Abstract
The distribution and abundance of the invasive ctenophore Mnemiopsis leidyi in the Bornholm Basin, an important spawning ground of several fish stocks, and in adjacent areas in the central Baltic Sea was studied in November 2007. The study showed that M. leidyi were relatively small (body length 18.6 ± 7.6 mm) and they were patchily distributed over a large part of the investigated area. Specimens were found on 68 and 59% of stations sampled with a Bongo net (n=39) and an Isaac-Kidd midwater trawl (n=51), respectively. Vertically, the highest densities of M. leidyi occurred at 40 to 60 m around the halocline. Horizontally, the highest abundances were found north and west of Bornholm, but relatively high densities were also observed in the Slupsk Furrow. The mean abundance was 1.58 ± 2.12 ind. m-2, the peak abundance was 8.92 ind. m-2, and the average and peak population density were 0.03 ± 0.05 and 0.28 ind. m-3, respectively. The abundances are low compared to densities recently observed in other areas of the Baltic region (e. g. Limfjorden, Aland Sea) and the estimated predation impact on zooplankton by M. leidyi was negligible in November 2007. However, because of the ctenophore’s wide distribution in the central Baltic Sea, its ability for rapid population growth, and its potential influence on fish stocks by competing for food and by preying on fish eggs and newly hatched larvae, close monitoring of the future development of M. leidyi in the Baltic Sea is strongly recommended.
Highlights
Several studies have recently described occurrences of the invasive lobate ctenophore Mnemiopsis leidyi A
The distribution and abundance of the invasive ctenophore Mnemiopsis leidyi in the Bornholm Basin, an important spawning ground of several fish stocks, and in adjacent areas in the central Baltic Sea was studied in November 2007
While the size distributions of M. leidyi in the catches using the Multinet and the Bongo net were similar, the size distribution of M. leidyi retained in the Isaac-Kidd midwater trawl (IKMT) and Bongo nets differed due to the different mesh sizes
Summary
Several studies have recently described occurrences of the invasive lobate ctenophore Mnemiopsis leidyi A. The ctenophore was accidentally introduced with cargo ballast water into the Black Sea in the early 1980s, where its explosive population growth coincided with a breakdown of, in particular, the anchovy fishery (Vinogradov et al 1989; Mutlu 1999; Kideys 2002). It spread into adjacent waters of the Mediterranean and to the Caspian Sea (Kideys and Niermann 1993; Shiganova 1993; Shiganova et al 2001). In 2007, observations of M. leidyi in Danish waters and the Baltic Sea commenced to become numerous, and in many cases there were indications of mass occurrences (Tendal et al 2007; Lehtiniemi et al 2007). These values considerably exceeded abundances reported from the northern Baltic Sea during spring and summer 2007 (Lehtiniemi et al 2007) and even most values reported from the Black Sea during the period when the zooplankton and fish stocks collapsed in this area (Purcell et al 2001b; Shiganova et al 2001)
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