Ecological investigations of the zooplankton community of Balsfjorden,northern Norway: Population structure and breeding biology of the chaetognath Sagitta elegans Verrill

The population dynamics of the chaetognath Sagitta elegans Verrill has been followed in Balsfjorden in 1976 and 1977. Seasonal variation in abundance, length-frequency distribution, growth in total length, and maturity stages are presented and discussed in relation to changes in hydrography.An annual generation of S. elegans was found, with a protracted and more or less continuous breeding season from May until October during 1977. The 1976 year-class consisted of two distinct length groups, both of which participated in the 1977 spawning. This spawning gave rise to possibly four sub-populations during 1977. The variation in numbers of sub-populations produced during the spawning season in 1976 and 1977 is discussed in relation to the hydrographical conditions in Balsfjorden. From November 1976 to March 1977 the abundance of S. elegans varied between 1 and 8 ind. · m^?3. The lowest value was recorded in May (0.9 ind. · m^?3). From September to December 1977 the population abundance was ≈2 ind. · m^?3.