Separating two herring stocks from growth data: long‐term changes in survey indices for Western Baltic Spring Spawning Herring (Clupea harengus) after application of a stock separation function |
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| Authors: | T Gröhsler M Schaber N Larson R Oeberst |
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| Institution: | 1. Thünen Institute of Baltic Sea Fisheries (TI‐OF), Rostock, Germany;2. Thünen Institute of Sea Fisheries (TI‐SF), Hamburg, Germany;3. University of Agricultural Sciences, Department of Aquatic Resources, Institute of Marine Research, Lysekil, Sweden |
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| Abstract: | In the Baltic Sea, herring stocks are surveyed and managed according to a spatial allocation based on ICES (International Council for the Exploration of the Sea) subdivisions. In the western Baltic, the distribution areas of two stocks overlap: the Western Baltic Spring Spawning Herring (WBSSH) and the Central Baltic Herring (CBH). Survey results of length‐at‐age data indicate in Subdivision (SD) 24, which is a part of the WBSSH management area, that a considerable fraction of CBH is present and correspondingly erroneously allocated to WBSSH stock indices. Accordingly, a stock Separation Function (SF) based on growth parameters was established to identify the fraction of CBH in the WBSSH area. In the present study, the SF was applied to 8 years of data from the GERman Acoustic Survey (GERAS), which is conducted annually in autumn in ICES subdivisions 21‐24. Results showed a distinct fraction of CBH in SD 24, and exclusion of the CBH greatly improved the quality of the GERAS index used in the assessment of the WBSSH stock. |
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