Decreased levels of the invasive ctenophore Mnemiopsis in the Marmara Sea in 2001

Mnemiopsis leidyi, an endemic ctenophore of the western Atlantic, was first reported in the Marmara Sea in summer 1992, at an average abundance of 27 ind. m−3. We investigated its abundance and distribution together with that of the mesozooplankton (most species are prey organisms of M. leidyi) at eight stations during August 2001, in order to evaluate recent population developments with regard to the impact of its predator, Beroe cf ovata. The abundance of M. leidyi was found to be quite low (range 0.5–8.8 ind. m−3, average 1.62 ind. m−3) compared to 1992. B. cf ovata was, although at very low abundance (0.1–1.1 ind. m−3), only present at stations where M. leidyi occurred. The zooplankton abundance was higher during our investigation than in previous years, which should at least partly be due to a decreased predation impact by M. leidyi.