Recruitment Estimates

Indices of 0-ringer abundance were available from larval surveys during the spawning season on the main spawning area (Table 3.5.9), and from the German Bottom Trawl Surveys during November-December in Sub-divisions (SD) 22 and 24 GBTS_ND (Table 3.5.3). Indices of 1-ringer abundance were also available from the GBTS_ND, (Table 3.5.3), and from the German Bottom Trawl Surveys during January-February in SD 24 GBTS_JF, (Table 3.5.4). Log transformed indices were compared by year class in Figure 3.6.1 The larval 0-ringer and GBTS_ND 0-ringer indices for the year classes 1977 to 2000 show some similar year-to-year variability (correlation R-square = 0.44). For the year classes 1978 to 1999 the GBTS_ND 0-ringer and the GBTS_JF 1-ringer showed co-variation (correlation R-square = 0.34) whereas the GBTS_ND 0-ringer and the GBTS_ND 1-ringer indices showed no co-variation. The indices illustrated in Figure 3.6.1 show the following general time trends: Poor recruitment of year classes 1980–82 was followed by an increase to a high level of recruitment for year classes 1983–88. From year class 1990 the recruitment declined until 1992 when recruitment was low. An increase in year classes 1993−1994 is indicated. The year class 1996 was below average but the estimates for 1998 and 1999 are comparable to historical high levels of recruitment. The high larval indices of the 1998 and 1999 year classes were followed by high values of in the subsequent 0-ringer GBTS_ND and the 1-ringer GBTS_JF indices. The very consistent signal of historical high recruitment of the 1998 and 1999 year-classes is further supported by 0-ringer and 1-ringer indices in the acoustic survey in Subdivisions 22–

24(Table 3.5.8).

After the 1998-1999 year-class peak there is an indicated significant drop in recruitment of the 2000 year-class. Both the larval index and the subsequent GBTS_ND and the acoustic survey in Subdivisions 22–240-ringer indices are far below average.

3.7 Data Exploration

3.7.1 Input data

Catch in numbers by age for spring spawners in Div. IVe, Div. IIIa and Sub-divisions 22-24 were available for 1991 to

Mean weights at age in the landings for spring spawning herring are found in Table 3.3.4. Mean weights for adults is conspicuously lower in Sub-division 22-24 compared to Div. IIIa. Mean weights of 2+ ringers in Div. IIIa have decreased since 1995 whereas mean weights in Sub-division 24 lacks an apparent trend between years.

Mean weights at age in the stock by year was derived as the mean weights representing catch in numbers during the 1^st quarter.

The maturity ogive used and proportions of F and M before spawning was assumed constant between years. F-prop.

was set to be 0.1 and M-prop. 0.25 for all age groups. The maturity ogive used was the same as that used at the HAWG meeting in 2000:

W-rings 0 1 2 3 4 5 6 7 8+

Maturity 0.00 0.00 0.20 0.75 0.90 1.00 1.00 1.00 1.00

Natural mortality was assumed constant at 0.2 for all years and 2+ ringers. A predation mortality of 0.10 and 16 was added to the 0 and 1 ringers, which resulted in an increase in their natural mortality to 0.3 and 0.46, respectively. The estimates of predation mortality were derived as a mean for the years 1977–1995 from the Baltic MSVPA (ICES 1997 CM/J:2).

Available survey indices were:

• Index 1: Hydroacoustic survey in Division IIIa, July 1989–99, 0–8+ ringers

• Index 2: Hydroacoustic survey in SD 22, 23 and 24, Oct. 1989–99, 0–8+ ringers

• Index 3: Larvae survey in SD 24 (Greifswalder Bodden), March-June 1977-99

• Index 4: German bottom trawl survey (GBTS) in SD 22, Nov. 1979–99, 0–3+ ringers

• Index 5: German bottom trawl survey (GBTS) in SD 24, Nov. 1978–99, 0–3+ ringers

• Index 6: German bottom trawl survey (GBTS) in SD 24, Feb. 1979–99, 1–8+ ringers

• Index 7: IBTS in Div. IIIa, Quarter 1, 1991-00, 1-5 ringers

• Index 8: IBTS in Div. IIIa, Quarter 3, 1991-99, 1–5 ringers

None of the indices covered the total spatial distribution of the WBSS stock and the indices represent different seasonal overlap:

Survey area Quarter 1 Quarter 2 Quarter 3 Quarter 4

Div. IIIa Index 7 Index 1,8

Sub-divisions 22-24 Index 3,6 Index 2,4,5

The observed year-class abundance varied similarly for yearclasses 1990 to 1998 but demonstrated also inconsistencies (negative correlations) between and within indices (between age-groups).

3.7.2 Exploration by individual survey indices

Exploratory runs were done by tuning catches by individual survey time series with ICA software (version 4). Contrary to the 2000 assessment, one separability period from 1997 to 2000 were selected. The period represents a fishery with a different selectivity pattern than before the international regulation in 1996. This period is also consistent with the separable period used for the North Sea assessment this year. The ICA settings were:

• The weighing factor to all indices (lambda = 1).

• A linear catchability model for all indices.

• The range of years for separable constraint (= 4 years from 1997 to 2000).

• The reference F set at age 4 and the selection 1 for oldest age.

• All available age groups were included and not down weighted.

• No shrinkage applied.

Results of these runs are presented in Figure 3.7.1. The hydroacoustic surveys in Div. IIIa, in Sub-div. 22-24 and the IBTS indices suggest high Fs of around 0.6, while the larval and trawl surveys in Sub-divisions 22-24 suggests considerably lower values. The hydroacoustic survey in Div. IIIa generally tracks the VPA estimates in numbers and have comparatively lower age and year residuals than the other indices. However, the survey showed high age residuals for the 0-and 1 ringers. The hydroacoustic survey and the German trawl survey in Sub-div. 24 showed high residuals for older ages. The runs indicated that the year-class strength in both 1998 and 1999 was above the average.

3.7.3 Exploration by combined survey indices

A combined ICA run (no 16) was performed with the following indices:

• Index 1: Hydroacoustic survey in Division IIIa, July 1989–99, 0–8+ ringers

• Index 2: Hydroacoustic survey in SD 22, 23 and 24, Oct. 1989–99, 0–8+ ringers

• Index 5: German bottom trawl survey (GBTS) in SD 24, Nov. 1978–99, 0–3+ ringers

• Index 8: IBTS in Div. IIIa, Quarter 3, 1991-99, 1–5 ringers

The choice of indices was based on the known migration pattern and spatial distribution of the stock between seasons.

A second combined run (no 17) was also performed on restricted age ranges of the chosen indices. Indices in the Div.

IIIa were truncated to include 2+ ringers only and indices in Sub-div. 22-24 were truncated to contain 0-1 ringers only.

The rationale was partly high age residuals for the deleted age groups, and partly an assumption about how well the age groups are represented in the surveys.

Model settings besides the age range were set as in section 3.7.2. Input data are shown in Tables 3.7.1 – 3.7.5.

Results are presented in:

ICA run Tables Figures

Combined run 16 (all available ages) 3.7.6 - 3.7.16 3.7.2 - 3.7.4

Both runs demonstrate a shallow SSQ response-curve. The SSB shows a stable level over the recent years. The F (3-6) of the combined run 16 was lower (0.50) than of the combined run 17 (0.58). However, runs indicate irregular catch abilities and high residuals. Moreover, the survey indices did not track the VPA estimates in stock numbers over time for some of the age-classes.

The working group concluded that the data exploration by the ICA software could not resolve the apparent incompatibility between surveys and catch data for the following reasons:

• Absence of an operative migration model (ICES 2000/ACFM:10).

• No survey fully covering the stock at any particular time.

• A limited time series of revised catch data (1991–2000).