Fishery Independent Information

2.3.1 Acoustic Surveys in VIa(N) and the North Sea in July 2004 Six surveys were carried out during late June and July 2004 covering most of the continental shelf north of 51^o 30’N in the North Sea and 56^oN to the west of Scotland to a northern limit of 62°N. The eastern edge of the survey area is bounded by the Norwegian, Danish, Swedish, German and Dutch coasts, and to the west by the shelf edge at approximately 200 m depth.

The individual surveys and the survey methods are given in the report of the Planning Group for Herring surveys (ICES 2005/G:04). The vessels, areas and dates of cruises are given below and in Figure 2.3.1.1:

ICES HAWG Report 2005 46

VESSEL PERIOD AREA

FV Enterprise 6 July – 25 July 56°- 60°N, 3° - 7° W R.V Johan Hjort 8 – 30 July 57°- 61° N, 2° - 6° E Scotia 1 - 22 July 58 15°- 62° N, 4° W - 2° E Tridens 28 June – 23 July 54°30 – 58° 15’ N, west of 3° E Walther Herwig III 28 June – 19 July 51° 30’ - 57° N, east England / 3° E /

6° E

Dana 29 June – 12 July North of 57°NS & 56° N, Kattegat east of 6° E

The data has been combined to provide an overall estimate. Estimates of numbers-at-age, ma-turity ogive and mean weights-at-age are calculated as weighted means of individual survey estimates by ICES statistical rectangle. The weighting applied is proportional to the survey track for each vessel that has covered each statistical rectangle. The data have been combined and the estimate of the stock surveyed is shown in Tables 2.3.1.1-3 by ICES subarea for North Sea autumn spawning herring.

Combined Acoustic Survey Results:

The estimate of North Sea autumn spawning herring SSB is 2.6 million tonnes which is 14,000 millions herring (Table 2.3.1.4). This data series is used as a relative index in the as-sessment of North Sea herring because the absolute abundance cannot be used directly due to uncertainties in target strength. The North Sea survey is reasonably consistent with previous years but shows a small decline, giving a total adult mortality of about 0.5 over the last 3 years, which is slightly higher than the estimates from the assessment. The North Sea herring SSB estimated from the survey rose from 2.6 million tonnes in 2001 (Table 2.3.1.5) to 2.9 million tonnes in 2002 and again to 3.0 million tonnes in 2003 and has now been seen to fall to 2.6 million. As observed last year the growth of the 2000 year class seems to be slower than for previously observed year classes. The herring are now 1.4 cm smaller, and 33g lighter than the similarly abundant 1998 year class at the same age (3-rings). Last year only 43% of this year class were mature at 2-ring compared to 77% and 86% for 1998 and 1999 year classes.

This year at 3-ring only 65% are mature compared to 97% and 93% for 1998 and 1999 year classes. If this year class had grown and matured as previous years, to 95% mature, and 177g the spawning stock biomass would have been 21% higher at 3.1 million tonnes, but if the 2003 estimate were treated in the same manner (i.e. use average growth and maturity) it would give an SSB of 3.8 million tonnes, still suggesting a decline in the last year. The survey shows again the two exceptional year classes of herring (the 1998 and 2000 year classes) in the North Sea, which is consistent with the observation of exceptionally large year classes observed in the MIK and IBTS surveys (ICES 2001/ACFM:12). The 2004 estimate of the 2000 year class suggests that it may be higher than the 1998 year class at 1.1 times at age 3-ring.

The numbers and biomass of adult autumn spawning herring can be seen in Figures 2.3.1.2, the numbers at 1, 2 and 3+ rings in Figure 2.3.1.3. The spatial distribution of mean weight at 1 and 2 ring, and fraction mature at 2 and 3 ring are given in Figure 2.3.1.4. These show a con-siderable spatial trend which is observed each year, with larger more mature fish found in the North and smaller less mature fish found in the south and particularly the eastern north Sea.

The relative spatial distributions of adult and juvenile autumn spawning herring can be seen in Figures 2.3.1.5 and 2.3.1.6 respectively. The mean weights-at-age and the fraction mature are used in the assessment, the influence of the precision of this data were discussed in detail in section 2.10 last year.

2.3.2 Larvae surveys

In 2004/05 The Netherlands and Germany participated in six surveys and managed to cover six out of ten areas. The survey effort is comparable to previous years. The areas and time periods (including numbers of samples, vessel-days in sampling and area coverage) are given in Table 2.3.2.1 and Table 2.3.2.2. The spatial extent of the surveys is shown in Figures 2.3.2.1 – 2.3.2.6. The historical background of the larvae surveys and the methods used for abundance calculation are described in the handbook for quality control. A more detailed de-scription is available in the manual for the international herring larvae surveys in the North Sea (ICES 2004/G:05).

Each surveys in 2004 resulted in high abundance estimates. In the Orkney/Shetlands area a large spatial extension of newly hatched larvae and high larvae aggregations were observed eastwards the Orkneys as in previous years (Figure 2.3.2.1). The overall abundance in this area varies greatly between years. In 2004 the estimates are average in recent time-series.

In the Buchan area (Figure 2.3.2.2) larval distribution was spread out compared to last year.

The LAI increased substantially during the last three years.

The LAI for the Central North Sea (Figure 2.3.2.3) has reduced compared to last sampling period, but the 2003 estimate was influenced by large catches at single stations. The CNS still yields a high abundance estimate. The LAIs in the CNS continuously rise over the last seven years.

Abundance estimates from the three surveys in the Southern North Sea resulted in a high in-dex which is almost comparable to last year. Spawning starts in the second half of December in a restricted area in VIId and then spread out into VIc during January (Figure 2.3.2.4-6). As usual, an area from the French coastline to the middle of the Channel contributed most to the abundance index in the Southern North Sea.

As a general pattern, herring seem to have recolonized the sampled spawning grounds in a broader range. An overview of the historic trends for a collection of sampling areas and peri-ods is given in Figure 2.3.2.7.

The model for the Multiplicative Larval Abundance Index (MLAI) was fitted to abundances of larvae less than 10 mm in length (11 mm for SNS) (Table 2.3.2.3). The analysis of variance and the parameter estimates are given in Table 2.3.2.4. The updated MLAI time-series is shown in Table 2.3.2.5. The estimated trend in spawning stock biomass from this model fit is plotted in Figure 2.3.2.8 versus the SSB values obtained from the ICA runs of the Herring Assessment Working Group (ICES 2004/ACFM:18).

The result of the survey in the 1^st period in the SNS is influenced by a large catch at one single station. Almost 12,000 larvae per m² were caught which contribute roughly 70% to the total catch. As a general rule, additional stations should be inserted in areas with high larval con-centrations to enable average calculation. Unfortunately this wasn’t done here. However, there are some routines in the MLAI calculation to make it robust against patchiness effect. Exclu-sion of the high catch leads to a difference less than 3% on the MLAI estimate. With respect to the general noise in survey data this impact can be neglected. Thus no data were excluded from the MLAI calculation. Both the LAI per unit as well as the MLAI from the larvae sur-veys in period 2004/2005 indicate that the SSB has slightly increased when compared to last years WG estimate.

2.3.3 International Bottom Trawl Survey (IBTS)

The International Bottom Trawl Survey (IBTS) started out as a young herring fish survey in 1966 with the objective of obtaining annual recruitment indices (abundance of 1-ringers in 1^st quarter) for the combined North Sea herring stocks. It has been carried out every year since,

ICES HAWG Report 2005 48

and presently the survey provides recruitment indices not only for herring, but also for round-fish species as well. Examinations of the catch of adult herring during the 1^st quarter IBTS have shown that this catch also indicates abundances of 2-5+ herring. Also during IBTS 1^st quarter, herring larvae are sampled during the night by small, fine-meshed nets. From 1977 to 1991 the gear was a small mid-water trawl (IKMT), but due to poor catchability of this gear, the standard gear was changed to a 2 metre ring net (MIK), used since the 1991 sampling. The total abundance of herring larvae in the survey area is used as an estimate of 0-ringer abun-dance of the stock. Hence, a series of herring abunabun-dance indices (0-5+ ringers) are available from the IBTS programme.

2.3.3.1 Indices of 2-5+ ringer herring abundances

Fishing gear and survey practices were standardised from 1983, and herring abundance esti-mates of 2-5+ ringers from 1983 onwards has shown the most consistent results in assess-ments of these age groups. This series is subsequently used in North Sea herring assessment.

Note that the abundances in Division IIIa are not included in the 2-5+ ringer indices. Table 2.3.3.1 shows the time-series of abundance estimates of 2-5+ ringers from the 1^st quarter IBTS for the period 1983-2005, when Table 2.3.3.2 contains area-disaggregated information on the IBTS indices for year 2005.

2.3.3.1 Index of 1-ringer recruitment

The 1-ringer index of recruitment is based on trawl catches in the entire survey area. Indices are available for year classes 1977 to 2003 (Table 2.3.3.3). This years estimate of the 2003 year class strength (1033) indicates a very low recruitment, among the lowest on record.

Figure 2.3.3.1 illustrates the spatial distribution of 1-ringers as estimated by the trawling in February 2003, 2004 and 2005. In 2005 the main concentrations of 1-ringers were found in the south-eastern part of the North Sea. The mean length of the 1-ringers in this area is relatively small, between 10 and 14 cm (Figure 2.3.3.2.).

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. A recruitment index of smaller sized 1-ringers is calculated based on abundance estimates of herring <13 cm (see discussion of pro-cedures in earlier reports (ICES CM 2000/ ACFM:12, and ICES CM 2001/ ACFM:12).

Table 2.3.3.3 includes abundance estimates of 1-ringer herring smaller than 13 cm, based on 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, and their relative proportions are also shown. In the time-series, the proportion of 1-ringers smaller than 13 cm (of total catches) is in the order of 20%, and the contribution from division IIIa to the overall abundance of <13 cm herring varies markedly during the period. (Table 2.3.3.3)

About 35% of this years group of 1-ringers is smaller than 13 cm. These are almost exclu-sively found in the North Sea area (Table 2.3.3.3)

2.3.3.2 The MIK index of 0-ringer recruitment

This years 0-ringer index is based on 544 depth-integrated hauls with a 2 metre ring-net (the MIK). Index values are calculated as described in the WG report of 1996 (ICES 1996/ACFM:10). The series of estimates is shown in Table 2.3.3.4, the new index value of 0-ringer abundance of the 2004 year class is estimated at 61.3.

This estimate indicates a very low recruitment, of the same size as estimated for the last two year classes, 2002 and 2003. The 0-ringers were distributed westerly and southerly in the North Sea with highest concentrations in the south-western areas. However, compared to the

preceding two year classes, which is also shown in Figure 2.3.3.3, the 0-ringers of this year class are distributed in a wider area of the North Sea. This is also apparent from Figure 2.3.3.4, which illustrates the changes in absolute and relative abundance of 0-ringers in the western part of the North Sea. The relative abundance is given as the number of 0-ringers in the area west of 2°E relative to the total number of 0-ringers in the given year class. Since the year class 1982, when the relative abundance was 25%, a general increase has been seen for the western part. In the last decade, the 0-ringer abundance in this area has dominated, during the preceding two years the relative abundance was in the order of 85%, while in 2005 the relative abundance declined to ca. 55%.