accused of ruining the bottom for trawling, especially when using
heavy drags (200 kg) equipped with pressure plates.
References
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dredge. J. Cons. perm. int. Explor. Mer 20, 290-291.
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