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
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.
Nutzung und Vervielfältigung:
Alle Rechte vorbehalten