.

Fishery independent surveys: North Sea herring

The size of the spawning stock biomass of the North Sea autumn spawning herring is assessed mainly by two surveys. These are the International Herring Larvae Surveys (IHLS) and the hydroacoustic summer surveys (HAS). In addition, the size of the recruiting year classes is assessed by sampling older larvae with the Method Isaac Kitt Mid Water Trawl in the frame of the International Bottom Trawl Survey (IBTS) (MIK index). By contrast, the IHLS aims at the very young stages of freshly hatched herring in the vicinity of the spawning areas. The abundance of the freshly hatched larvae allows back-calculation of the biomass of parental fish having produced the offspring. The abundance index calculated from herring larvae abundance proved to be a very good and reliable estimator for the spawning stock size which is totally independent of the data provided from the fishery.

International Herring Larvae Surveys (IHLS)

The IHLS has begun already in 1967 and was coordinated by ICES from the beginning (Saville, 1968). However, in 1972 the IHLS was newly defined and further standardised (Anon., 1985, 1990). At the beginning of the survey series a great number of countries participated regularly, i.e. Denmark, Germany, England, Ireland, The Netherlands, Norway, Scotland and Sweden. In the 80ies however, the international participation declined and since the early 90ies only The Netherlands and Germany carry out the survey. For this reason the survey effort has been decreased by 4/5th of its original size.

Stratification. Since 1972 the cruises are carried out from September until end of January, following a fixed station grid. The temporal pattern is defined by half month survey periods and the geographical pattern by standard sampling areas. These areas originally covered most of the western part of the North Sea and the English Channel (Fig.1). Each sampling unit is one statistical rectangle of 30 x 30 nm and contains 9 stations, thus providing a dense sampling grid over the entire spawning area. However, due to the serious reduction in invested effort, the survey is now concentrated on the Shetland, Buchan, Central North Sea and English Channel area only.

Fig. 1: Areas of the International Herring Larvae survey. Zentrale Nordsee: Central North Sea, Südliche Nordsee: Southern North Sea.	Fig. 2: The Gulf Sampler "Nackthai", a modified Gulf III sampler, as used for the International Herring Larvae survey in the North Sea.

The sampling is done with a modified Gulf III sampler, essentially lacking the casting behind the entrance cone (model "Nackthai”) (Fig.2). Methodological details are given in Anon. (1985, 1990). The collected larvae under 24 mm total length are sorted into 1 mm length classes. For calculating the Larvae Production Estimate (LPE) the number of larvae under 17 mm length are used, whereas for the Larvae Abundance Index (LAI) only very young larvae of under 10 mm length are used. Until 1986 only the LAI was calculated and the LPE afterwards. For the calculation of a reliable LAI a reasonably good coverage in time and space must be given, since youngest fish larvae are highly patched. With shrinking effort invested into the survey difficulties appeared in sufficient coverage. Reacting to this, the LPE-Index was developed (Christensen 1985, 1989; Lassen & Pedersen 1985; Lough et al., 1985). For the LPE a specific daily hatch rate is calculated for each single mm-class of larvae. In the light of methodological problems with the inherent assumptions of the LPE calculations and the decline in survey participation and coverage the Multiplicative Larval Abundance Index was used since 1993, compensation mathematically for the gaps in coverage in time and space by utilizing multiple analysis of variance (Patterson & Beverage, 1994),


References

Anonymous (1985) Manual for the International Herring Larvae Surveys South of 62°North. ICES mimeo, 1985.

Anonymous (1990) Manual for the International Herring Larvae Surveys South of 62°North. ICES, mimeo, 1990.

Christensen, V. (1985) Estimation of herring larvae production. ICES CM 1985/H:60.

Christensen, V. (1989) Programmes for the International Herring Larvae Survey Working Group – Larvae Production Estimates (LPE). ICES mimeo.

Lassen, H., Pedersen, S.A. (1985) Growth, mortality and larvae production in the Shetland area estimated from the International Herring Larvae Surveys in 1972 – 1983. ICES CM 1985/H:48.

Lough, R.R., Bolz, G.R., Pennington, M. and Grosslein, M.D. (1985) Larval abundance and mortality of Atlantic herring (Clupea harengus L.) in the Georges Bank and Nantucket Shoals areas, 1971-78 seasons, in relation to spawning sizes. J. NW Atl. Fish. Soc., 6: 21-35.

Patterson, K. and Beverage, D.S. (1994) Report of the herring larvae surveys in the North Sea and adjacent waters in 1994/1995. ICES CM 1994/H:21.

Saville, A. (1968) Report on the International Herring Larvae surveys in the North Sea and adjacent waters, 1967/68. ICES C.M. 1968/H:20.

Entered/Updated by (Date)
Cornelius Hammer (15 Aug 00)