Short-term projection by fleets .1 Method .1 Method

Last year, the work by the SGHAEP (2001) lead the WG to abandon the area-based predictions using local partial fishing mortalities. Instead, the WG decided to give predictions by fleet, assuming that the fleetwise partial fishing mortalities apply to the stock as a whole. The standard tool that is currently available (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. To allow for exploring such options, a short-term prediction program (MFSP) was developed during last year’s meeting. This program has been somewhat refined for this year’s meeting (Skagen 2003; WD11) and was used for the predictions this year.

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 industrial fisheries by Norway are included.

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 fleets are basically the same as last year, but the definitions have been modified slightly to bring them in accordance with actual practise. In previous years the Norwegian by-catches in the industrial fishery in the North Sea were not reported separately. Rather, the whole Norwegian catch was allocated to the A-fleet. This year, these by-catches were reported separately, although without an age distribution based on samples. Traditionally, these by-by-catches have mainly consisted of adult fish. Therefore, the WG decided to still include the Norwegian by-catches in the industrial fishery in the catches of the A-fleet, even though this fishery is by small-meshed trawl. The B-fleet then covers the by-catches of herring in fisheries in the North Sea under EU regulations. It has also been managers’ practise to base the quota shared by EU on Norway on the advice for the A-fleet. This is in principle a TAC for directed fisheries for herring, but Norway uses part of this TAC to cover the by-catches in its industrial fisheries. The by-catch ceiling for the industrial fisheries in the EU-Norway agreement, which is decided based on the advice for the B-fleet, is allocated to EU.

Input Data for Short-term Projections

All the input data for the short-term projections are summarised in Table 2.7.2.1.

The starting point for the projection is the stock of North Sea autumn spawners in the North Sea and Division IIIa combined at 1 January 2003.

Stock Numbers: For the start of 2003 the total stock number was taken from ICA (Population Abundance year 2003, Table 2.6.2.2).

For 0-ringers in 2004 and 2005, the stock number was set to 48 800 million which is the geometric mean of the recruitments in the period 1983 – 1999.

Fishing Mortalities: Selection by fleet at age was calculated by splitting the total fishing mortality for each age proportional to the catches by fleets at that age. Due to the change of fleet allocations of some catches this year, (see Section 2.2.3) only fishing mortalities and catches for 2002 were used.

Mean Weights-at-age in the stock: Since the weights used in the assessment are already smoothed, the values for 2002 (Table 2.6.2.2) were used in the prediction.

Maturity-at-age: The average maturity-at-age for 2001 and 2002 was used (Table 2.6.2.2).

Mean weights in the catch by fleet: The revisions in allocation of catches between fleets C and A (Section 2.2.3), also implies that the weights-at-age in the catch for these fleets needs to be revised. Revised fleetwise weights in the catch

could so far only be provided for 2002. Accordingly, these weights were used (Table 2.2.6). The previous practise of taking a weighted mean of the last two years should be resumed when the data have been updated.

Natural Mortality: Unchanged from last year (Table 2.6.2.2).

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

The input file to the prediction program is shown as Table 2.7.2.1.

2.7.3 Prediction for 2003 and management option tables for 2004 Assumptions and Predictions for 2003

Two sets of predictions are presented, one assuming Fstatus quo in 2003, and one assuming that the agreed TACs are taken. In previous years, overshoot of the TAC was assumed. Due to the large increase in TACs in 2003, the WG decided not to make this assumption for 2003.

The partial fishing mortalities at Fstatus quo appear in Tables 2.7.3.1 a-c, and the catches assumed appear in Tables 2.7.3.2 a-c.

Management Option Tables for 2004

The EU-Norway agreement specifies fishing mortalities for juveniles (F 0-1) and for adults (F 2-6). With four fleets there are innumerable combinations of fleetwise fishing mortalities and catches that satisfy this constraint.

In each set, a range of fixed catches were assumed for fleets C and D (20 000-70 000 t in steps of 10 000 t for fleet C and 5 000 – 30 000 t in steps of 5 000 t for fleet D). For each combination of these, the catches by the fleets A and B were adjusted to give an F0-1 and an F2-6 at specified values (0.10 or 0.12 for F0-1 and 0.20 or 0.25 for F2-6).

The text table below is an overview of the options tables (Tables 2.7.3.1 a-c and 2.7.3.2a-c):

Assumption for

2003 F 0-1

2004 F2-6

2004 Catch fleet C

2004 Catch fleet D

2004 Table

F status quo

0.10 0.20 20,30,40,50,60,70 5,10,15,20,25,30 2.7.3.1a

0.10 0.25 20,30,40,50,60,70 5,10,15,20,25,30 2.7.3.1b

0.12 0.25 20,30,40,50,60,70 5,10,15,20,25,30 2.7.3.1c

TAC constraint 0.10 0.20 20,30,40,50,60,70 5,10,15,20,25,30 2.7.3.2a

0.10 0.25 20,30,40,50,60,70 5,10,15,20,25,30 2.7.3.2b

0.12 0.25 20,30,40,50,60,70 5,10,15,20,25,30 2.7.3.2c

In addition, a limited number of management options with the corresponding estimated fishing mortalities and catches by fleet as well as the predicted SSB is presented in Table 2.7.3.3. The table contains predictions corresponding to three scenarios: the combination of Fst quo in 2003 and the fishing mortalities specified in the EU - Norway agreement, assuming roll-over TACs in 2003 and in 2004, and assuming Fstatus quo for both 2003 and 2004.

All scenarios presented (Tables 2.7.3.1 a-c, Tables 2.7.3.2 a-c) indicate a continued increase in spawning biomass and in yield. This is mainly caused by the 1998 and 2000 year classes. The weak 2002 year class leads to comparatively low catches in the fleets exploiting mainly juveniles. The catches by the A fleet are estimated close to 500 000 tonnes 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 approximately 110 – 130 000 tonnes with an F0-1 = 0.12. The difference in the results for 2004 between the F status quo and the TAC constraint assumptions for 2003 is small.

2.7.4 Comments on the short-term projections

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

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