Short term projection by fleets

2.7.1 Method

The program used (MFSP) was developed three years ago in the HAWG. The version used this year was the same as at last years meeting (Skagen 2003). The standard tool that currently is available for short term predictions (the MFDP program) has some limitations with regard to management options that can be covered. In particular, when varying the fishing mortality for one fleet, the fishing mortalities for the other fleets are assumed constant at status quo F.

For the North Sea herring, managers have agreed to constrain the total outtake at levels of fishing mortalities for ages 0-1 and 2-6, and need options to show the trade-off between fleets within those limits. The MFSP program was developed to cover these needs.

2.7.2 Input data Fleet Definitions

The current fleet definitions are:

North Sea

Fleet A: Directed herring fisheries with purse seiners and trawlers. By-catches in in-dustrial fisheries by Norway are included.

ICES HAWG Report 2005 54

Fleet B: Herring taken as by-catch under EU regulations.

Division IIIa

Fleet C: Directed herring fisheries with purse seiners and trawlers Fleet D: By-catches of herring caught in the small-mesh fisheries The fleet definitions are the same as last year.

Input Data for Short Term Projections

All the input data for the short term projections are shown in Table 2.7.1, which is the input file for the predictions.

Stock Numbers: For the start of 2005 the total stock number was taken from ICA (ica.n – file)

For 2006 and 2007, the recruitment was set to 49 960 million which is the geometric mean of the recruitments of the year classes 1981 – 2001.

Fishing Mortalities: Selection by fleet at age was calculated by splitting the total fishing tality in 2004 for each age proportional to the catches by fleets at that age. These fishing mor-talities were used for all years in the prediction.

Maturity at age: For all the year classes except the 2000 year class, the average maturity at age for 2001 to 2004, calculated without the 2000 year class, was used (Table 2.6.2.2). For the 2000 year class, which so far has matured more slowly than usual, the maturity was predicted by fitting a logistic function to the maturities at age observed so far. That gave a fraction ma-ture of 0.91 as 4-ringers in 2005 and 0.98 as 5-ringers in 2006. For 2007, this year class was assumed to be fully mature.

Mean Weights at age in the stock: A similar procedure as for maturities was followed for mean weights at age in the stock. Again a 4 year average of the annual weights, excluding the 2000 year class was used for all year classes except the 2000 year class. The weights at age for the 2000 year class were obtained by fitting a von Bertalanffy function. The weights used for this purpose were the raw annual weights, while smoothed weights are used in the assessment.

Mean weights in the catch by fleet: The mean weights by fleet for the years 2002 – 2004, excluding the 2000 year class was used for all year classes except the 2000 year class. For the 2000 year class, the procedure outlined for weights in the stock was followed, assuming von Bertalanffy growth. Separate values for t0 were estimated for each fleet. The Winf and k were estimated assuming they were equal for all fleets.

Natural Mortality: Unchanged from last year, equal to those assumed in the assessment.

Proportion of M and F before spawning: Unchanged from last year at 0.67.

2.7.3 Prediction for 2005 and management option tables for 2006 Assumptions and Predictions for 2005

In 2004, the TAC for the A-fleet was overfished by approximately 15%, while the B- and C- fleets caught less than half their TAC. Catches in 2005 may be predicted with some confi-dence. The retrospective error has been low in recent years. It therefore seems most reasonable to use assumed catches to account for the removal in 2005. It is assumed that the TAC of 535 000 tonnes of the A-fleet will be taken, and that the bycatch by the B-fleet will increase somewhat in 2005, because it is expected to target sprat to a larger extent than in recent years due to shortage of sandeel, and closure of the fishery for Norway pout..

The alternative option assuming Fstatus quo is also presented. The partial fishing mortalities at Fstatus quo appear in tables 2.7.1. It should be noted, however, that Fstatus quo gives far lower catch by the A-fleet than the agreed TAC, and also lower catch by the B-fleet than can be expected.

Management Option Tables for 2006

The EU-Norway agreement on management of North Sea herring was updated in 2004. The revised rule specifies fishing mortalities for juveniles (F 0-1) and for adults (F 2-6) not to be exceeded, at 0.12 and 0.25 respectively, for the situation where the SSB is above 1.3 million tonnes. In addition, it now has a rule specifying reduced fishing mortalities when the SSB is below 1.3 million tonnes. Moreover, the current agreement has a constraint on year-to-year change of 15% in TAC, but allows for a stronger reduction in TAC if necessary.

With four fleets there are innumerable combinations of fleetwise fishing mortalities and catches that satisfy the agreed rules. The predictions presented are in accordance with the agreed arrangement.

Since the North Sea autumn spawning (NSAS) stock was rebuilt, the advise has been that the primary limiting factor for the fishery in IIIa should be the concern for the Western Baltic spring spawning (WBSS) stock. Using that as a guideline, and in order to reduce the number of possible options, a range of fixed catches were assumed for the fleets C and D derived from the likely recommended outtake of WBSS. The procedure for obtaining these catch limitations are described in detail in Section 3.10. In brief, the historical fractional distribution of the WBSS catches on IIIa and the other areas is used to translate the total recommended TAC for WBSS into outtake of WBSS in IIIa. Then, the mix of WBSS and NSAS in the IIIa catches is used to derive the outtake of NSAS in IIIa. Assuming a total catch of WBSS of 95 000 tonnes (see Section 3.7) led to a catch of 16 600 tonnes of NSAS herring for the C-fleet and 11100 tonnes of NSAS herring for the D-fleet by this procedure.

It has become increasingly clear that in previous years, large parts of the catches reported for IIIa were actually taken in the North Sea. For 2004, Norway was allowed to transfer all of its quota in IIIa to IV, while the EC could transfer 50% of its quota. For 2005, Norway could again transfer its quota in IIIa to IV, while the EC could not. Furthermore, the last 3 year classes of NSAS have been weak, implying relatively small amounts of NSAS in IIIa. There-fore, it seems likely that the current fleet behaviour, with relatively small catches of NSAS in IIIa will be continued in the coming years.

In each set of predictions, a range of fixed catches were assumed for fleets C and D (8300, 12450 and 16600t for fleet C and 5500, 7750 and 11100t for fleet D). For each combination of these, the catches by the fleets A and B were adjusted to give an F0-1 at either 0.05, which is close to the F status quo, or the agreed value of 0.12, and to an F2-6 at 0.25. In addition, be-cause these predictions lead to a reduction of catches by the A-fleet of slightly more than 15%

compared to the 2005 TAC, a similar set of predictions were done with a fixed catch by the A-fleet at 85% of the 2005 TAC, i.e. 455 000 tonnes.

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The text tables below is an overview of the options.

Predictions with Fstatus quo for 2005 Status quo F-values by fleet:

A: F2-6 = 0.240, B: F0-1 = 0.036, C: F0-1 = 0.003, D: F0-1 = 0.010

Predictions with catch constraint for 2005

Catch constraints by fleet: A: 535, B: 25, C: 20, D: 15

All predictions are for North Sea autumn spawning herring only.

The results are presented in Table 2.7.2. In addition, runs were made screening over narrow intervals of catch options for all fleets, as requested in the Memorandum of Understanding between ICES and its client commissions. The results file is too extensive to be included in the report, but is available on the WG directory.

2.7.4 Comments on the short-term projections

Making fleetwise predictions for 4 fleets that are more or less independent remains problem-atic, in particular when it comes to presenting results in a way that allows managers to over-view the range of possible trade-offs between fleets.

It is also worth noticing that the realised F2-6 in the past have exceeded that intended when setting the TACs for many years. If managers wish to avoid exceeding the agreed limits, op-tions with lower F-values may be preferable.

All scenarios presented indicate a decrease in spawning biomass and in yield. This is mainly caused by the weak 2002, 2003 and 2004 year classes taking over from the strong 1998 and

2000 year classes. The catches by the A fleet are estimated close to the lower bound implicit in the constraint on TAC reduction agreed by EU and Norway at F2-6 = 0.25, while the catches by the B, C and D fleets is a trade-off between these fleets, the sum of which will be approxi-mately 70 – 80 000 tonnes with an F0-1 = 0.12.

The predictions presented here account for the delayed maturation of the large 2000 year class.

To what extent the increased stock size will lead to slower growth and maturation in the future remains to be seen. There are some indications that this was the case when the stock was large prior to 1960 (ICES 1998/ACFM:14), but there are no indications of reduced growth of the 2001 and 2002 year classes.

The estimated impact of the juvenile fishery depends on the assumed value for natural mortal-ity. It has not been investigated to what extent changes in natural mortality would affect the current advise, or if indeed such changes are taking place.