Time series of the Ruegen Larval Herring N20 Index and potential reasons for a declining larval production
The inshore waters of Strelasund/Greifswalder Bodden (ICES SD 24) are considered a major spawning area of Ruegen herring which represents a significant component of the Western Baltic spring spawning (WBSS) herring stock. The German Institute of Baltic Sea Fisheries (TI-OF), Rostock monitors the density of herring larvae as a proxy of recruitment success within the frame work of the Ruegen Herring Larvae Survey (RHLS). As a fishery-independent indicator of stock development, the recruitment index is incorporated into the ICES Herring Assessment Working Group (HAWG) advice since 2008 as the only 0-group recruitment index for the assessment of Western Baltic Spring Spawning herring. Additionally, the collected data provide an important baseline for detailed investigation of herring spawning- and recruitment ecology. This Working Document intents to provide an update on the status of the N20 time series under the light of recent declines of larval herring production. Based on ecological case studies, new insights and additional hypotheses on potential reasons for the decline are presented.