Abundance, breeding and temporal size distribution of the chaetognath Sagitta setosa in the Kattegat

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Abstract
During a hydrographical survey in the northern Kattegat, 1975-1977, vertical plankton hauls were taken monthly at 4 stations between Goeteborg and Frederikshavn. Hydrographical parameters were measured at 10 stations along the transect. S. setosa J. Mueller was by far the most dominant chaetognath (99% of individuals). S. elegans Verrill and Eukrohnia hamata Morebius occurred simultaneously with inflowing high saline water during autumn and winter. The transport of S. setosa by currents was an important factor influencing the abundances and the breeding period in the Kattegat. The abundance peak in 1977 was only 1/5 of that found in 1975. The start and the duration of the breeding period (defined as presence of stage 1 individuals < 3 mm) varied between the 3 yr. Breeding occurred between the middle of July and Dec. with a peak in Aug. and Sept. Changes in size distribution were small during winter and spring, possibly due to low temperature. S. setosa has a life span of probably 1 yr in the Kattegat. Some earlier results regarding the number of breeding periods of S. setosa per year are questioned. The number of generations per year of S. setosa and the start and the duration of the breeding period(s) in western European waters should be regarded as an open question until further information is available.

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