- Weight-at-age in stork is set equal to weight-at-age in catch.
- Time-series (1986-2005) average used for maturation-at-age without density-dependence.
- No uncertainty in weight-al-age, maturity-at-age, or natural mortality-al-age.
- Exploitation patlern: 1997-2005 averages used for all age groups in all years.
- Implementation of catch: First, the catch-at-age is calculated from the perceived stock using the fishing mortality derived from the harvest control rule and the given exploitation pattern. This catch-at-age is then applied to the actual slock.
- The implementation error and bias is estimated using the sante percentage for all age groups.
Recruitment estimation
The recruitment dynamics show some relatively clear changes over time. A Beverton-Holt relationship with a log-normal error distribution was used for the long-term evaluations.
Рисунок не приводится
Figure 3.3.3.1.1 Spawning-Stock Recruitment (age 3) plot for Northeast Arctic saithe.
Reality check
The model was exposed to a reality check using F(4-7) = 0.38 for all SSB levels, a 50% maximum year-to-year change in TAC, and three options for assessment error. F(4-7) = 0.38 is equal to the average fishing mortality for the period 1960-2005. The runs indicate that the model performs reasonably well at this level of fishing mortality.
Scenarlos
The various settings used in the long-term simulations are presented in Table 3.3.3.1.1 and the results of the simulations are described in Table 3.3.3.1.2. The starting point for these simulations is a stock in healthy condition.
| Table 3.3.3.1.1 | Settings for long-term simulation runs. |
Run No. | F | 3-year rule | Implementation error | Option for assessment error | Fishing pattern | Percent change in TAC | F below B(pa) |
1 | 0.35 | No | No | 1 | 97-05 av. | 15 | Flat |
2 | 0.35 | Yes | No | 1 | 97-05 av. | 15 | Linear |
3 | 0.35 | Yes | Yes | 1 | 97-05 av. | 15 | Linear |
4 | 0.35 | Yes | Yes | 2 | 97-05 av. | 15 | Linear |
5 | 0.35 | Yes | Yes | 3 | 97-05 av. | 15 | Linear |
6 | 0.35 | Yes | Yes | 2 | 97-05 av. | 10 | Linear |
7 | 0.35 | Yes | Yes | 3 | 97-05 av. | 10 | Linear |
8 | 0.35 | Yes | Yes | 2 | 97-05 av. | 20 | Linear |
9 | 0.35 | Yes | Yes | 3 | 97-05 av. | 20 | Linear |
10 | 0.30 | Yes | Yes | 1 | 97-05 av. | 15 | Linear |
11 | 0.30 | Yes | Yes | 2 | 97-05 av. | 15 | Linear |
12 | 0.30 | Yes | Yes | 3 | 97-05 av. | 15 | Linear |
13 | 0.25 | Yes | Yes | 1 | 97-05 av. | 15 | Linear |
14 | 0.25 | Yes | Yes | 2 | 97-05 av. | 15 | Linear |
15 | 0.25 | Yes | Yes | 3 | 97-05 av. | 15 | Linear |
16 | 0.25 | Yes | Yes | 3 | 76-79 av. | 15 | Linear |
| Table 3.3.3.1.2 | Results of long-term simulations. Catch, TSB and SSB in 1000 tonnes, recruits in millions. The options tor the error are 1: No assessment error. 2: assessment error estimates based on the period 1999-2005, i.e. underestimation of stock size, and 3: Opposite trend in assessment error. |
Run No. | Oplion for assessment error | Input F | Realised F | Catch | TSB | SSB | Recr. | % years SSB < B(lim) | % years SSB < B(pa) | Average year-to-year change in TAC |
1 | 1 | 0.35 | 0.35 | 194 | 844 | 439 | 203 | 0 | 0 | 5 |
2 | 1 | 0.35 | 0.35 | 195 | 846 | 440 | 204 | 0 | 0 | 3 |
3 | 1 | 0.35 | 0.37 | 194 | 813 | 413 | 202 | 0 | 0.001 | 3 |
4 | 2 | 0.35 | 0.29 | 185 | 1015 | 602 | 209 | 0 | 0 | 10 |
5 | 3 | 0.35 | 0.48 | 193 | 703 | 317 | 195 | 0 | 3 | 8 |
6 | 2 | 0.35 | 0.29 | 184 | 1016 | 602 | 209 | 0 | 0 | 8 |
7 | 3 | 0.35 | 0.48 | 193 | 704 | 318 | 195 | 0.005 | 3 | 7 |
8 | 2 | 0.35 | 0.29 | 185 | 1017 | 603 | 210 | 0 | 0 | 11 |
9 | 3 | 0.35 | 0.48 | 193 | 702 | 317 | 195 | 0 | 3 | 9 |
10 | 1 | 0.30 | 0.32 | 196 | 917 | 499 | 206 | 0 | 0 | 3 |
11 | 2 | 0.30 | 0.25 | 181 | 1140 | 713 | 212 | 0 | 0 | 10 |
12 | 3 | 0.30 | 0.41 | 198 | 790 | 384 | 201 | 0 | 0.049 | 8 |
13 | 1 | 0.25 | 0.26 | 194 | 1044 | 609 | 210 | 0 | 0 | 3 |
14 | 2 | 0.25 | 0.21 | 174 | 1291 | 850 | 214 | 0 | 0 | 9 |
15 | 3 | 0.25 | 0.33 | 200 | 897 | 473 | 205 | 0 | 0 | 8 |
16 | 3 | 0.35 | 0.44 | 157 | 477 | 203 | 182 | 2.6 | 68 | 8 |
To study the performance of the rule in a stock recovery situation runs were made starting in 1986 and ending in 1991: 1986 was chosen because it was a year when the stock size was fairly low. Settings for the recovery simulation runs are presented in Table 3.3.3.1.3 and the results of the simulations are described in Table 3.3.3.1.4.
| Table 3.3.3.1.3 | Settings for recovery simulation runs. The options for the error are 1: No assessment error, 2: assessment error estimates based on the period 1999-2005, i.e. underestimation of stock size, and 3: Opposite trend in assessment error. |
Run No. | F | 3-year rule | Recruitment | Implementation error | Option for assessment error | Percent change in TAC | F below B(pa) |
1 | 0.35 | Yes | Modelled | Yes | 1 | 15 | Linear |
2 | 0.35 | Yes | Modelled | Yes | 3 | 15 | Linear |
3 | 0.35 | Yes | Lowest obs. | Yes | 3 | 15 | Linear |
| Table 3.3.3.1.4 | Results of simulations for 1986-1991. Catch, TSB, and SSB in 1000 tonnes, recruits in millions. The fishing mortality in 1986 is as assessed 0.54. |
Run No. | Input F | Mean Realised F SSB1991 | Range Mean Catch 1987->1991 | SSB 1987->1991 | Probability realisations with 1987->1991 | Probability realisations SSB1991 |
1 | 0.35 | 0,21->0,39 | 43->136 | 87->279 | 1->0 | 1->0.06 |
2 | 0.35 | 0,33->0,51 | 57->138 | 87->231 | 1->0 | 1->0.4 |
3 | 0.35 | 0,30->0,41 | 35-> 78 | 84->162 | 1->0.04 | 1->1 |
Results of the evaluation
Initial stock in healthy conditions
The rule is considered consistent with the precautionary approach and shows only a very small risk of SSB falling below B(lim).
Most of the results of the simulations are quite similar. Catches range from 157 000 to 200 000 tonnes, recruits from 182 to 214 millions, while the variations in estimated biomasses are larger. SSB ranges from 203 000 to 850 000 tonnes.
The historic implementation error (the difference between TAC and catch) is low (3% on average). Consequently, the result is not significantly different whether or not the implementation error is included.
The part of the HCR limiting the annual change in TAC to 15% is probably not too restrictive and large enough to maintain SSB above B(lim) in practically all the simulated cases.
Initial stock depleted
The simulations indicate that management following the HCR can rebuild the slock to above B(lim) within three years.
Source of information
ICES. 2007. Report of the Arctic Fisheries Working Group. 18-27 April 2007. ICES CM 2007/ACFM:16.
Vedlegg 14/appendix 14
Clarification of the effects of overfishing in 2007 on the TAC for NEA cod for 2008
Report from a sub-group of the Scientific working group during 36th session of the NRFC upon request from the heads of delegations. The request is only to clarify the effects of various levels of catches in 2007 on the TAC for 2008 and onwards.
IUU (illegal, unregulated and unreported) catches creates considerable problems for fish stock assessment and predictions, and leads to increased uncertainty and hence lower quality of the scientific advice. Unfortunately, this problem has been an inherent part of the stock assessment of NEA cod in recent years. To generate a prognosis as basis for TAC-calculation, assumptions about the catches in the assessment year has to be made. Basically, this can be done by assuming a certain total catch in the assessment year, for instance the TAC. However, in some cases the TAC is considered not to be appropriate basis for such analysis. For NEA cod, ICES has used a so-called F(status quo) projection. This implies an assumption that the same fishing mortality coefficient (F) as calculated for a recent period of years (1-3) is applied also for the assessment year. This approach has been found to produce prognoses, which are better in accordance with reality when considered on a retrospective basis, than the use of TAC. This technique incorporates a possible overfishing of TAC, but also other underlying deficiencies of data and methods.
The use of F(status quo) implies an assumption that the fisheries in the assessment year is carried out in a relatively stable manner compared to a recent period (similar fishing effort, exploitation pattern etc.). If there are reasons to believe that these assumptions are not valid for the assessment year, another approach could be used when giving prognoses and TAC for the coming years. Several intermediate options between a TAC constraint and an F(status quo) approach could be proposed. In the tables below, the effects of several levels of exploitation in 2007, ranging from the agreed TAC (424 000 t.) to the F(status quo) level (530 000 t.) is applied to the assessment made by ICES in June 2007.
The members of the subgroup undertaking these calculations have not yet seen any estimates of the anticipated total catch for 2007.
Table 1. Level of TAC for the three coming years for different catch levels in 2007, and fishing according to the catch rule during 2008-10. All numbers are in '000 tonnes.
Prognosed catch in 2007 ('000 tonnes) | TAC | ||
2008 | 2009 | 2010 | |
| 424 (= TAC) | 440 | 480 | 516 |
| 435 | 437 | 478 | 514 |
| 445 | 434 | 476 | 512 |
| 466 | 428 | 471 | 508 |
| 488 | 423 | 466 | 505 |
| 509 | 417 | 461 | 501 |
| 530 (ICES progn. based on Fsq) | 409 | 455 | 496 |
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Figure 1. TAC in 2008-2010 as a function of catch in 2007, when fishing according to the catch rule is implied for the following years.
Table 2. Level of Total stock size for the four coming years for different catch levels in 2007, and fishing according to the catch rule during 2008-10. All numbers are in '000 tonnes.
Prognosed catch in 2007 ('000 tonnes) | TSB | |||
2008 | 2009 | 2010 | 2011 | |
| 424 (= TAC) | 1640 | 1777 | 1929 | 2070 |
| 435 | 1628 | 1767 | 1920 | 2063 |
| 445 | 1616 | 1757 | 1911 | 2055 |
| 466 | 1592 | 1736 | 1894 | 2042 |
| 488 | 1568 | 1715 | 1877 | 2027 |
| 509 | 1544 | 1695 | 1860 | 2014 |
| 530 (ICES progn. based on Fsq) | 1513 | 1668 | 1838 | 1996 |
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Figure 2 Total stock biomass in 2008-2010 as a function of catch in 2007, when fishing according to the catch rule is implied for the following years.
Vedlegg 14/appendix 14
Table 3. Level of Spawning stock size for the four coming years for different catch levels in 2007, and fishing according to the catch rule during 2008-10. All numbers are in '000 tonnes.