9.3.2.1
European Commission (EC) request on evaluation of management plan for
NEA mackerel
Request
ICES was requested to develop multi-annual management plans for North East Atlantic (NEA) mackerel.
ICES is requested to identify multi-annual plans of the following form, and assuming that egg surveys of mackerel
continue on a triennial basis:
1.
2.
3.
The sum of the regulated catches for the combined stock of NEA mackerel (covering all areas where mackerel
are caught) shall be set according to a fishing mortality of [A].
Not withstanding paragraph 1 above, the sum of the regulated catches for the combined stock of mackerel
shall no be altered by more than [B]% with respect to the sum of the regulated catches for the combined stock
of the previous year.
Not withstanding paragraph 1 and 2, in the event that the spawning stock size for mackerel shall be estimated
at less than [C tonnes / appropriate model specified units], the sum of the regulated catches for the combined
stock of mackerel, and other conservation measures as appropriate, shall be adapted to assure rebuilding of
the spawning stock size to above [C] without incurring the restriction referred to in paragraph 2.
ICES is asked to identify combinations of values for A, B and C that would assure management of mackerel stock that
would conform to the precautionary approach, i.e. a low risk of stock depletion, stable catches and sustained high yield.
Values of A in the range 0.15 to 0.2 values of B in the range 5% to 20% and values of C above the present Bpa are of
particular interest to managers, but ICES should explore other relevant scenarios on its own initiative as appropriate.
ICES are also invited to suggest other approaches to multi-annual management of mackerel on its own initiative.
After consultation with the EC, ICES launched a dialogue process involving stakeholders and ICES scientists.
A subgroup of scientists from the ICES Working Group on the Assessment of Mackerel, Horse Mackerel, Sardine, and
Anchovy (which conducts the assessment of the NEA mackerel stock) was appointed to plan and carry out the work.
The ICES advice for 2008 included some preliminary results of the mackerel evaluation. Below, the results of the full
analyses are presented.
ICES response
ICES evaluated several types of harvest control rules for managing the Northeast Atlantic mackerel stock. Any of the
types of harvest control rules would be consistent with the precautionary approach if the appropriate parameters were
incorporated within the harvest control rule. As such, ICES does not recommend any specific harvest control rule from
the three types analyzed. Any harvest control rule that is consistent with the precautionary approach could be an option
for management and the choices of specific parameters might be based on preferences of stakeholders or managers.
Scenarios
ICES conducted simulations to explore the trade-offs among three different types of harvest control rules (HCRs):
•
•
•
F-rule, as requested by the EC, where the TAC is derived by projecting the stock forwards and applying a
fishing mortality (F) in accordance with the current Coastal States agreement.
Fixed TAC rule where the TAC is established as a fixed value, except when spawning stock biomass (SSB)
was estimated to be below the trigger point.
Fixed harvest-rate rule where the TAC is a fraction (harvest-rate, HR) of the estimated SSB.
When SSB was above the biomass trigger point in the simulations, fixed values were used in the TAC, harvest rate, and
fishing mortality HCRs. These values were linearly reduced when SSB was below the trigger point (Figure 1). For each
of the HCRs, ICES evaluated the effects of limiting the interannual change in TAC when SSB was above the trigger
biomass.
ICES Advice 2008, Book 9
1
Figure 1.
Schematic representation of Harvest Control Rules. Note: the interannual constraint on
changes in TAC cannot be depicted in the figure.
For the TAC and fixed-harvest HCRs, ICES also evaluated two additional effects:
•
•
setting TACs annually vs. once every three years;
applying the constraint on interannual TAC changes in all cases (‘always’) or just when the SSB was above the
trigger biomass (‘only’).
All the simulations were based on the recorded catches of NEA mackerel. However, there is strong evidence that
catches have been underreported in the past. Hence, an unknown uncertainty exists in the parameterization of the
population model used in the simulations due to the uncertainty in the unreported removals. As such, the simulation
results represent a minimum potential for yield and maximum risk provided that current enforcement is maintained or
improved. Conversely, if unreported removals increase, the probabilities of SSB being below 1.7 Mt are
underestimated.
Interpretation of precautionary approach and biological reference points
The EC request asks ICES to evaluate mackerel HCRs that would be consistent with the precautionary approach. ICES
interpretation of “consistent with (or conform to) the precautionary approach” is the de-facto accepted definition of a
low (5%) probability that the stock is below the limit biomass reference point (Blim) within the time period of the
simulation evaluations.
Blim for mackerel has not been previously defined. The NEA mackerel stock has not exhibited reduced recruitment at
SSBs down to the lowest observed SSB of 1.7 Mt. Recruitment below 1.7 Mt SSB is unknown. Based on this ICES
considers that an SSB of 1.7 Mt can be used as a potential Blim for NEA mackerel.
Thus ICES interprets “consistent with (or conform to) the precautionary approach” for NEA mackerel to mean less
than a 5% probability that the SSB is below 1.7 Mt during the simulation time periods.
Results of evaluations
The trade-off between catches and interannual TAC variability is illustrated in Figure 2. The red zone is a region where
strategies have probabilities greater than 5% of SSB being lower than 1.7 Mt. Fixed TAC strategies lead to low
interannual TAC variability with reduced maximum catches. Higher catches with higher TAC variability occur with the
F-rule and fixed harvest-rate strategies. The highest catches at any level of interannual TAC variability occur near the
boundary between the blue and green regions and in the red region.
2
ICES Advice 2008, Book 9
60
Strategies with risk >5% of SSB below 1.7Mt
TAC Inter-annual Variability (%)
50
F rule or HR strategies --- HR strategies
40
30
Fixed TAC strategies
20
10
0
400
450
500
550
600
650
700
Mean Catch
Figure 2.
Summary of performance of F-rule, HR-rule and TAC-rule with respect to mean catch
and average interannual variability in TAC for the NEA mackerel stock. Selected
scenario results depicted in this figure are presented in further detail in the subsequent
text tables.
F-rule
Selected results for the F-rule HCR are shown in Table 1 for various sets of HCR parameters (A – the target F; B – the
maximum limit on interannual TAC variation; and C – the SSB that triggers a change to the HCR) and associated
performance indicators. The scenarios included in the table were selected because they resulted in a risk of less than 5%
of SSB being below 1.7 Mt at any time during the simulation period. These strategies all lie near the boundary between
the green and red areas in Figure 2. The simulated strategy that gave the highest overall catch has also been included in
the table, but is shaded because the risk in this strategy of SSB being below 1.7 Mt was greater than 5%. More detailed
results are presented in Table 4.
All scenarios in Table 1 (except for the highest catch) generated expected average annual catches (Mean Catch) below
650 thousand tonnes, with expected interannual TAC variability (IAV) ranging between 10% and 33%. In these
scenarios, the expected average SSB (Mean Stock Size) was above the current Bpa (2.3 Mt) and the target F ranged
between 0.12 and 0.26.
The TAC in any year depends on the target F, the estimated SSB, and the TAC in the previous year. The realised F will
generally differ from the target F in the HCR. The right-most column in Table 1 (the percentage of time below the
trigger biomass) indicates the proportion of time during the simulation period when the target F would be expected to
have been reduced and the constraint on the maximum interannual change in TAC lifted.
Very restrictive constraints on interannual TAC changes occasionally generate sharp reductions in TACs when SSB
declines below the trigger biomass. This is followed by a slow rise in TACs as SSB increases.
Higher average catches are generally associated with higher interannual variability in TACs because management must
adapt quickly to changes. The upper fishing mortality limit specified in the current management plan (F=0.2) was
evaluated using an unconstrained TAC policy and generated a 4.3% probability of mackerel SSB being lower than
1.7 Mt during the simulations. Hence, this scenario would be considered precautionary (i.e., <5% probability).
ICES Advice 2008, Book 9
3
Table 1
Selected results of F-rule evaluations according to the criteria in the left column within the range of
parameters that was explored. Shaded scenarios have a larger than 5% probability of SSB being below 1.7
Mt (weight units in ‘000 tonnes).
lowest IAV
10%
constraint,
F < 0.23
Highest catch
IAV<12.5%
Highest catch
IAV<15%
Highest catch
IAV<17.5%
Highest catch
IAV<20%
Highest catch
IAV<22.5%
Highest catch
IAV<25%
Highest catch
risk<5%
Upper limit
current
management
plan
Highest catch
HCR Parameters
A
B
C
Target % limit Biomass
F
to
Trigger
change
in TAC
0.12
10
2000
0.22
10
2000
Mean
Catch
496
604
Summary of Performance1
Mean
Realised Mean
Risk
interannual
F
Stock
of
variability
(SSB) SSB<
in TAC
Size
1.7
(%)
Mt
9.7
0.11
4122
0.1
12.3
0.19
3241
3.8
prop.
of age
7+ in
catch
% time
below
Biomass
Trigger
0.55
0.44
0.1
3.5
0.24
10
2000
613
12.4
0.20
3214
3.8
0.43
3.7
0.24
10
2100
609
12.8
0.20
3189
3.8
0.43
5.5
0.22
15
2000
624
16.1
0.21
3078
4.2
0.42
3.7
0.22
20
2000
631
19.4
0.22
2989
3.7
0.40
4.0
0.22
20
2200
636
20.4
0.22
3021
3.3
0.40
8.2
0.22
20
2500
638
23.5
0.22
2949
3.5
0.40
24.1
0.26
20
2700
650
26.9
0.23
2906
4.7
0.38
39.7
0.20
-
-
649
32.4
0.23
2828
4.3
0.37
-
0.3
20
2000
665
22.7
0.28
2615
21.7
0.33
16.9
Fixed TAC rule
Expected average catches (Mean Catch) of up to about 600 thousand tonnes were achieved with low interannual
variability in TACs (<5%) for scenarios which had less than a 5% probability of SSB being below 1.7 Mt (Table 2,
more details in Table 5). Attempting to get higher average catches with a less than 5% probability of being below
1.7 Mt results in much higher interannual variability in TACs.
1
Mean Catch = expected average catch over the period of simulation.
Mean interannual variability in TAC = IAV.
Realised F = Mean fishing mortality that occurs (could be different from target F or perceived F).
Mean stock (SSB) = average SSB over the period of simulation.
Risk of SSB< 1.7 Mt = proportion of simulated populations with SSB<1.7 Mt at least once during the simulation.
prop. of age 7+ in catch = average proportion of catch by weight that consists of age 7 and older fish.
% time below trigger biomass = average proportion of time during the simulation period during which SSB was below
the trigger biomass.
4
ICES Advice 2008, Book 9
Table 2
Selected results of TAC-rule evaluations according to the criteria in the left column within the range of
parameters that was explored. Shaded scenarios have a larger than 5% probability of SSB being below 1.7
Mt (weight units in ‘000 tonnes).
HCR Parameters
Target % limit Biomass
TAC
to
Trigger
change
in TAC
lowest IAV
Highest catch
IAV<5%
Highest catch
IAV<7.5%
Highest catch
IAV<10%
Highest catch
IAV<12.5%
Highest catch
risk<5%
Highest catch
Mean
Catch
400
15
2300
421
Mean
interannual
variability
in TAC
(%)
1.0
550
25
2300
565
600
25
2300
650
25
700
Summary of Performance
Realised Mean
Risk
F
Stock
of
(SSB) SSB<
Size
1.7 Mt
prop.
of age
7+ in
catch
% time
below
Biomass
Trigger
0.09
4292
0.3
0.56
1.0
4.3
0.17
3347
3.4
0.45
9.9
603
6.5
0.21
3032
8.5
0.40
17.5
2300
635
8.8
0.25
2758
14.9
0.36
28.6
15
2300
657
10.9
0.29
2536
26.5
0.33
39.0
700
15
3200
628
14.4
0.21
3018
3.8
0.41
64.3
700
15
2300
657
10.9
0.29
2536
26.5
0.33
39.0
Fixed harvest rate rule
The fixed-harvest-rate HCR generated catches between 518 and 645 thousand tonnes and interannual TAC variability
between 3% and 35% for scenarios having a less than 5% probability of SSB being below 1.7 Mt are shown in Table 3.
Table 3
Selected results of HR-rule evaluations according to the criteria in the left column within the range of
parameters that was explored. Shaded scenarios have a larger than 5% probability of SSB being below 1.7
Mt (weight units in ‘000 tonnes).
HCR Parameters
Target % limit Biomass
HR
to
Trigger
change
in TAC
lowest IAV
Highest catch
IAV<15%
Highest catch
IAV<17.5%
Highest catch
IAV<20%
Highest catch
IAV<22.5%
Highest catch
IAV<25%
Highest catch
risk<5%
Highest catch
Mean
Catch
0.1
15
2300
450
Mean
interannual
TAC
variability
(%)
12.7
0.16
15
2300
571
0.18
15
2600
0.24
15
0.26
Summary of Performance
Realised Mean
Risk
F
Stock
of
(SSB) SSB<
Size
1.7 Mt
prop.
of age
7+ in
catch
% time
below
Biomass
Trigger
0.091
4443
0
0.59
0.1
14.5
0.15
3639
0.3
0.5
1.9
582
17.2
0.161
3503
0.5
0.48
9.1
2300
629
19.5
0.22
2982
7.5
0.40
15.6
15
2300
646
20.7
0.235
2869
10.1
0.39
20.5
0.3
15
2300
658
24.5
0.267
2664
23.4
0.35
31.5
0.24
25
2900
642
31.0
0.221
2959
4.2
0.4
49.9
0.3
25
2900
675
38.4
0.276
2605
18.8
0.34
71.9
ICES Advice 2008, Book 9
5
Additional considerations
The explorations of different HCRs suggest that the main factor influencing exploitation is the target F, although other
factors also influence the exploitation rate. Lower variability in catch is associated with lower catches, and can be
obtained by selecting lower trigger biomasses with lower F targets and having more restrictive constraints on
interannual TAC changes.
Any constraint on the maximum allowable change in annual TACs should be applied only with an HCR in which
maximum TAC change has been explicitly considered and risk evaluated. Some strategies which generate very stable
catches maximise catch by requiring a substantial reduction in TAC when SSB falls below the biomass trigger.
Managers might wish to consider whether this approach to setting TACs is desirable before adopting such an approach.
Very restrictive interannual TAC constraint strategies (5%) produce gradual increases in TAC over time followed by an
abrupt reduction when SSB falls below the biomass trigger point. Although this is associated with moderate risk, the
risk is only moderate if managers actually implement the required severe TAC cuts.
Under the conditions explored in the simulations, an annual TAC regime generates higher yields and lower risks than a
3-year TAC regime because actions can be taken more quickly.
Source of information
Report on NEA mackerel long-term management scientific evaluation (ICES CM 2008/ACOM:54).
6
ICES Advice 2008, Book 9
Table 4
Target
0.16
Results of F-rule evaluations for different combinations of HCR parameters within the range that was
explored. Shaded scenarios have a larger than 5% probability of SSB being below 1.7 Mt (weight units in
‘000 tonnes).
%limit to
change
in TAC
10
Trigger
biomass
15
20
0.2
10
15
20
0.22
10
15
20
0.24
10
15
20
0.26
10
15
20
ICES Advice 2008, Book 9
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
Data
Mean catch
550
543
529
527
573
567
557
546
585
572
567
559
582
580
574
567
607
596
589
588
615
609
600
596
593
586
579
587
618
614
600
601
631
618
612
610
599
604
597
599
631
618
618
616
635
630
627
623
616
611
608
612
639
625
629.0
624.0
651.0
638.0
636.0
637.0
mean IAV in
TAC
Fishing
mortality
Mean stock
(SSB)
Risk of
SSB<1.7 Mt
% of catch
age 7+
11.3
12.5
14.0
16.4
15.4
16.6
18.4
20.6
18.4
19.7
21.4
23.6
12.8
14.2
16.4
19.6
16.6
18.7
20.9
24.0
20.0
21.9
24.9
27.5
13.5
15.6
18.3
22.1
17.7
19.9
23.1
25.5
20.9
23.2
26.2
29.1
14.3
16.2
20.1
25.1
18.4
20.4
24.5
28.6
22.1
24.4
28.0
32.1
15.1
17.5
22.0
26.9
19.1
22.3
25.7
31.2
22.9
26.6
29.6
33.3
0.14
0.14
0.13
0.13
0.15
0.15
0.14
0.14
0.16
0.16
0.15
0.15
0.17
0.17
0.16
0.16
0.19
0.18
0.17
0.17
0.19
0.19
0.18
0.18
0.18
0.18
0.17
0.17
0.20
0.19
0.19
0.18
0.21
0.20
0.19
0.19
0.19
0.19
0.18
0.19
0.21
0.20
0.20
0.20
0.22
0.22
0.21
0.20
0.21
0.20
0.20
0.20
0.23
0.22
0.2
0.2
0.2
0.2
0.2
0.2
3743
3812
3849
3851
3585
3649
3656
3708
3479
3522
3564
3599
3464
3517
3528
3548
3289
3298
3374
3395
3166
3245
3286
3321
3318
3352
3374
3387
3151
3225
3232
3284
3044
3109
3147
3187
3249
3272
3291
3246
3046
3120
3135
3126
2939
2970
3041
3082
3125
3156
3142
3124
2962
2978
3030.0
3026.0
2839.0
2885.0
2927.0
2961.0
0.0
0.1
0.0
0.1
0.7
0.3
0.3
0.0
0.3
0.0
0.3
0.1
0.9
0.9
0.6
0.0
1.7
1.2
0.5
0.5
2.1
1.4
0.3
1.0
2.1
1.0
1.0
0.8
2.3
2.0
1.9
1.7
2.9
2.0
1.5
1.0
2.6
2.5
1.7
1.6
4.0
3.2
2.5
3.1
5.7
4.3
3.5
2.6
4.6
3.5
4.2
2.2
8.7
5.2
4.7
3.3
8.4
6.4
5.0
3.8
0.51
0.51
0.52
0.52
0.49
0.49
0.50
0.50
0.48
0.48
0.49
0.49
0.47
0.47
0.47
0.48
0.44
0.45
0.46
0.46
0.43
0.44
0.45
0.45
0.45
0.46
0.46
0.46
0.43
0.44
0.44
0.44
0.41
0.42
0.43
0.43
0.44
0.44
0.44
0.44
0.41
0.42
0.42
0.42
0.40
0.40
0.41
0.42
0.42
0.42
0.42
0.42
0.40
0.40
0.4
0.4
0.4
0.4
0.4
0.4
%time below
trigger
biomass
1.6
4.8
10.3
20.9
2.1
5.3
14.1
24.5
2.2
6.8
15.0
28.8
5.3
11.0
21.0
36.3
6.1
15.4
26.0
42.9
6.4
15.9
29.5
45.4
7.5
15.9
28.9
44.7
9.7
18.8
34.5
50.0
11.3
21.6
37.7
54.7
9.8
18.5
33.1
52.4
11.8
24.1
40.4
58.5
15.6
29.4
43.5
62.0
13.1
26.1
41.6
59.5
17.4
31.9
46.3
65.3
19.7
35.3
52.9
68.2
7
Table 5
Target
0.16
Results of HR-rule evaluations for different combinations of HCR parameters within the range that was
explored. Shaded scenarios have a larger than 5% probability of SSB being below 1.7 Mt (weight units in
‘000 tonnes).
%limit to
change
in TAC
15
25
0.2
15
25
0.22
15
25
0.24
15
25
0.26
15
25
8
Trigger
biomass
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
Data
Mean catch
571
556
545
541
582
577
564
560
610
601
592
594
614
611
608
596
627
613
610
608
637
627
626
614
629
625
621
618
654
647
642
630
646
637
632
633
662
655
656
644
mean IAV in
TAC
Fishing
mortality
Mean stock
(SSB)
Risk of
SSB<1.7 Mt
% of catch
age 7+
14.5
16.0
17.8
19.7
19.8
21.2
23.0
24.9
16.2
18.5
21.1
24.2
22.0
24.4
26.9
30.2
17.8
20.3
23.5
27.0
23.4
26.4
28.3
32.4
19.5
21.7
25.9
30.1
25.4
27.4
31.0
35.2
20.7
24.0
27.8
33.5
26.6
29.7
33.2
37.0
0.15
0.14
0.14
0.13
0.16
0.15
0.15
0.14
0.19
0.18
0.17
0.17
0.20
0.19
0.18
0.18
0.20
0.19
0.19
0.18
0.22
0.21
0.20
0.19
0.22
0.21
0.20
0.20
0.24
0.22
0.22
0.21
0.23
0.23
0.22
0.22
0.25
0.24
0.24
0.22
3639
3695
3728
3796
3526
3565
3594
3639
3266
3344
3380
3427
3107
3179
3232
3287
3112
3176
3212
3279
2964
3003
3092
3134
2982
3044
3090
3115
2844
2935
2959
2987
2869
2919
2943
2968
2716
2777
2841
2884
0.3
0.4
0.1
0.0
0.6
0.1
0.1
0.3
1.4
1.2
0.7
0.5
2.5
0.8
0.9
0.8
3.9
2.4
1.4
1.1
4.1
3.3
2.1
1.6
7.5
4.4
3.3
3.6
8.2
6.6
4.2
2.9
10.1
9.2
5.9
4.8
13.9
10.5
8.5
6.7
0.50
0.51
0.51
0.52
0.48
0.49
0.49
0.50
0.45
0.46
0.46
0.46
0.43
0.44
0.44
0.45
0.43
0.44
0.44
0.45
0.40
0.41
0.42
0.43
0.40
0.41
0.42
0.42
0.38
0.39
0.40
0.40
0.39
0.39
0.39
0.40
0.36
0.37
0.38
0.39
%time below
trigger
biomass
1.9
5.4
12.7
20.9
2.1
5.9
13.9
26.8
6.2
13.2
26.3
38.9
8.4
17.7
30.6
46.3
10.1
19.9
35.3
49.8
12.0
26.3
40.4
57.4
15.6
27.6
42.7
59.6
18.8
30.3
49.9
67.0
20.5
33.5
51.7
68.3
25.1
40.6
56.7
73.1
ICES Advice 2008, Book 9
Table 6
Target
400
Results of TAC-rule evaluations for different combinations of HCR parameters within the range that was
explored. Shaded scenarios have a larger than 5% probability of SSB being below 1.7 Mt (weight units in
‘000 tonnes).
%limit to
change
in TAC
15
Trigger
biomass
25
450
15
25
500
15
25
550
15
25
600
15
25
650
15
25
700
15
25
ICES Advice 2008, Book 9
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
2300
2600
2900
3200
Data
Mean catch
421
419
415
410
421
419
416
413
471
468
463
456
472
468
464
457
520
514
508
497
521
515
508
500
563
556
547
536
565
558
549
541
600
593
583
573
603
594
585
574
632
624
613
600
635
625
616
602
657
652
637
628
657
650
641
628
mean IAV in
TAC
Fishing
mortality
Mean stock
(SSB)
Risk of
SSB<1.7 Mt
% of catch
age 7+
1.0
1.7
2.7
4.3
1.0
1.9
2.8
4.2
1.7
2.7
3.9
5.3
1.8
2.8
4.0
5.8
2.5
3.7
5.1
7.1
2.7
4.0
5.7
7.2
4.0
5.6
7.0
8.9
4.3
5.9
7.4
9.0
6.2
7.4
8.9
10.7
6.5
7.9
9.7
11.5
8.6
9.6
11.1
13.1
8.8
10.4
12.0
13.6
10.9
11.8
13.7
14.4
11.9
13.1
14.3
16.0
0.09
0.09
0.09
0.09
0.09
0.09
0.09
0.09
0.11
0.11
0.11
0.10
0.12
0.11
0.11
0.11
0.14
0.14
0.13
0.12
0.14
0.14
0.13
0.12
0.17
0.16
0.15
0.15
0.17
0.16
0.16
0.15
0.21
0.19
0.18
0.17
0.21
0.20
0.18
0.17
0.25
0.22
0.20
0.19
0.25
0.23
0.21
0.19
0.28
0.26
0.23
0.21
0.29
0.26
0.24
0.22
4292
4353
4402
4409
4312
4307
4375
4443
3972
4046
4090
4160
3943
4014
4085
4162
3676
3735
3828
3889
3636
3717
3791
3889
3345
3425
3557
3620
3347
3439
3509
3660
3054
3200
3315
3423
3032
3183
3287
3384
2763
2953
3095
3175
2758
2896
3031
3163
2536
2723
2871
3018
2496
2664
2823
2982
0.3
0.2
0.0
0.1
0.2
0.2
0.2
0.0
1.0
0.4
0.0
0.1
1.0
0.7
0.2
0.1
1.9
0.9
0.5
0.5
2.4
1.4
0.3
0.2
5.0
1.9
1.5
0.5
3.4
2.4
0.9
0.5
8.4
5.6
2.3
1.3
8.5
6.3
2.8
0.9
16.3
9.3
4.1
3.0
14.9
10.0
4.4
2.9
26.5
15.4
9.8
3.8
28.4
17.4
9.7
3.7
0.56
0.56
0.57
0.58
0.56
0.56
0.57
0.57
0.53
0.53
0.54
0.55
0.52
0.53
0.54
0.55
0.49
0.50
0.51
0.52
0.49
0.50
0.51
0.52
0.45
0.46
0.48
0.49
0.45
0.46
0.47
0.49
0.41
0.43
0.45
0.46
0.40
0.42
0.44
0.46
0.37
0.39
0.41
0.43
0.36
0.39
0.41
0.43
0.33
0.36
0.38
0.41
0.32
0.35
0.38
0.40
%time below
trigger
biomass
1.0
2.2
5.4
10.6
0.8
2.3
5.4
9.6
2.6
4.9
8.4
14.9
2.6
4.9
8.6
16.0
4.8
8.1
14.1
24.0
5.4
8.6
16.2
22.1
10.2
15.3
23.0
32.7
9.9
15.8
23.6
31.3
18.1
23.9
32.0
42.6
17.5
25.6
32.8
45.2
28.1
34.3
43.4
55.4
28.6
37.0
45.7
55.6
39.0
45.6
56.6
64.3
41.7
49.7
58.5
67.5
9
Technical annex to the ICES response
General
Background to the mackerel fishery and management
Management of Northeast Atlantic mackerel is under an agreement between Coastal States to set fishing mortalities in
the range 0.15 to 0.2 aimed at maintaining the SSB above 2.3 Mt (source: agreed record of negotiations between
Norway, Faroe Islands, and EU in 1999).
Approach to the answering the request
In agreement with the EC, ICES launched a dialogue process according to the guidelines set by the ICES Study Group
on Management Strategies. A subgroup of ICES scientists from the Working Group on Pelagic and Widely Dispersed
Stocks (WGWIDE) were appointed to plan and carry out the work. This group has had two meetings in 2007 and one
meeting in 2008. Managers and stakeholders were invited for the 2007 meetings and outlined possible alternative
harvest control rules. ICES was unable to provide a full response to this request in 2007 because the work was not
finalized.
A Technical Report (ICES CM 2008/ACOM:54) circulated to stake-holders for comments and to reviewers was
prepared by a subgroup of scientists. The Technical Report consists of the following:
•
•
•
•
•
General.
Methods: Model Conditioning, Simulation tools, Harvest Control Rules, Model validation, Management
scenarios, Performance Statistics, and Reference points.
Results: Model validation, Interpretation of acceptable risk, F-rule; Harvest rate and Target TAC rules.
Discussion of results.
Conclusions.
Various annexes, including a more detailed description of the work undertaken and additional sensitivity tests, were
attached to the main body of the Technical Report (ICES CM 2008/ACOM: 54).
Reference points and criteria for precautionary approach
Reference points from ICES advice 2007 (ICES 2007b):
Type
Blim
Bpa
Flim
Value
no biological basis for defining Blim
2.3 million t
Precautionary
0.26, the fishing mortality estimated
approach
to lead to potential stock collapse
Fpa
0.17
Unchanged since 1998, target reference points added in 1999
Technical basis
Bloss in Western stock raised by 15%: = 2.3 million t
Floss = 0.26
Flim * 0.65
During the evaluation, the study group examined the basis of existing reference points.
Biomass reference points
For NEA mackerel, Blim has not previously been defined. Following the recent benchmark assessment, small revisions
occurred to the estimates of spawning stock biomass. More importantly, the time-series of data available to define Blim
and Bpa was extended beyond that available in 1998. There is now a need to re-evaluate biomass reference points for
this stock. Until this is completed however, the lowest observed SSB of 1.7 Mt is considered a potential candidate for
Blim. There has been no indication of reduced recruitment down to this biomass level, but fitted recruitment models
indicate increasing probability of reduced recruitment below this SSB.
Fishing Mortality reference points
The fishing mortality limit reference point of Flim = 0.26 is based on a previous estimate of Floss. There are now more
years of data available to ICES encompassing a wider range of fishing mortalities. Using this more complete data set,
the expert subgroup re-estimated Floss to be 0.42. This estimate was based on a segmented regression fit to stock and
recruitment data and on SSB per recruit analyses.
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ICES Advice 2008, Book 9
An updated estimate of Fpa may be derived from Floss by taking into account the precision of the F estimate. Evaluations
conducted by WGMHSA (ICES, 2005) indicated that a factor of 0.55 should be applied to Floss, providing a resultant
potential estimate of Fpa = 0.23. Flim and Fpa for this stock should be fully evaluated by WGWIDE in September
2008.
Basis for precautionary advice
The EC request asks ICES to evaluate mackerel HCRs that would be consistent with the precautionary approach. ICES
interpretation of “consistent with (or conform to) the precautionary approach” is the de-facto accepted definition of a
low (>5%) probability that the stock is below the limit biomass reference point (Blim) within the time period of the
simulation evaluations.
Blim for mackerel has not been previously defined. The NEA mackerel stock has not exhibited reduced recruitment at
SSBs down to the lowest observed SSB of 1.7 Mt. Recruitment below 1.7 Mt SSB is unknown. Based on this ICES
considers that an SSB of 1.7 Mt can be used as a potential Blim for NEA mackerel.
Thus ICES interprets “consistent with (or conform to) the precautionary approach” for NEA mackerel to mean less
than a 5% probability that the SSB is below 1.7 Mt during the simulation time periods.
Data used
A full description of the data is presented in Section 2.1 of the Technical Report (ICES CM 2008/ACOM:54). The
quantitative evaluation of the proposed HCRs are all based on the most recent assessment dataset for NEA Atlantic
mackerel (ICES, 2007a).
Recruitment. A probabilistic hybrid model was used to generate recruitment as a function of SSB based on the available
time-series of data. The uncertainty in estimates of SSB in the early years was explicitly considered in the analysis. The
errors only affected estimates of SSB and these were shown to be small. It was preferable to include these values in the
analysis because they contain the only data on recruitment when SSB is above 3 Mt. A Bayesian analysis was used to
evaluate the combined uncertainty in parameter estimates and probability of the different functional S/R models and
stochastic distributions (Figure TA1).
Figure TA1 NEA Mackerel observed (red) and simulated recruitment against SSB. Superimposed on the graphs are
the 5, 50, and 95 percentiles of the simulated recruitments.
ICES Advice 2008, Book 9
11
Methods
Evaluation of the proposed HCR was performed by simulation. Simulations were carried out using three different tools:
FLR (http://flr-project.org/doku.php?id=doc:biblio:evaluation): FLR was used to characterize the errors in the data
collection and assessment process. It provided estimates of overall precision and correlation between years. FLR
was also used to evaluate the differences between the F rule using a short-term forecast and the HR rule which sets a
TAC based on a proportion of the SSB in the assessment year.
F-PRESS (Codling & Kelly, 2006): F-press was used to provide validation of HCM through comparison of
simulation output and to provide more detailed evaluation of fixed TAC options.
HCM (Harvest Control rule evaluation for Mackerel): The HCM models were the basis of the quantitave results that
are presented in this report.
A description of each tool can be found in Section 2.2 of the Technical Report (ICES CM 2008/ACOM:54). The
simulation period was always 21 years (i.e. up to and including 2027). 1000 iterations were run and statistics
calculated for the simulation period 2017–2027. This period was selected to reduce the influence of the initial stock
condition on the results of the HCRs.
Results for the unconstrained fixed TAC rule from the HCM and F-PRESS frameworks were compared, showing that
the differences between frameworks were negligible.
Type of harvest control rules
F-rule proposed by the EC
This rule sets the TAC according to three parameters: A – the target fishing mortality, C – the level of SSB that triggers
a response in the rule (Btrig), and B – which constrains interannual TAC variation. Once those parameters are set F is
derived as follows:
If SSB > Btrig(parameter B) then F= target F (parameter A), but TAC in year y shall at most deviate by C%
from the TAC in year y−1.
If SSB < Btrig, the F is set at F=Ftarg*SSB/Btrig, and the constraint on TAC change does not apply.
Points of interpretation:
1.
2.
The action below the trigger biomass is a simplification of the request, which required rebuilding to above the
trigger biomass within an unspecified time.
The SSB that is used for decision was the SSB projected through the intermediate year and into the TAC
year.
Fixed TAC rule
This is a rule where the target TAC is set as a function of the SSB in the year y−1 before the TAC year y. The rule has 3
parameters: target TAC (Ctarget), trigger SSB (Btrig) and constraint in TAC interannual variation (TACconstraint). The SSB
that is used in the TAC-rule, is the estimated SSB in the year before the TAC year. The TAC rule has the following
form:
If SSB > Btrig, TAC = Ctarget
If SSB < Btrig, TAC = Ctarget*SSB/Btrig
If
abs{(TAC(y−1)−TAC(y))/TAC(y−1)} > TACconstraint
and (optionally) SSB > Btrig
TAC(y) = TAC(y−1)*(1+TACconstraint)
TAC(y) = TAC(y−1)*(1−TACconstraint)
if TAC(y)>TAC(y−1)
if TAC(y)<TAC(y−1)
The rule was applied either each year or every three years. In the latter case, the same TAC was applied unchanged for
the whole three-year period (HCM) but the approach with F-PRESS was to allow a maximum of 15% change in each
year. The rule was tested with and without the option of applying the TAC constraint only at SSB >Btrig.
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ICES Advice 2008, Book 9
Fixed Harvest Rate (HR) rule
This is another rule where the TAC is set as a function of the SSB and the TAC in the year y-1 before the TAC year y.
Basically, the TAC is set as a fraction (the HR) of the observed SSB. The rule has 3 parameters, HRtarget, Btrig, and
TACconstraint. The SSB that is used in the HR rule, is the estimated SSB in the year before the TAC year. The HR rule
has the following form:
If SSB > Btrig, TAC = HRtarget*SSB
If SSB < Btrig, TAC = HRtarget*SSB*SSB/Btrig
If
abs{(TAC(y−1)−TAC(y))/TAC(y−1)} > TACconstraint
and (optionally) SSB > Btrig
then
TAC(y)= TAC(y−1)*(1+TACconstraint)
if TAC(y)>TAC(y−1)
TAC(y)= TAC(y−1)*(1−Cconstraint)
if TAC(y)<TAC(y−1)
Results
The results for the three strategies (Target TAC, Target F, and Target HR) can be compared in terms of the main
outcomes in Figures TA2 and TA3.
The relationship between catch and risk is shown in Figure TA2. Higher catches are available at lower risks from Target
HR and F strategies.
300
TargC
400
500
600
TargF
TargHR
Prob < 1.7 Mt (%)
30
20
10
0
300
400
500
600
300
400
500
600
catch ('000 t.)
Figure TA2 NEA Mackerel simulation results for scenarios of three different types of rules (left: TAC rule, middle: F
rule, and right: HR rule) and where constraints in TAC variation only applied above the trigger biomass.
Dotted lines indicate the 5% probability level and the 600 000 t catch level.
The influence of maintaining (“always”) or removing (“only”) the constraints on change in TAC is shown in Figure
TA3 The risks increase if the constraint on change is maintained when the SSB drops below the trigger level. However,
strategies that allow this are available with risks less than 5%.
ICES Advice 2008, Book 9
13
300
only
TargC
400
500
600
700
only
TargF
only
TargHR
40
30
20
Prob < 1.7 Mt (%)
10
0
always
TargC
always
TargF
always
TargHR
40
30
20
10
0
300
400
500
600
700
300
400
500
600
700
catch ('000 t.)
Figure 3
14
NEA Mackerel simulation results for scenarios of three different types of rules (left: TAC rule, middle: F
rule, and right: HR rule) and where constraints in TAC variation “only” applied above the trigger biomass
(top) or when constraints in TAC variation “always” applied (bottom). Dotted lines indicate the 5%
probability level and the 600 000 t catch level.
ICES Advice 2008, Book 9
F-rule
The simulations with the F-rule, as proposed by the EC, considered a range of values for the parameters for Target F
(A), Trigger SSB (C), and Percentage constraint on TAC variation (B). The constraint on TAC variation was only
applied when it led to an SSB above the trigger biomass. The results are presented as means over the years 10–20 and
over 1000 iterations for each combination of the parameters (see also: Table 3.9 of ICES CM 2008/ACOM:54). The
main trends in these results can be summarized as follows:
•
•
•
•
•
The risk to SSB <1.7 Mt increases with increasing Target F, and is reduced with increasing Trigger SSB.
A stronger constraint on the TAC variation reduces the risk (but this may depend on the initial TAC).
The realized catch increases with increasing Target F.
A stronger constraint on the TAC variation decreases the catch.
The interannual TAC variation increases with increasing Trigger SSB and with increasing Target F.
The influence of constraining the change in TAC by a maximum percentage is shown in Figure TA4. The risks increase
with increased constraint. However, particularly for the 5% constraint these results may depend on the starting point for
the simulations.
400
5
450
500
550
600
650
400
10
15
450
500
550
600
650
20
Prob < 1.7 Mt (%)
20
15
10
5
0
400
450
500
550
600
650
400
450
500
550
600
650
catch ('000 t.)
Figure TA4 NEA Mackerel simulation results for F-rule scenarios where constraints in TAC variation are only
applied above the trigger biomass. Catch vs. Probability of the stock below 1.7 Mt. The constraints in
maximum TAC variation in the scenarios were 5, 10, 15, and 20%. Dotted lines indicate the 5%
probability level and the 600 000 t catch level.
The variability in TAC and the mean catch are shown in Figure TA5. Constraining the TAC reduces variability but also
reduces overall catch.
ICES Advice 2008, Book 9
15
400
450
5
500
550
600
650
400
10
450
500
15
550
600
650
20
40
IAV
30
20
10
400
450
500
550
600
650
400
450
500
550
600
650
catch ('000 t.)
Figure TA5 NEA Mackerel simulation results for F-rule scenarios and where constraints in TAC variation only
applied above the trigger biomass. Catch vs. Inter Annual Variation in TAC. The constraints in maximum
TAC variation in the scenarios where 5, 10, 15, and 20%. Dotted lines indicate the 10% variation level
and the 600 000 t catch level.
The proportion of age 7 and older fish in the population is given in Figure TA6. The main influence on the proportion of
older fish is increased if mean catch is reduced.
400
5
450
500
550
600
650
400
10
15
450
500
550
600
650
20
0.60
X7?? (%)
0.55
0.50
0.45
0.40
0.35
400
450
500
550
600
650
400
450
500
550
600
650
catch ('000 t.)
Figure 6
NEA Mackerel simulation results for F-rule scenarios where constraints in TAC variation are only
applied above the trigger biomass. Catch vs. X7 (proportion of fish at age 7 and older in the catch). The
constraints in maximum TAC variation in the scenarios where 5, 10, 15, and 20%. Dotted lines indicate
the 50% level and the 600 000 t catch level.
To obtain a high mean catch, a high target F should be used with a high trigger biomass and a weak constraint on TAC
variation. This will lead to a high risk and a high interannual variation of the TACs. The maximum stability is achieved
with a low trigger SSB, a low Target F, and a strong constraint on TAC variation. This will lead to a low risk, but the
catches will also be lower.
To show the trade-off between TAC stability and average catch, a subset of HCRs with high catch and low TAC
variability was selected. All the selected options have a less than 5% probability of SSB <1.7 Mt. This procedure selects
options with high catches that conform to the precautionary approach. Options with lower risks are associated with
lower catches. With this level of risk, catches are in the range of 580–640 thousand tonnes and the IAV between 10%
and 30%. The target F is in the range of 0.24–0.30. Some examples of parameter choices are given in the text table
shown in the previous section (ICES response). One outstanding result is that to have a catch near the maximum, the
16
ICES Advice 2008, Book 9
IAV has to be quite high, well above 15%. More stability requires a substantial reduction in catch, which will in general
also imply a lower risk.
Simulations conducted using full assessment feedback showed that setting the TAC based on the traditional approach,
i.e. a short-term forecast in compliance with the Coastal States agreement, results in higher risk and lower yield. This
implies that an HCR for NEA mackerel is more optimally conditioned with a direct estimate of the stock, even if this is
in the past, than the use of a short-term forecast to “project” the stock to the year in which the decision is implemented.
A complete list of the HCR parameters explored and associated performance statistics is shown in Table 3.9 of the
Report on NEA mackerel long-term management (ICES CM 2008/ACOM:54).
ICES Advice 2008, Book 9
17
Fixed harvest rate rule (HR-rule)
HR-rule scenarios that were associated with a less than 5% risk of SSB <1.7 Mt and with the constraint only applied
above the trigger biomass, led to catches up to 645 thousand tonnes, IAV in the range of 3–35%, and target harvest rates
up to 26%.
The evaluations presented in the report (Table 3.1 from the report) indicate that for the same yields lower risks were
achievable for HR strategies that did not use a short-term forecast with an evaluation of F but used instead a harvest
rate that set a TAC based only on a fraction of the evaluated SSB.
With the HR-rule the average long-term catch, the risk, and the variability will all increase with increasing target
harvest-rate. The impact of the trigger biomass is small on risk and catch, but the variability increases with increasing
trigger biomass, in particular with a weak constraint on TAC change. A stronger constraint on TAC variation (10%)
leads to lower catches.
Maintaining the constraint to changes in TAC always (irrespective of whether SSB is above or below trigger biomass)
gives substantially lower overall interannual variability in TAC and delivers only slightly lower maximum yield for the
same risks. The difference between achievable maximum catch and interannual variability for annual and triennial
regimes is small, except that by maintaining the triennial regime the only lower interannual variability options are
available. These effects are illustrated in Figure TA7. With a strong constraint <10%, both catches and risk are higher
with the ‘Always’ constraint, and the catch is higher with a one-year rule than with a 3-year rule, in particular if the
constraint only applies above the trigger.
300
3
always
400
500
600
3
only
30
20
10
IAV
1
always
1
only
30
20
10
300
400
500
600
catch ('000 t.)
Figure TA7 Trade-off between catch and stability in a HR-rule evaluation of NEA Mackerel.The points represent the
outcome of scenarios for various combinations of trigger biomass and target harvest-rate. Only results
that imply a probability below 5% of SSB <1.7 Mt are shown. Constraints in TAC variation only applied
above the trigger biomass (right) or independent of stock size (right) and with one-year decision cycle
(below) or a three-year cycle. Dotted lines indicate the 10% variability in TAC and the 600 000 t catch
level.
18
ICES Advice 2008, Book 9
Fixed TAC rule
At the risk level of less than 5% for SSB <1.7 Mt, catches up to about 600 000 tonnes can be achieved with low
interannual variability (Figure TA8). Attempting to get higher average catches with an acceptable risk results in much
higher interannual variability.
There are a range of options that lead to a less than 5% risk of SSB <1.7 Mt For the harvest rules tested, results
demonstrate the strong influence of the Btrig parameter. Assuming a maximum acceptable risk of <5% the maximum
yields available for a trigger point ~ 2.6 Mt are of the order of 560 kt with an associated F of up to 0.17. If the stock was
to be exploited with a trigger biomass ~ 3.2 Mt, then target TACs of up to 700 kt are feasible although average yields
are unlikely to exceed 625 kt. Further, IAV increases with increasing SSB trig. This is consistent with the HCR being
implemented more frequently as the trigger point is increased to stock levels well above the current level.
To provide a stable fixed TAC regime with estimated less than 10% chance of requiring a change to the TAC requires a
target TAC of around 550 kt with a trigger biomass of 2300.
The effects of the HCR period and limiting annual TAC changes to +/−15% are less pronounced than variation in the
target TAC or SSB trigger parameter.
Common to all methods is that more variable regimes give higher catches. The average catch in the long term increases
with increasing target TAC, and decreases with increasing trigger biomass. The risk of SSB <1.7 Mt increases with
increasing target TAC and decreases with increasing trigger biomass. The variability, expressed as IAV, increases with
increasing target TAC and with increasing trigger biomass. The level of constraint on TAC variation matters little for
the average catch and for the risk, but the IAV increases with a weaker constraint.
The difference between the option to constrain the catches at all levels of SSB or only at SSB above the trigger biomass
is small except when the constraint is very strong (5%). Likewise, the difference between annual and triennial advice is
small except with a very strong constraint, although the risk is generally somewhat higher with a triennial regime.
300
3
always
400
500
600
3
only
30
20
10
IAV
1
always
1
only
30
20
10
300
400
500
600
catch ('000 t.)
Figure TA8 Trade-off between catch and stability in a TAC-rule evaluation of NEA Mackerel. The points represent
the outcome of scenarios for various combinations of trigger biomass and target harvest-rate. Only results
that imply a probability below 5% of SSB <1.7 Mt are shown. Constraints in TAC variation only applied
above the trigger biomass (right) or independent of stock size (right) and with one-year decision cycle
(below) or a three-year cycle. Dotted lines indicate the 10% variability in TAC and the 600 000 t catch
level.
ICES Advice 2008, Book 9
19
Section 3.5 and Annexes 3 and 4 in the Report provide a wide range of options for the possible implementation of a
target TAC strategy for the exploitation of the fishery. An extensive presentation of all simulation results can be found
in Section 3 of the Technical Report (ICES CM 2008/ACOM:54).
Further work required
The Ad-hoc expert group recommends that WGWIDE should re-evaluate the precautionary reference points for NEA
mackerel.
Source of information
Report on NEA mackerel long-term management scientific evaluation (ICES CM 2008/ACOM:54).
References
Codling, E. A., and Kelly, C. J. 2006. F-PRESS: A Stochastic Simulation Tool for Developing Fisheries Management
Advice and Evaluating Management Strategies. Irish Fisheries Investigations No. 17.
(http://www.marine.ie/NR/rdonlyres/8442D077-7E7B-4679-AF0FCAD7A1B9B2C9/0/FPRESSCodlingKelly2006MIFisheriesInvestigationSeriesNo171.pdf )
ICES. 2005. Working Group on Mackerel, Horse Mackerel, Sardine, and Anchovy. CM 2005/ACFM:36.
ICES. 2007a. Working Group on Mackerel, Horse Mackerel, Sardine, and Anchovy. ICES CM 2007/ACFM:31.
ICES. 2007b. Report of the ICES Advisory Committee on Fishery Management, Advisory Committee on the Marine
Environment, and Advisory Committee on Ecosystems, 2007. ICES Advice. Book 9. 129 pp.
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ICES Advice 2008, Book 9