Biological Composition of the Catch

Biological information (numbers, weight, length, catch (SOP) at age and relative age composition) on the catch as obtained by sampling of commercial catches is given for the whole year and per quarter in Tables 2.2.1 to 2.2.5. Where available, data is displayed separately for herring caught in the North Sea (including a minor amount of Western Baltic Spring Spawners taken in IVa East), IVa East (total; Western Baltic Spring Spawners (WBSS) only – see Section 2.2.2;

North Sea Autumn Spawners only), IVa West, IVb, VIId/IVc as well as for North Sea Autumn Spawners (NSAS) caught in Div. IIIa, and the total NSAS stock, including catches made in IIIa.

Biological information for North Sea Autumn Spawners caught in Division IIIa was obtained using splitting procedures described in Section 3.2. The total catches (SOP Figures), mean weights and numbers at age by fleet are given in Table 2.2.6. Note that fleet D includes the former fleet E from 1999 on.

Data on catch numbers at age and SOP catches are shown for the period 1990–2000 in Tables 2.2.7 (herring caught in the North Sea), 2.2.8 (WBSS taken in the North Sea, see below), 2.2.9 (NSAS caught in Div. IIIa) and 2.2.10 (total numbers of NSAS). Mean weights at age are given for the same period (1990–2000) separately for the different Divisions where NSAS are caught (Table 2.2.11).

2.2.1 Catch in numbers at age

North Sea catches in numbers at age over the years 1990–2000 are given in Table 2.2.7. The total number of herring taken in the North Sea in 2000 (2.9 billion) has slightly decreased as compared to the corrected Figure for last year (3.1 billion, see below); the numbers of North Sea Autumn spawners have increased by 4%. Catches of 0-ringer NSAS have decreased by 30%, while those of 1-ringers have increased by factor 3.5 (as compared to the corrected Figures for last year, see below). This was expected and is likely to be caused by high catches of the strong 1998 year class. 0- and 1-ringers contributed more than half of the total catch in numbers of North Sea autumn spawners in 2000. Figure 2.2.1.

shows the relative proportions on the total catch numbers for different periods (1960–2000, 1980–2000 for the total area, and 2000 for different Divisions).

The following Table summarises the total catch in tonnes of North Sea autumn spawners. After the splitting of the North Sea Autumn Spawners in Div. IIIa and the Western Baltic Spring Spawners caught in the North Sea, the amount of the total catch used for the assessment was 373,000 tonnes, which is almost exactly the Figure used for 1999:

Area Allocated Unallocated Discards Total

IVa West 124,157 29,581 5,841 159,579

IVa East 52,950 -4 - 52,946

IVb 85,641 -13,769 317 72,189

IVc/VIId 23,262 20,966 196 44,424

2.2.2 Treatment of Spring Spawning herring in the North Sea

Norwegian Spring Spawners are taken close to the Norwegian coast under a separate TAC. These catches are not included in the catch tables. Coastal Spring Spawners in the southern North Sea (e.g. Thames estuary) are caught in small quantities regulated by a local TAC. These catches are given in Table 2.1.1 and 2.1.5.

Western Baltic and Division IIIa spring spawners (WBSS) are taken in the eastern North Sea during the summer feeding migration. These catches are included in Table 2.1.1 and listed as IIIa type. Table 2.2.8 specifies the estimated catch numbers of WBSS caught in the North Sea, which are transferred from the North Sea assessment to the assessment of Division IIIa/Western Baltic in 1990–2000.

The method of separating these fish, as described in former reports from this Working Group (ICES 1990/ Assess:14) assumes that for autumn spawners, the mean vertebral count is 56.5 and for spring spawners 55.80. The fractions of spring spawners (fsp) are estimated from the formula (56.50-v)/0.7, where v is the mean vertebral count of the (mixed) sample. The method is quite sensitive to within stock variation (e.g. between year classes) in mean vertebral counts. The same method has been applied to separate the two components in the summer acoustic survey.

To calculate the proportion of spring spawners caught in the transfer area, 17 samples that have been taken in May and June 2000 were used for the second quarter. For the third quarter, 16 samples taken in July and August were used (Figure 2.2.2).

The resulting proportion of spring spawners and the quarterly catches of these in the transfer area in 2000 are as follows:

Quarter 2 wr (%) 3 wr

(%) 4+ wr

(%) Catch in the

transfer area (t) Catch of WBSS in the North Sea (t)

Q 2 12 14 17 6,956 929

Q 3 14 17 20 14,527 5,720

total 21,483 6,649

The quarterly age distribution in Sub-division IVa East was applied to the catches of the second and third quarters in the whole area. The numbers of spring spawners by age were obtained by applying the estimated proportion by age.

2.2.3 Data revisions

The numbers at age and mean weights at age in the catch were revised for Div. IIIa for 1999, which made updating of a number of tables for North Sea autumn spawners and Western Baltic spring spawners (see Section 3) necessary. The revision was due to corrections in proportions of catches by fleets of the Swedish landings.

A revision of the historic data caused by the application of new splitting factors for catches in Div. IIIa was thought to have minor influence on the North Sea autumn spawner assessment. Due to time constraints, it was postponed to next year’s working group.

The catch tables (Tables 2.1.1 to 2.1.6) were transferred from a word processing program into a spreadsheet program to ease crosschecking. Some minor corrections, mostly attributable to rounding errors, were corrected during this process.

However, some discrepancies could not be resolved during the working group’s meeting and will be dealt with intersessionally.

2.2.4 Quality of catch and biological data

As in the previous years, it was possible in 2000 to get information on misreportings and unallocated catches from several countries fishing on herring in the North Sea and adjacent areas. Catches made in IVa were mainly misreported to VIa North and IIa, but misreporting also occurred within Area IV, to IIIa and from VIId to IV and IIa.

Only The Netherlands and Scotland provided estimates of discards, but discards are known to occur in the fisheries of most countries and they could represent a significant amount of the total catch, which is so far not included in the assessment. In this respect, there is still a need to improve the quality of the catch data for the North Sea herring. As mentioned earlier, some of the differences in the distribution of catches estimated by the working group are attributable

In general, sampling of commercial landings for age, length and weight was at the same level as last year (Table 2.2.12) and is below the previously recommended level of one sample per 1,000 t landed, if unallocated and misreported catches are taken into account. As it is known that by-catches of herring in other fisheries occur and most of the countries have not implemented a sampling scheme for monitoring these fisheries, Table 2.2.12 can not be used to judge whether a country has met the recommended sampling levels or not.

It should be observed that “sampled catch” refers to the proportion of the reported catch to which sampling was applied.

This Figure is limited to 100% but might in fact exceed the official landings due to sampling of discards, unallocated and misreported catches. It is not possible to judge the quality of the sampling from this Figure alone. Of 90 different reported metiers (each combination of fleets/nations/areas/quarters – this is likely to be an underestimate of the possible number of metiers), only 30 were sampled. This introduces uncertainties in the biological composition of the catches, which affects the quality of the assessment. The working group repeats that there is a need for an increased sampling effort, especially to assure that catches landed abroad are reasonably sampled.

2.3 Recruitment

2.3.1 The IBTS index of 1-ringer recruitment

The 1-ringer index of recruitment is based on the IBTS, 1^st quarter (trawl catches at daytime February 2001). The index is calculated for the entire survey area, weighting statistical rectangles as described in the WG report of 1995 (ICES CM.1995/ Assess:13).

The indices based on surveys from the period 1979 to 2001 (estimates of the strength of year classes 1977 to 1999) are given in Table 2.3.1. and the temporal trend in indices is illustrated in Figure 2.3.1. The estimate of the 1999 year class (2674) indicates a recruitment of intermediate strength, 20% above the average for the last 20 years.

Figure 2.3.2 illustrates the spatial distribution of 1-ringers as estimated by the trawling in February during 1999, 2000 and 2001. In 2001 the primary concentrations of 1-ringers were found in the German Bight and in the Skagerrak/Kattegat area. (div. IIIa). The enhanced abundance in Division IIIa resembles the findings in 2000, while the present 1-ringer concentration in the German Bight contrasts the more central North Sea distribution pattern found in 2000.

2.3.2 The MIK index of 0-ringer recruitment

The 0-ringer index is based on night time catches by a fine-meshed ring net (the MIK) during the February survey of the IBTS. Index values are calculated as described in the WG report of 1996 (ICES 1996a). The index value indicating the abundance of 0-ringers in 2001, the 2000 year class, is estimated to 214.8 (Table 2.3.2).

This estimate of the 2000 year class is high, and indicates a recruitment well above average. It is based on extraordinary high concentrations of 0-rings along the coast of UK. The spatial distribution of the 0-ringers follows the trend of a westerly displacement which has been observed during the last years (Figure 2.3.3), however, this year the north-westerly distribution is very marked. Unusual high catches were made, for example off Moray Firth and Buchan.

2.3.3 Relationship between the MIK 0-ringer and the IBTS 1-ringer indices

The relationship between the two indices is illustrated in Figure 2.3.4 and described by the inserted linear regression.

The comparison between the indices for the 1999 year class reveals a relation that is in accordance with the long-term trend. Both indices indicate an intermediate 1999 year class.

2.3.4 Trends in recruitment as estimated by the assessment

The long-term trend in recruitment of 1-ringers to the stock of North Sea autumn spawners is illustrated by Figure 2.3.5.

Recruitment estimates are based on the present 2001 ICA assessment. The Figure illustrates the decline during the sixties and the seventies, followed by a marked increase in the early eighties. After the strong 1985 year class a new decline was observed followed by relatively strong year classes in the most recent years with ICA estimates of 1-ringer

2.3.5 Separate recruitment indices of the Downs herring

In last year’s report of the working group (ICES CM 2000/ACFM 10) the possibilities of separating the Downs herring from the indices of recruitment were investigated. The Downs herring hatch later than the other autumn spawned herring and generally appears as a smaller sized group during the 1^st quarter IBTS.

Polymodal length distributions of the 1-ringers during some years indicate that the group could be separated as 1-ringers smaller than 13 cm. In other years however, as noted in last year’s report, a mode of smaller juveniles is less distinct.

Table 2.3.3. shows the abundance of 1-ringer herring smaller than 13 cm, estimated from a standard retrieval of the IBTS database, i.e. the standard index is in this case calculated for herring<13 cm only. Indices for these small 1-ringers are given either for the total area or the area excluding division IIIa. The proportion of 1-ringers in the total catches that are smaller than 13 cm is in the order of 20%, with a maximum proportion of 57% for year class 1996 (Table 2.3.3).

The contribution of small 1-ringers from division IIIa also varies significantly (Table 2.3.3), for example two prominent peaks in the abundance estimates (year classes 1986 and 1991) are due to high relative abundance in IIIa. Some of the variability in small 1-ringers in division IIIa might be due to a variable occurrence of small herring from local stocks of Kattegat winter spawners. This year’s estimate of the proportion of small 1-ringer herring (in the 1999 year class) is low, the lowest during the last decade.

The 0-ringed Downs herring are found in the Southern Bight, and due to their later time of hatch they can be distinguished by their smaller mean length. Because of the restricted area of distribution, the previous sampling (until 2000) has not covered the group with sufficient precision, and the sporadic catches of these larvae have not been included (i.e. samples of a mean length of less than 21 mm in the Southern Bight are excluded) in the standard 0-ringer index shown in Table 2.3.2. At the 1999 meeting of the IBTS working group it was decided to increase the intensity of the MIK sampling in the relevant rectangles, and this procedure was implemented in 2000. During the MIK sampling in 2000 only minor densities of small larvae were observed, while the present year’s MIK data (2001) showed extraordinary high concentrations of small larvae along the coast of the Netherlands (Figure 2.3.6). The markedly higher value of last year’s observation is evident from the comparison of density estimates shown in Table 2.3.4 and Figure 2.3.7.

In contrast to the indices based on MIK 0-ringer and IBTS 1-ringer catches of the whole catch, the indices based on the Downs component in these catches show considerable differences over time which is expressed by a correlation coefficient (R²) of only 0.002 for the relationship between these indices (Figure 2.3.7). The increased MIK sampling intensity in the southern area may result in an improved index of the Downs component in the MIK catches.