Scottish Industry Science
Partnership Report
Report no. 01/10
SISP project 004/09
Trials to reduce cod by-catches in Shetland mixed demersal
whitefish trawl fisheries by putting large meshes (300 and 600mm)
in the front portion of a commercial trawl
R J Kynoch, F G O’Neill and R J Fryer
February 2010
Trials to reduce cod by-catches in Shetland mixed demersal
whitefish trawl fisheries by putting large meshes (300 and
600mm) in the front portion of a commercial trawl
R J Kynoch, F G O’Neill and R J Fryer
Marine Laboratory,
Marine Scotland – Science,
Aberdeen.
SUMMARY
During 2009 experimental trials were conducted to assess the extent to which cod
by-catches in the Shetland mixed whitefish fishery would be reduced by replacing the
120mm netting in the front sections of a trawl with 300 and 600mm netting. A further
aim was to assess the effect of these modified trawls on the catches of monkfish and
megrim, which are economically important to the Shetland fleet.
The results showed that both test gears caught significantly fewer cod, hake, megrim
and ling than the control trawl across all length classes and significantly fewer
monkfish below 76 and 83cm respectively than the control trawl. The 300mm trawl
caught significantly more haddock than the other trawls and catches of saithe greater
than 53cm were reduced in both test gears.
INTRODUCTION
During 2008 the Orkney Fisheries Association (OFA) conducted catch comparison
trials using a modified whitefish trawl designed to reduce the capture (and
consequently potential discarding) of cod (Campbell et al., 2010). The ‘Orkney Gear’
(as the trawl became known) replaced the 160mm diamond mesh netting in the front
portion of the trawl (wings, cover and first belly section) with 300mm diamond mesh
netting. The Orkney Gear retained fewer cod: the relative catch rate was length
dependent with 45% fewer cod caught at 35cm but no significant reduction for
lengths >78cm. The Orkney Gear also caught fewer monkfish and megrim: the
relative catch rate of monkfish was length dependant with 37% fewer monkfish
caught at 37cm, but no significant reduction for lengths >55cm; the relative catch rate
of megrim did not depend on length, with about 43% fewer megrim caught at all
lengths. Under the 2009 Conservation Credit scheme a ‘buy back’ was offered to
whitefish vessels using the Orkney Gear. The incentive offered was either 20 days or
12% depending if the vessel was on flat rate or track record allocations.
Owing to the importance of monkfish and megrim to the Shetland mixed whitefish
trawl fleet, concerns were raised by local skippers about the viability of fishing the
Orkney Gear in their fishery. To assess the impact of this new trawl the Shetland
Fishermen's Association (SFA) submitted a Scottish Industry/Science Partnership
(SISP) proposal to carry out a catch comparison study similar to the Orkney trials but
using a vessel and on fishing grounds typical of the Shetland fishery. The vessel
used for the Orkney trial was a 2600hp twin-rig trawler, whereas those typical of the
Shetland fleet are in the range 800 -1200hp. Furthermore, two different mesh sizes
were proposed, one of 300mm netting and another of 600mm. There was provision
under the 2009 Conservation Credit scheme to fish trawls with 600mm diamond
mesh netting in their front portion, which would receive an additional 24 days or 15%
allocation.
This report presents the results of these trials, carried out in the summer of 2009.
VESSEL AND FISHING GROUNDS
The Resilient (LK195), a 593kW twin rig whitefish trawler (Figure 1), was chartered
for the trials. The charter ran for 22 days from 25 June to 16 July 2009. Two Shetland
fishing grounds were targeted, one about 15 nautical miles east of Unst and the other
20 to 25 nautical miles North of Muckle Flugga. The species mix was of cod, megrim,
monkfish, ling, hake, haddock and saithe, with small numbers of whiting, lemon sole
and plaice.
Figure 1: Fishing vessel Resilient (LK195).
FISHING GEAR – CONTROL TRAWL
The vessels own commercial whitefish trawl was used as the control/standard trawl.
The design had a 468 meshes x 120mm (nominal) fishing circle with 160mm
(nominal) diamond meshes in the upper wings, lower wings and top-square
(Figure 2). The rockhopper ground gear had 350mm discs throughout, all rigged onto
28mm wire with an overall length of 45.73m. The wire rig had 36.58m single wire
sweep, 36.58m of single rubber leg sweep and 27.44m double bridles (top – wire and
lower – chain). The gear was fished with a three-warp towing system with a 1500kg
roller clump centre weight and spread using 1000kg Morgere Ovalfoil otterboards.
During the trials all trawls, the standard and the two test trawls, were rigged with a
tickler chain made from 16mm mid-link chain and a flip-up rope assembly, as used by
Resilient when fishing commercially.
Figure 2: Resilient standard trawl.
FISHING GEAR – EXPERIMENTAL TRAWLS
Two test trawls were made for these trials, one with a maximum mesh size of 300mm
(nominal) diamond mesh (Figure 3) and the other with 600mm (nominal) diamond
mesh (Figure 4). The test trawls had the same frame rope lengths (e.g. headline,
footrope and fish tails), trawl stretched length and positioning of guard/strengthening
meshes as the standard trawl. Both trawls were rigged with similar ground gear and
flotation specifications as the standard trawl.
Figure 3: 300mm test trawl.
Figure 4: 600mm test trawl.
To simplify rigging of the 600mm meshes to the headline and footrope combination
wire, a 5 mesh deep x 300mm mesh DBL twine guard was used which was intended
to minimise slippage of the large meshes and to prevent the 200mm floats fouling on
the large meshes (Figure 5). To improve strengthening further, the same 300mm
mesh guard was also used in the upper and lower wing tips. To enable the rearmost
meshes of the 600mm first belly section to connect to the following 120mm (nominal)
mesh panel a 4 mesh deep x 300mm DBL bating section was used (Figure 6).
Figure 5: Guard meshes to connect 600mm meshes to headline/footrope.
Figure 6: 300mm batings section connecting 120mm to 600mm meshes.
During the trials all trawls (standard and test) were rigged with new 120mm (nominal)
codends. The codends were made from 5mm DBL high tenacity PE twine with 94
open meshes in the circumference and 100 meshes deep. The 120mm extensions
were made from 5mm single high tenacity PE twine with 94 open meshes in the
circumference and 100 meshes deep. The codend mesh sizes were measured,
whilst the netting was wet, using an EU standard wedge gauge (Table 1).
TABLE 1
Mesh measurements of 120mm codends using EU wedge gauge
Codend
Control trawl
300mm trawl
600mm trawl
Mesh size (mm)
126
126
126
FISHING TRIALS PROCEDURE
Tow duration was 4 hours except for hauls 11 and 34 which were 3.1 and 2.2 hours
respectively due to the tickler chains starting to dig into the seabed. Towing speed
over the ground ranged between 2.7 to 3.1 knots, the usual towing speed for Reliant
when fishing commercially for monkfish and megrim. Simrad ITI net monitoring
acoustic instrumentation was used to monitor otterboard spread during most hauls.
Only one headline height sensor was available so height measurements were
recorded for each trawl on alternate days. A full haul summary can be found in
Tables 2 and 3.
The twin trawl technique (Wileman et al., 1996) was used to assess the relative catch
rates of the test gears. This involves towing two similar trawls in parallel, with the test
gear fished on one side of the twin trawl system and the unmodified control trawl
fished on the other side. To reduce any port/starboard bias the test trawls were
fished on both sides of the twin trawl system during the trials. As standard practice
aboard Resilient, the starboard trawl’s codend was always emptied into the vessel’s
fish hopper first. To prevent washout the port codend was hauled up and left
suspended on the stone trap above the fish hopper until the starboard catch had
been processed.
At the end of each haul, catches from the test and control trawls were kept separate
with the target species separated into baskets, weighed and then all measured. A
breakdown of weights for the target species is given in Tables 4 and 5. The amounts
of non-target species caught during the trials were very small and estimated to be no
more than 60kg per haul in either test or control codends. Most of the non-target
species caught were skate, rays, gurnards and lesser spotted dogfish.
TABLE 2
Haul summary table for the 300mm test trawl
Haul
number
R09/01
R09/02
R09/03
R09/04
R09/05
R09/06
R09/07
R09/08
R09/09
R09/10
R09/11
R09/12
R09/13
R09/14
R09/15
R09/16
R09/17
R09/18
R09/19
R09/20
R09/21
R09/22
R09/23
R09/24
R09/25
R09/26
R09/27
R09/28
R09/29
R09/30
Test trawl
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
300mm
Side test
Fished
(P or S)
P
P
P
P
P
P
P
P
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
P
P
P
P
P
P
Area
fished
Haul
Duration
(Hrs)
Unst
Unst
Unst
Unst
Unst
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
N Flugga
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
3.1
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
Mean
speed Made
good
(kts)
3.0
3.0
3.1
3.0
2.8
2.9
3.0
2.8
2.7
2.7
2.8
3.1
2.9
2.9
2.8
2.8
2.7
2.8
2.9
2.7
3.0
2.9
2.8
2.7
2.7
2.8
3.1
3.0
2.8
3.0
Mean
headline
height
test (m)
5.0
4.9
N/R
N/R
N/R
4.2
4.2
4.3
N/R
N/R
N/R
4.7
4.9
4.9
N/R
N/R
N/R
N/R
N/R
4.6
N/R
N/R
N/R
4.9
4.5
4.6
N/R
N/R
N/R
4.4
Mean
headline
height
control (m)
N/R
N/R
3.3
3.2
3.4
N/R
N/R
N/R
3.0
3.0
3.0
N/R
N/R
N/R
3.0
2.9
2.9
N/R
N/R
N/R
2.9
3.5
3.1
N/R
N/R
N/R
3.0
3.3
2.7
N/R
Mean
speed Made
good
(kts)
2.8
2.9
2.9
2.7
2.8
2.9
2.9
2.9
3.1
3.0
2.8
2.8
2.9
3.0
Mean
headline
height
test (m)
5.2
4.9
5.0
4.9
4.7
4.8
4.7
N/R
N/R
N/R
N/R
N/R
N/R
N/R
Mean
headline
height
control (m)
N/R
N/R
N/R
N/R
N/R
N/R
3.3
3.1
3.4
3.0
3.2
2.9
2.9
3.1
TABLE 3
Haul summary table for the 600mm test trawl
Haul
number
R09/31
R09/32
R09/33
R09/34
R09/35
R09/36
R09/37
R09/38
R09/39
R09/40
R09/41
R09/42
R09/43
R09/44
Test trawl
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
600mm
Side test
Fished
(P or S)
P
P
P
P
P
P
S
S
S
S
S
S
S
S
Area
fished
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Unst
Haul
Duration
(Hrs)
4.0
4.0
4.0
2.2
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
TABLE 4
Summary of target species catch composition for 300mm test and control trawl codends
Haul
No
R09/01
R09/02
R09/03
R09/04
R09/05
R09/06
R09/07
R09/08
R09/09
R09/10
R09/11
R09/12
R09/13
R09/14
R09/15
R09/16
R09/17
R09/18
R09/19
R09/20
R09/21
R09/22
R09/23
R09/24
R09/25
R09/26
R09/27
R09/28
R00/29
R09/30
300mm test trawl codend catch (kg)
HAD
60
89
62
55
41
24
16
23
8
11
8
74
82
68
45
60
34
45
38
19
23
19
9
23
18
26
31
20
27
32
WHI
24
43
29
11
19
0
0
0
0
0
0
35
23
34
23
30
15
27
5
11
0
0
0
0
0
0
3
0
3
0
COD
134
92
131
124
242
42
23
17
7
11
30
74
128
183
168
120
185
118
152
172
5
15
14
40
21
46
61
72
68
57
SAI
48
15
89
36
25
47
83
38
36
30
57
93
33
69
58
19
29
26
33
60
55
22
42
45
45
40
127
66
60
107
HAK
30
75
50
53
83
5
9
5
1
2
5
63
93
53
71
56
22
41
30
18
21
5
6
8
7
7
25
10
22
20
LIN
30
45
93
104
89
38
63
13
32
43
34
33
78
63
38
22
33
33
45
45
23
36
24
44
46
31
21
21
9
19
Control trawl codend catch (kg)
ANG
80
58
78
124
94
89
78
69
128
82
106
77
55
72
121
65
70
41
76
79
63
97
102
77
78
90
73
84
92
94
MEG
1
3
2
4
1
11
16
10
20
12
9
1
1
1
0
2
3
2
6
2
10
17
14
16
15
14
5
4
4
2
PLA
4
5
6
4
9
0
0
0
0
0
0
8
12
1
6
5
2
2
5
4
0
0
0
0
0
0
0
0
0
0
LSO
2
1
1
2
1
0
0
0
0
0
0
0
2
0
1
1
0
2
2
1
1
0
0
0
0
0
0
0
0
0
Total
413
426
541
517
605
256
288
175
232
191
264
458
507
544
531
380
393
337
392
311
201
121
211
253
210
254
346
277
285
331
HAD
44
52
30
27
45
18
13
7
10
4
7
47
51
29
31
44
31
30
14
10
9
10
7
21
16
16
20
27
10
24
WHI
40
33
18
10
18
0
0
0
0
0
0
33
30
16
32
20
10
17
9
11
0
0
0
0
0
0
1
5
7
2
COD
171
118
289
215
168
26
36
24
27
53
50
168
107
310
311
280
316
179
298
254
50
42
25
42
76
57
181
208
92
182
SAI
53
23
72
87
50
106
60
33
86
50
86
134
33
108
120
27
32
38
33
27
103
40
54
58
91
56
211
35
103
178
HAK
92
61
105
104
74
11
19
9
8
4
6
71
70
58
68
78
49
59
66
18
20
1
13
20
19
7
41
19
25
34
LIN
60
52
117
158
122
60
53
57
63
101
61
48
60
103
53
52
66
51
111
57
68
52
36
32
43
50
49
29
14
17
ANG
107
98
85
120
107
108
103
103
77
143
130
81
103
86
110
99
82
89
102
141
141
152
97
116
112
123
142
160
148
104
MEG
8
9
7
6
4
59
74
59
75
76
61
3
6
8
9
11
8
11
16
9
68
71
46
64
75
66
16
13
2
16
PLA
7
12
14
14
15
0
1
0
0
0
0
19
11
10
19
7
5
6
5
5
0
0
0
0
0
0
0
0
0
0
LSO
2
2
2
3
3
1
0
0
0
0
0
3
2
1
2
3
1
4
1
1
1
1
0
0
0
1
1
0
0
0
Total
584
460
739
744
606
389
359
292
346
431
401
607
473
729
755
621
600
484
655
533
460
369
278
353
432
376
662
496
401
557
TABLE 5
Summary of target species catch composition for 600mm test and control trawl codends
Haul
No
R09/31
R09/32
R09/33
R09/34
R09/35
R09/36
R09/37
R09/38
R09/39
R09/40
R09/41
R09/42
R09/43
R09/44
600mm test trawl codend catch (kg)
HAD
23
14
0
13
11
17
6
9
10
10
14
15
30
19
WHI
1
2
0
4
1
3
1
1
0
0
2
2
4
2
COD
22
48
30
42
41
40
18
29
9
25
21
33
44
50
SAI
11
9
8
10
9
9
10
15
7
27
12
26
40
25
HAK
23
50
24
35
14
46
39
22
24
20
12
14
16
14
LIN
4
16
6
0
25
7
14
4
9
8
2
10
21
6
Control trawl codend catch (kg)
ANG
25
20
5
23
25
37
47
51
55
41
35
20
47
35
MEG
1
0
0
1
0
2
2
0
0
0
1
0
2
1
PLA
0
4
1
0
1
0
4
0
2
2
3
1
0
0
LSO
2
0
0
0
2
1
1
1
2
1
1
1
2
1
Total
112
163
74
128
129
162
142
132
118
134
103
122
206
153
HAD
23
0
0
12
14
15
6
2
10
9
23
7
15
15
WHI
4
0
0
3
1
3
0
1
0
0
3
2
5
1
COD
113
118
146
61
127
173
186
76
98
103
92
138
163
159
SAI
26
15
4
4
5
18
17
30
34
29
22
11
58
60
HAK
22
45
35
42
28
85
99
77
84
54
35
41
21
37
LIN
44
15
21
21
27
39
39
20
46
20
15
40
25
17
ANG
64
100
57
38
77
100
93
99
131
104
68
121
100
90
MEG
7
7
5
1
5
16
11
13
10
7
7
7
11
9
PLA
3
11
11
3
11
9
5
11
3
19
15
1
6
0
LSO
1
0
0
0
2
3
3
1
4
2
2
5
4
0
Total
307
311
275
185
297
461
459
330
420
347
284
368
408
388
RESULTS
Gear Performance
Mean headline heights for the control trawl were about 1.6m less than for both test
trawls. The otterboard-to-clump spreads for the side fishing the test trawl were also
generally higher, possibly indicating that the larger mesh test trawls had less drag
and were easier to tow. There were some problems operating the 600mm trawl. The
600mm netting in the belly and top sheets consistently got caught on the 200mm
floats while shooting. On several occasions it took 3-4 attempts to clear the floats,
even after a temporary float rope was rigged. A solution was eventually devised
using a rope run through the headline centre and then wound back around the
powerblock. This kept the headline clear of the deck as the trawl was being shot until
the wingends were clear of the stern roller. This technique was used for the rest of
the trials but would not be considered practical for commercial fishing.
Catch comparison
A total of 30 hauls were completed with the 300mm trawl and 14 with the 600mm
trawl. The 300mm trawl was fished on the Unst and Flugga grounds but the 600mm
trawl only on the Unst grounds. Adequate numbers of cod, hake, monkfish, megrim
saithe and ling were encountered during the trials, but the numbers of whiting, plaice
and lemon sole were low and insufficient for subsequent analysis.
The catches retained in the test and control trawl codends were analysed in two
ways: using the smoother based methodology of Fryer et al. (2003); and using a
generalised linear mixed model (Holst & Revill, 2009) (see Appendix for details).
Both methods estimated and compared the catch rates of the test gears relative to
the control gear and only differed in the assumptions they made about the shape of
the catch rate curves. The results were virtually identical and the fitted relative catch
rates from the generalised linear mixed modelling approach are shown in Figure 7.
Cod, hake, megrim, and ling: the relative catch rates did not depend on length.
Both test gears caught significantly fewer fish than the control gear. The 600mm
gear caught significantly fewer fish than the 300mm gear.
Haddock: the relative catch rates did not depend on length. The 300mm gear
caught significantly more haddock than the control gear. The catch rates of the
600mm gear and the control gear did not differ significantly.
Monkfish: the relative catch rates increased with length. The 300 and 600mm gears
caught significantly fewer monkfish than the control gear below 76 and 83 cm
respectively. The relative catch rate of the 600mm gear was significantly below that
of the 300mm gear below 78 cm.
Saithe: the relative catch rates decreased with length. Both test gears caught
significantly fewer saithe than the control gear above 53 cm. The relative catch rate
of the 600mm gear was significantly below that of the 300mm gear above 61 cm.
Figure 7: Estimated catch rates of the test gears relative to the control gear from the
generalised linear mixed modelling approach. The lines are solid when the relative catch rate
is significantly different from unity at the point-wise 5% significance level and dashed
otherwise. The catch rates are plotted for lengths that were present in most hauls.
DISCUSSION
In general, the catch rates of the 300mm trawl were significantly below the control
gear and those of the 600mm trawl were significantly below the 300mm trawl. The
one exception was for haddock where significantly more were caught in the 300mm
gear than the control gear and there was no difference between the 600mm and the
control gear. The catch rates of cod, megrim, ling, hake and haddock did not depend
on length whereas that of monkfish increased with length while that of saithe
decreased.
Cod, megrim and monkfish are of prime importance in the Shetland whitefish fishery.
The 300 and 600mm trawls caught 49 and 74% less cod respectively than the control
trawl at all lengths (although very few cod > 91cm were caught). The respective
losses of megrim were estimated to be 80 and 93%, again across all length classes.
The relative catch rates for monkfish were length dependent. The 300mm trawl
caught ~50% fewer monkfish at 30cm with no significant differences >76cm. The
600mm trawl caught 85% fewer monkfish at 30cm with no significant differences
>83cm.
The results for the 300mm trawl are broadly similar to those of the ‘Orkney Gear’
(Campbell et al., 2010). The Orkney Gear caught 45% fewer cod at 35cm with no
significant differences >78cm, about 43% fewer megrim at all lengths, and 37% fewer
monkfish at 37cm with no significant differences >55cm. The 300mm trawl might
have caught less megrim than the Orkney Gear relative to the control, because the
mesh size of the control trawl during the Orkney trials was 160mm whereas here it
was 120mm.
Ling, hake and saithe are also important to the Shetland fleet. The 300mm gear
caught 28% fewer hake and 36% fewer ling than the control gear while the 600mm
gear caught 53% fewer hake and 68% fewer ling. Catches of saithe greater than
53cm were also reduced. The relative catch rate decreased with larger fish and with
increasing mesh size, suggesting that saithe are increasingly more able or prepared
to penetrate netting the bigger they are and the bigger the mesh size.
As found during the Orkney trials, haddock catch rates were significantly higher for all
lengths in the 300mm trawl compared to the control trawl. This is possibly due to the
higher headline height of the 300mm trawl (~1.6m) compared to the control trawl.
However, there was no difference in catch rates between the 600mm and control
trawls, which may indicate that haddock are escaping through the 600mm meshes.
These results show that the gear modifications made here will result in large catch
reductions of some species that are of economic importance to the Shetland
whitefish fleet. The extent to which these gears are deemed viable will depend on a
range of factors such as target species and species mix, available fishing
opportunities and market prices etc. Given, however, that the rationale for these trials
was to reduce the by-catch of cod, further research should be directed towards the
development of gears that reduce catches of cod but retain species such as
monkfish, megrim, ling, saithe and hake and which, presumably, would be more
attractive economically.
CONCLUSIONS
The main aims of these trials were to assess the effect of increasing the mesh size in
the front sections of a trawl from 120mm to 300 and 600mm on the ground fish and
cod catches of the Shetland mixed whitefish fishery.
•
The 300 and 600mm trawls caught 49 and 74% fewer cod respectively than
the control trawl at all lengths
•
The 300 and 600mm trawls caught 80 and 93% fewer megrim respectively
than the control trawl at all lengths.
•
The catch rates for monkfish were length dependent. The 300mm trawl
caught ~50% fewer monkfish at 30cm with no significant difference > 76cm.
The 600mm trawl caught 85% fewer monkfish at 30cm with no significant
difference > 83cm.
•
The 300mm trawl caught significantly more haddock than the 600mm and
control trawls which had similar catch rates.
•
The 300 and 600mm trawls caught 36 and 68% fewer ling respectively than
the control trawl at all lengths
•
The 300 and 600mm trawls caught 28 and 53% fewer hake respectively than
the control trawl at all lengths
•
Catches of saithe greater than 53cm were also reduced, with the catch rate
decreasing with larger fish
ACKNOWLEDGEMENTS
The authors would like to thank the skipper and crew of the fishing vessel Resilient
for their technical help and expertise, the staff at L.H.D Lerwick who provided shore
side facilities, the Shetland Fishermen’s Association and Shetland P. O. for advice
and assistance throughout the project, and Iain Penny and Findlay Burns, Marine
Scotland – Science and Paul MacDonald, NAFC Marine Centre, Shetland.
APPENDIX
The catches from the test and control codends were analysed in two ways: using the
smoother based methodology of Fryer et al. (2003) and using a generalised linear
mixed model (Holst & Revill, 2009).
The smoother based analysis was in three stages:
1.
a smoother was used to model the log catch rate of the test gear relative to
the control gear for each haul;
2.
the fitted smoothers were combined over hauls to estimate the mean log
relative catch rate for each gear;
3.
bootstrap hypothesis tests using the statistic Tmax were used to assess
whether the mean log relative catch rates depended on gear, the side that the
test gear was fished (port or starboard) or fishing area, and to compare the
mean log relative catch rates to zero (or equivalently the mean relative catch
rates to unity).
The generalised linear mixed model was based on the usual SELECT model of
selection (Millar & Fryer, 1999). Let yhl be the number of fish of length l in haul h
measured in the test codend, and nhl be the number of fish of length l in haul h
measured in both codends. Then, conditional on nhl, yhl was assumed to have a
binomial distribution with probability rh(l) where logit rh(l) is a linear function of length
logit rh (l ) = α h + β h l
and is interpreted as the log catch rate of the test gear relative to the control gear for
haul h. There was no sub-sampling to consider. The parameters αh, βh were
assumed to vary between hauls as:
αh ~ N(α g , σ 2α )
β h ~ N(β g , σ β2 )
Thus, the selection parameters αh, βh are normally distributed about intercepts αg and
slopes βg which depend on the fixed effects (typically gear, side and area) with
standard deviations σα and σβ respectively. The haul selection parameters αh, βh
were assumed to be correlated. The significance of the fixed effects on selection
was assessed by Wald tests in a backwards stepwise procedure.
The smoother analysis makes fewer assumptions than the mixed model and in
particular assumes no specific shape for the relative catch rate curves. Because the
smoothers use local weighting, the fitted catch rates track the data, even at lengths
with few fish. The mixed model can be more powerful than the smoother analysis
and can incorporate haul-specific covariates such as catch size (although these were
not considered here). However, care is needed when interpreting the fitted catch
rates from a mixed model since their shape is often determined by the lengths where
there are many fish and might not be appropriate elsewhere. For example, in catch
comparison trials, the two gears might have similar catches once fish get above a
certain size. Such behaviour is usually well described by the smoother, but a linear
mixed model will force the relative catch rate to continue increasing (or decreasing)
at large lengths.
For the data presented here, the generalised linear mixed modelling approach seems
appropriate. The smoothers found little evidence of nonlinearity in the individual haul
or mean relative catch rate curves so linear logistic functions of length can be used.
Further, the mean relative catch rates (from both approaches) were often well below
unity or showed no length dependence, so inferences about catch rates at large
lengths were not an issue. Attempts were made to incorporate quadratic terms in
length into the mixed model to account for any nonlinearity, but these were not
pursued as there was only one instance (monkfish) where a quadratic term was
significant and then only marginally.
Both analyses were on the logistic scale, but the results have been back-transformed
for presentation.
RESULTS
The proportions of fish retained in the test codend (of those retained in both codends)
are shown below for each species and haul. The black and red lines are the fitted
values from the smoother analysis and the mixed model respectively. The grey
shaded areas are pointwise 95% confidence bands on the fitted smoothers. The
megrim data were sparse in fishing area 1, so the smoothers were only fitted to the
data for fishing area 2 and a mean relative catch rate could only be estimated for the
300mm gear. For most hauls, the fitted values from the mixed model are broadly
comparable with those from the smoother. The largest discrepancies occur in sparse
hauls, where the width of the smoother confidence bands gives due warning about
over-interpretation.
The estimated mean relative catch rates for each species and gear are shown below:
the black lines are the fitted values from the smoother analysis (with pointwise 95%
confidence bands indicated by the grey shaded areas) and the red lines are the fitted
values from the mixed model. The points are the proportion of fish retained in the
test codend (of those retained in both codends) pooled over hauls. There is strong
agreement between the results from the smoother analyses and the mixed models.
megrim
600mm
monkfish
saithe
1.0
0.5
0.0
300mm
1.0
0.5
0.0
20
40
60
50
cod
600mm
100
50
haddock
100
hake
ling
1.0
0.5
0.0
300mm
1.0
0.5
0.0
50
100
20
40
60
50
100
50
length (cm)
There was no strong evidence that the relative catch rate of any species depended
on the side that the test gear fished or the fishing area. A couple of terms were
marginally significant (at levels between 1 and 5%) but these were higher order
interactions and were treated as spurious given the number of tests made in the
model selection procedure.
Detailed results for each species are given below.
100
Cod: the smoother analysis found that the catch rates of both test gears differed from
that of the control gear (p < 0.001 in both cases). Both test gears caught significantly
fewer cod than the control gear at all lengths (pointwise 5% level). The relative catch
rates of the two gears differed (p < 0.001) with the relative catch rate of the 600mm
significantly below that of the 300mm gear at all lengths above 37 cm (pointwise 5%
level). The relative catch rates showed no suggestion of length dependence, which
was confirmed by the mixed model (p = 0.64). The mixed model also confirmed the
significant difference between the relative catch rates of the two gears (p < 0.001),
estimating the relative catch rate of the 300mm and 600mm gears to be 51% and
26% respectively (with 95% confidence intervals of 44-59% and 20-32%).
Haddock: the smoother analysis found that the catch rate of the 300mm gear
differed from that of the control gear (p < 0.001) but that the catch rate of the 600mm
gear did not differ from that of the control gear (p = 0.78). The 300mm gear caught
significantly more haddock than the control gear at all lengths (pointwise 5% level).
The relative catch rates of the two gears differed (p = 0.042) with the relative catch
rate of the 600mm gear below that of the 300mm gear for haddock between 40 and
53 cm (pointwise 5% level). The relative catch rates showed no suggestion of length
dependence, which was confirmed by the mixed model (p = 0.22). The mixed model
also confirmed the significant difference between the relative catch rates of the two
gears (p = 0.007), estimating the relative catch rate of the 300mm and 600mm gears
to be 142% and 105% respectively (with 95% confidence intervals of 130-157% and
87-128%).
Hake: the smoother analysis found that the catch rates of both test gears differed
from that of the control gear (p = 0.005, < 0.001 for the 300mm and 600mm gears
respectively). The 300mm and 600mm gears caught significantly fewer hake than
the control gear between 46 and 72 cm and below 80 cm respectively (pointwise 5%
level). The relative catch rates of the two gears differed (p = 0.012) with the relative
catch rate of the 600mm gear below that of the 300mm gear at all length below 70
cm (pointwise 5% level). The relative catch rates showed no suggestion of length
dependence, which was confirmed by the mixed model (p = 0.55). The mixed model
also confirmed the significant difference between the relative catch rates of the two
gears (p = 0.005), estimating the relative catch rate of the 300mm and 600mm gears
to be 72% and 47% respectively (with 95% confidence intervals of 62-84% and 3760%).
Ling: the smoother analysis found that the catch rates of both test gears differed
from that of the control gear (p < 0.001 in both cases). Both test gears caught
significantly fewer ling than the control gear at all lengths up to 100 cm (pointwise
95% level). The relative catch rates of the two gears differed (p < 0.001) with the
relative catch rate of the 600mm gear below that of the 300mm gear at all length
below 80 cm (pointwise 5% level). The relative catch rates showed no suggestion of
length dependence, which was confirmed by the mixed model (p = 0.62). The mixed
model also confirmed the significant difference between the relative catch rates of the
two gears (p < 0.001), estimating the relative catch rate of the 300mm and 600mm
gears to be 64% and 32% respectively (with 95% confidence intervals of 57-72% and
25-42%).
Megrim: the smoother analysis found that the catch rate of the 300mm gear differed
from that of the control gear (p < 0.001), with the 300mm gear catching fewer megrim
at all lengths (pointwise 5% level). The relative catch rate showed no suggestion of
length dependence, which was confirmed by the mixed model (p = 0.61). The mixed
model could be fitted to the data from both test gears. There was a significant
difference between the relative catch rates of the two gears (p < 0.001), with the
relative catch rate of the 300mm and 600mm gears estimated to be 20% and 7%
respectively (with 95% confidence intervals of 18-23% and 4-13%).
Monkfish: the smoother analysis found that the catch rates of both test gears
differed from that of the control gear (p < 0.001 in both cases). The 300mm and
600mm gears caught significantly fewer monkfish than the control gear below 72 and
80 cm respectively (pointwise 5% level). The relative catch rates of the two gears
differed (p < 0.001) with the relative catch rate of the 600mm significantly below that
of the 300mm gear at all lengths below 76 cm (pointwise 5% level). The mixed
model showed that the relative catch rates increased with length (p < 0.001) and
estimated that the 300mm and 600mm gears caught significantly fewer monkfish
than the control gear below 76 and 83 cm respectively (pointwise 5% level). The
mixed model also confirmed the significant difference between the relative catch
rates of the two gears (p < 0.001), with the relative catch rate of the 600mm gear
significantly below that of the 300mm gear at all lengths below 78 cm.
Saithe: the smoother analysis found that the catch rates of both test gears differed
from that of the control gear (p = 0.003, 0.008 for the 300mm and 600mm gears
respectively). Both gears caught significantly fewer saithe than the control gear at
lengths above 53 cm (pointwise 5% level). The relative catch rates of the two gears
did not differ significantly (p = 0.18). The mixed model showed that the relative catch
rates decreased with length (p < 0.001) and again estimated that the 300mm and
600mm gears caught significantly fewer saithe than the control gear above 53 cm
(pointwise 5% level). However, the mixed model also estimated that the 600mm
gear caught significantly more saithe than the control gear below 34 cm (pointwise
5% level). This is probably spurious as there were few saithe at these lengths, and
the fitted values were being driven by the linear fit to the rest of the data. The mixed
model found marginal evidence that the relative catch rates of the two gears differed
(p = 0.054), with the relative catch rate of the 600mm gear significantly below that of
the 300mm gear at all lengths above 61 cm.
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