| INTER- AND INTRA-ANNUAL VARIABILITY IN LARVAL RETENTION OF WESTERN BALTIC SPRING SPAWNING HERRING AND ITS EFFECTS ON RECRUITMENT SUCCESS |
| BAUER, R. K., Stepputtis, D., Hammer, C., Zimmermann, C., Thnen-Institute of Baltic Sea Fisheries, Rostock, Germany, 18069, rbauer@gmx.com, daniel.stepputtis@vti.bund.de, cornelius.hammer@vti.bund.de, christopher.zimmermann@vti.bund.de; Grwe, U., Leibniz Institute for Baltic Sea Research Warnemnde, Rostock, Germany, 18119, ulf.graewe@io-warnemuende.de |
| The stocks of Atlantic and Pacific spring spawning herring migrate to coastal, often sheltered and enclosed waters for spawning, likely to ensure a match of larval emergence and plankton blooms. Consequently, larval retention within these areas is considered to favor larval survival, contrary to larval drift to the lower productive surrounding deeper waters. Although larval retention of spring spawning herring has been observed at some spawning grounds, there is a general lack of knowledge about its inter- and intra-annual variability and related effects on recruitment success, as well as an understanding of the magnitude of importance of the retention in relation to other factors important for larval survival. Within this study, this problem was assessed for Western Baltic Spring Spawning Herring (WBSSH) in its main spawning area, the Greifswalder Bodden (GWB), a shallow lagoon in Northern Germany. The retention was analyzed using a two-dimensional Lagrangian particle tracking model. Simulating larval dispersal, this model was forced by depth-integrated flow fields of seven years, 2003 till 2009, which were derived from a high resolution three-dimensional hydrodynamic model. This period encompasses a period of dramatic, continuous decline in WBSS recruitment success from 2004 till 2008.
Weekly cohorts of virtual larvae were seeded in the entire GWB and at particular macrophyte covered areas where herring is known to spawn. Associated retention probabilities were related to larval abundances found in the GWB during the Rgen Herring Larvae Survey, which rendered the definition of a larval retention index. With this index the annual percentage of remained larvae of each model year is estimated.
The results impressively demonstrate that larval retention within the GWB is extremely high throughout the investigated model years, regardless the considered seeding area. Supplementary analysis on the effects of different wind fields further demonstrate that even at unfavorable wind conditions a significant proportion of larvae remain in the GWB. Thus, larval drift can not explain the observed variability of WBSS recruitment success. By contrast, the results suggest that larval retention is a crucial and stable feature of the life strategy of WBSS.
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