Changes in distribution and lengths of Mnemiopsis leidyi in the central Baltic Sea between fall and spring

In March 2008, we conducted a survey to investigate the distribution and abundance of the invasive ctenophore Mnemiopsis leidyi in the Bornholm Basin after the winter period. Compared to the situation in November 2007 the centre of the distribution had shifted towards the deeper parts of the Basin where temperatures were higher than at the surface. Furthermore, we found a decrease in average size from 18.6 mm ± 7.6 SD in November to an average of 10.5 mm ± 4.9 SD in March, which may indicate the emergence of a new generation of M. leidyi between the two sampling dates.

Since the first observations of Mnemiopsis leidyi A. Agassiz 1865 in Northern Europe (Faasse and Bayha 2006;Hansson 2006;Javidpour et al. 2006;Oliveira 2007), several studies have described occurrences of this invasive lobate ctenophore in the Baltic, spanning from Kattegat to the Bothnian Sea Janas and Zgrundo 2007;Kube et al. 2007;Lehtiniemi et al. 2007;Tendal et al. 2007;Javidpour et al. 2008;Viitasalo et al. 2008). In November 2007, Huwer et al. (2008 investigated the horizontal and vertical distribution of Mnemiopsis leidyi in the central Baltic Sea. The studied area covered the Bornholm Basin, an important spawning ground for cod and sprat (Köster et al. 2005). Four months later, in March 2008, we conducted a survey in the same area to investigate if M. leidyi had survived the winter and how the distribution and abundances of this invasive species had changed during wintertime.
M. leidyi were sampled during a cruise with the Danish research vessel 'DANA' from March 1st to 17th 2008, on a station grid covering the Bornholm Basin. Additional sampling was conducted on a transect through the Arkona Basin consisting of 6 stations ( Figure 1). To assess abundances and the horizontal distribution of M. leidyi, hauls with a Bongo net were taken on a total of 75 stations. The gear was equipped with two nets of different mesh size (335 μm and 500 μm) and with flowmeters to measure the volume of filtered water. Double oblique hauls from the surface to 2 meter above the seafloor were conducted at a towing speed of 3 knots.
Upon retrieval of the gear, specimens of M. leidyi were immediately sorted from the samples, counted and the body oral-aboral length of each individual was measured to the nearest 1.0 mm with a sliding caliper. All samples were analysed within 30 minutes after capture. Counts of M. leidyi were standardised to 1 m 2 surface area by accounting for the filtered water volume and the maximum depth of the tow.
A total of 208 and 261 specimens of M. leidyi were caught in the 335 µm and 500 µm net, respectively. There was no significant difference (P = 0.634) between the size ranges of specimens caught with the two different mesh sizes, as average lengths were 10.2 mm ± 4.6 SD and 10.7 mm ± 5.1 SD for the 335 μm and 500 μm net, respectively. Accordingly, samples from the two nets have been pooled in the further analysis.
For a comparison of length distributions of M. leidyi between fall and spring, length data from November 2007 were used. The sampling procedure for these data is described in Huwer et al. (2008).

Horizontal distribution and abundance
Low temperatures have been found to be a limiting factor for the distribution and growth of M. leidyi (Kremer 1994;Mutlu 1999;Purcell et al. 2001;Shiganova et al. 2001;Purcell and Decker 2005). Therefore, we were interested in investigating the abundance and distribution of M. leidyi in the central Baltic after the winter period. Huwer et al. (2008) found M. leidyi to be present over large areas of the central Baltic Sea in November 2007. In the present study in March 2008 the species was again distributed over large parts of the area. However, in comparison to the situation in fall 2007, the centre of the distribution had shifted to the deeper central parts of the Bornholm Basin (Figure 1). During the present study in spring 2008, no ctenophores were found on the 18 stations with depths <50 meters. The abundances of M. leidyi increased with increasing depth and almost 90% of the specimens were registered on stations with depths >70 meter (Table 1).
A possible reason for this change may be colder winter temperatures above the halocline in the Bornholm Basin. Figure 2 shows temperature and salinity profiles from a station in the central Bornholm Basin (Station 23 in Figure 1 and Annex 1; 55.292°N, 15.750°E) in November 2007 and March 2008. The situation in March showed a well mixed surface layer with a temperature of 4°C, and higher temperatures of 6 to 8°C in the bottom layer below 50 m depth. In November no such distinct temperature difference was detected and the surface water was much warmer with temperatures of about 9.5°C. In contrast to temperature, salinity did not seem to have an influence on the change in distribution in the present study. The salinity profiles showed little differences between fall and spring due to the permanent halocline in the Bornholm Basin. Kube et al. (2007) have shown that M. leidyi was able to survive the winter in the Baltic Sea. Even though abundances at a sampling station in the Bornholm Basin were generally low, they found a decrease in abundances from February to May.  Abundance peaks were observed in late summerearly autumn, which is in good correspondence to population dynamics in native habitats of M. leidyi (Kremer 1994   The maximum abundance of M. leidyi in the Bornholm Basin in November 2007 was 8.92 ind. m -2 , with an average level of 1.58 ± 2.12 ind. m -2 (Huwer et al. 2008). In the present study four months later, abundances were lower with a maximum abundance of 4.83 ind. m -2 , and an average level of 0.43 ± 0.86 ind. m -2 .
However, even though abundances had decreased from fall to spring, M. leidyi has survived the second winter after its introduction to the central Baltic.

Conclusions
It can be concluded that M. leidyi was still present in the Bornholm Basin after the winter 2007/2008 and that the size distribution had shifted to smaller specimens, which may indicate the emergence of a new generation. Furthermore, there has been a shift in the horizontal distribution towards deeper parts of the Basin from fall to spring. Accordingly, M. leidyi may migrate from the colder surface layer to the warmer deep water during winter, while expanding its distribution to warmer upper layers and coastal areas during summer and fall. A similar temperature dependent vertical distribution was reported by Javidpour et al. (2008) for the M. leidyi population in Kiel Fjord. Other possible reasons for the changing distribution might be passive advection or differences in food availability. Thus, the present study provides further support for the hypothesis that M. leidyi uses the deep layers of the Bornholm Basin as overwintering refuge (Kube et al. 2007;Huwer et al. 2008).
However, to obtain a better understanding of M. leidyi distribution and population dynamics in the central Baltic, a temporally explicit monitoring is highly advisable. M. leidyi is known to prey on fish eggs and larvae (Cowan and Houde 1993;Purcell and Arai 2001;Purcell et al. 2001) and to compete with fish for zooplankton prey. Therefore, the seasonal development of this invasive predatory species receives special interest in the light of spatio-temporal interactions with ichthyoplankton in the Bornholm Basin Huwer et al. 2008). This area is at present the most important spawning ground for Eastern Baltic cod, a fish stock already suffering from unfavorable environmental conditions and heavy exploitation (Köster et al. 2005).