. Separation of Spring and Autumn Spawning herring in the Skagerrak and Kattegat

Acronym:

Funding agency: European Union (EU CFP Study 96/073)

Project duration: 24 months (03/97-02/99)

Objectives
• to evaluate scientific methods to discriminate herring stocks in the Kattegat and Skagerrak into spring and autumn spawning stock components with the specific aim to develop routine methods of such discriminations for assessment purposes. The methods to be evaluated are vertebrae counts, microstructure otolith analysis and otolith macrostructural (annuli) analysis.

• to estimate the partitions of spring and autumn spawning herring in the sea and in the commercial landings of the Kattegat and Skagerrak during the last 5 years. The anlysis will be based on otolith samples and metric measurements obtained from research surveys and commercial catches in 1991-1995 by the participating Swedish, Danish and German fishery research institutes.

• to use the results of the analysis for the ICES Herring Assessment Working Group for the Area South of 62°N (HAWG) as a basis for a reliable analytical assessment of the spring spawning herring stock in the Southwest Baltic and the Kattegat and Skagerrak.
Due to the urgent need of data for stock assessment purposes the analysis will proceed in two steps:
Firstly, existing data on vertebrae counts will be sub-sampled with the objective to identify variations by season, sub-area and depth strata. The vertebrae data will be compiled and compared to estimates obtained by previous ICES working groups, study groups and from literature data. Preliminary results will be presented to the Herring WG in 1997.
Secondly, results of the initial vertebrae analysis will also be used to sub-sample otoliths for the macro- and microstructural analysis. Thereafter analysis by the three methods will proceed in parallel, Progress and annual compilations of results will be reported to the relevant ICES working groups.

Outline:  
Catches of herring in the Kattegat and Skagerrak are taken from a mixture of two main spawning stocks (Anon., 1991): the Baltic spring spawners and the North Sea autumn spawners. In addition several local stocks have been identified (Jensen, 1957). These are however considered to be less abundant and therefore of minor importance to the herring fisheries (Anon., 1991).
The North Sea autumn spawners (NSAS) enter Skagerrak and Kattegat as larvae and migrate back to the North Sea at an age of 2-3 years (Rosenberg & Palrnen, 1982). The Baltic spring spawners (BSS) spawn around the Baltic island Rügen. They enter the Belt Sea, Kattegatt and Skagerrak as adults after spawning (Biester; 1979).
The herring stocks in the Kattegatt and the Skagerrak have traditionally been separated by the average counts in number of vertebrae in herring samples (Rosenberg & Palmen, 1982; Gröger & Gröhsler, 1995). NSAS have a mean number of 56.5 vertebrae while the BSS are represented by a lower mean number, 55.7 vertebrae. The most prominent local spring spawning herring, the Skagerrak spring spawners (SSS), are represented by a higher number, 57.0 vertebrae.
Following the tradition from Heinke (1898) several other morphometric and metric variables have been used to seprarate herring stocks (Rosenberg & Palmen, 1982). The use of most of these variables was evaluated by an ICES study group in 1992 (Anon., 1992). The group concluded that a simple modal length analysis of the relevant 1-2 age groups would be precise enough for routine assessment purposes.
However, modal length analysis has proved to be an imprecise measure requiring a large sampling effort. Experience within the Herring assessment working group showed that the separation procedure often failed. The amounts of herring catches that was allocated to the NSAS stock have varied between 30 to 50% of total annual landings during the last 10 years. Errors in the estimate of this withdrawal will clearly affect the quality of the assessment of the BSS stock. A more precise measure is needed.
The diameter of the first winterring (annuli) on the otoliths of autumn spawrers are significantly larger than for spring spawners (Rosenberg & Palmen, 1982). The analysis of otolith annuli has however not been applied on a routine basis in the Kattegat -Skagerrak area, may be since it has been more labour demanding. New image analysis systems can however remove this obstacle:
Microstructural otolith analysis have also been tested to separate spring and autumn spawners (Moksness & Fossum, 1991). Otolith growth, which can be inferred from microscopical examination, are significantly slower for autumn spawners. Mosegaard & Popp-Madsen showed in a working paper (1996) that the processing speed of the method can be accelerated by image analysis and training. The disadvantage of a lower number of measurements are outweighted by a very high precision. Efficient grinding methods opens the possibility to include ages over 2 years old in a routine examination.

References

Anon, 1991. Report of the Herring Assessment Working Group for the Area South of 62°N. ICES, C.M. 1991,Assess:15.
Anon 1992. Report of the Workshop on Methods of Forecasting Herring Catches in Div. IIIa and the North Sea. ICES, C.M. 1992/H:5.
Biester, E. 1979. Der Frühjahrshering Rügens. Doctoral Thesis. Wilhelm-Pieck Universität Rostock.
Gröger, J. & T. Gröhsler 1995. On the discrimination of herring stocks in Division IIIa. ICES, C.M. 1995/J:22.
Heinke, F., 1898. Naturgeschichte des Herings. Teil 1. Deutscher Seefischerei Verein. Band III.
Jensen, A.J.C., 1957. Danish herring investigations in the Skagerrak, Kattegat, Belts and Baltic. Ann. Biol. 14: 191-194.
Moksness, F & P. Fossum, 1991. Distinguishing spring and autumn spawned herring larvae (Clupea harengus L.) by otolith microstructure. ICES J. Mar. Sci. 48:61-66.
Mosegaard, H. & K. Popp-Madsen, 1996. Racial discrimination of herring stocks comparing vertebral counts and otolith microstructure analysis. Manuscript presented to the Herring Assessment W.G. S62°, April, 1996.
Rosenberg, R. & L.-E. Palmen, 1982. Composition of herring stocks in the Skagerrak-Kattegat and the relations of these stocks with those of the North Sea and adjacent waters. Fish. Res. 1:83-104.

Participating institutions:

Data entered/updated by (Date)
Tomas Gröhsler (21-07-00)