Plattform Platform	Reise-Nr. Cruise-No.	Zeitraum Period	Projekt Project	Arbeitsgebiet Working area
Clupea	280	11.03.2014 - 25.06.2014	RHLS	Baltic Sea Greifswalder Bodden

Fahrtleiter Chief Scientist	Institut Institute	Auslaufhafen Port of departure	Einlaufhafen Port of return	Stationskarte Station map	Schiffsroute Trackchart
Polte Patrick	Thunen-Institut fur Ostseefischerei	Stralsund	Sa?nitz	Karte Map

Ziel der Reise / Objectives of Cruise:

The waters of the Greifswalder Bodden (ICES area 24) are considered a major spawning area of Western Baltic spring spawning (WBSS) herring. The German Thunen Institute of Baltic Sea Fisheries (TI-OF), Rostock, and its predecessor monitors the density of herring larvae as a vector of recruitment success since 1977 within the framework of the Rugen Herring Larvae Survey (RHLS). It delivers a unique high-resolution dataset on the herring larvae ecology in the Western Baltic, both temporally and spatially. Onboard the research vessel "FFS Clupea" a sampling grid including 35 stations is sampled weekly using ichthyoplankton gear (Bongo-net, mesh sizes 335 μm; 780 μm) during the main reproduction period from March to June. The weekly assessment of the entire sampling area is conducted within two days (detailed description of the survey design can be found in Polte 2013, ICES WD08). The data collected provide an important baseline for detailed investigation of spawning and recruitment ecology of WBSS herring stocks. As a fishery-independent indicator of stock development, the recruitment index is then incorporated into the assessment of the ICES Herring Assessment Working Group. The rationale for the N20 recruitment index is based on strong correlation found between the amount of 20 mm (TL) herring larvae in the Greifswald Bay and abundance data of juveniles (1wr and 2 wr fish) as determined by acoustic surveys in the Arkona and Belt Seas (GERAS). Those recurring correlations support the underlying hypotheses that i) major variability of natural mortality occurs at early life stages before larvae reach a total length of 20 mm and ii) annual recruitment strength of the WBSS herring stock is not independent of larval production in the particular Greifswald Bay area. The N20 recruitment index is calculated every year based on data obtained from the RHLS. This is done by correcting weekly growth of larvae for seasonal temperature change and taking the sum of larvae reaching ≥ 20 mm by every survey week until the end of the investigation period. On the spatial scale, the 35 sampling stations are assigned to 5 strata and mean values of stations for each stratum are extrapolated to the strata area (for details see Oeberst et. al 2009). The sum of 20 mm larvae caught over the investigation period in the entire area results in the N20 recruitment index for those herring that enter the fishery as adults two to three years later. Calculation procedures have been reviewed and re-established in 2007 and the recalculated index for the time series from 1991 onwards is used by HAWG since 2008 as the sole 0-group recruitment index for the assessment of Western Baltic Spring Spawning herring.

Messungen / Measurements