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

During a hydrographical survey in the northern Kattegat, 1975to 1977, vertical plankton hauls were taken monthly at fourstations between Göteborg and Frederikshavn. Hydrographicalparameters were measured at 10 stations along the transect.Sagitta setosa J. Mailer was by far the most dominant chaetognath(99% of individuals). Sagitta elegans Verrill and Eukrohniahamata Möbius occurred simultaneously with inflowing highsaline water during autumn and winter. The transport of S. setosaby currents was an important factor influencing the abundancesand the breeding period in the Kattegat. The abundance peakin 1977 was only one fifth of that found in 1975. The startand the duration of the breeding period (defined as presenceof stage 1 individuals <3 mm) varied between the three years.Breeding occurred between the middle of July and December witha peak in August and September. Changes in size distributionwere small during winter and spring, possibly due to low temperatures.S. setosa has probably a life span of one year in the Kattegat.Some earlier results regarding the number of breeding periodsof S. setosa per year are questioned. It is proposed that thenumber of generations per year of S. setosa and the start andthe duration of the breeding period(s) in western European watersshould be regarded as an open question until further informationis available.