Rapid Establishment of the Alien Crab Rhithropanopeus Harrisii (Gould) in the Gulf of Riga/ Mudakrabi Rhithropanopeus Harrisii Kiire Naturaliseerumine Liivi Lahes

Abstract

INTRODUCTION Owing to its low salinity and short evolutionary history, the Baltic Sea virtually lacks native top predatory crabs. This contrasts to true oceanic waters where crabs constitute an essential element of the nearshore benthic ecosystems (e.g. Lee, 1998). To date, the only 'iconic' species in the Baltic Sea range is the Chinese mitten crab Eriocheir sinensis (H. Milne-Edwards). Despite occasional findings of this invasive species all over the coastal Baltic Sea, the reproduction of the mitten crab in the central, northern, and eastern Baltic regions is considered unlikely due to the low salinity, and the individuals caught are assumed to have actively migrated into the region over 1500 km distance (Ojaveer et al., 2007). However, the Baltic Sea also hosts the Harris mud crab Rhithropanopeus harrisii (Gould) (Fig. 1). This species has a native distribution from New Brunswick (Canada) to Veracruz (Mexico). In Europe the invasive species was first found in 1874 in the Netherlands, in the Baltic Sea area it was first observed in 1936 (Nikolaev, 1951). In contrast to E. sinensis, the mud crab is capable of reproducing in the diluted Baltic Sea and can form high-density populations (Maiju Lehtiniemi, pers. comm.); thus, it may exert strong pressure on the local benthic macrophyte and invertebrate communities. Despite its long invasion history, until very recently the distribution of R. harrisii was confined to Mecklenburg Bay, the Oder Estuary, the Gulf of Gdansk, and the Curonian Lagoon only (Jazdzewski & Konopacka, 1993; Bacevicius & Gasiunaite, 2008; Czerniejewski, 2009; Jazdzewski & Grabowski, 2011). Seemingly, R. harrisii has a large between population variability in Europe with more recent populations showing a tendency for increased genetic diversity (Projecto-Garcia et al., 2010). This suggests that R. harrisii is still in the process of expansion in Europe and its sudden northwards expansion seems to be a result of new introductions. In this study we report the sudden expansion of this alien invasive species over 500 km northwards and provide information on its distribution. [FIGURE 1 OMITTED] MATERIAL AND METHODS Crabs are not systematically monitored in the Estonian coastal sea. Nevertheless, since 1995 the Estonian National Monitoring Programme surveys benthic macrophytes and the associated invertebrates around the whole Estonian coastal range. In addition, the programme also surveys pelagic communities. The phytobenthos and zooplankton sampling and sample analysis follow the guidelines developed for the HELCOM COMBINE programme (HELCOM, 2012). Although not specifically targeted towards large and mobile invertebrates, the national monitoring programme is potentially capable of sighting the mud crab both in benthic and pelagic ecosystems. In addition, there is another local long-term activity that provides semiquantitative data on crabs. Specifically, this is done through the provision of artificial spawning substrata for the commercially valuable fish pike-perch in Parnu Bay. This activity was started already in the late 1980s and has been done every year since then. Most of these artificial spawning substrata consist of linen gillnets of small mesh size. Such substratum may also provide a suitable habitat for the Harris mud crab and, therefore, may provide occurrence information on the species. These artificial spawning substrata were checked for the mud crab occurrence in 2011-2012, also with SCUBA diving, which was also performed in several additional localities in Parnu Bay to map the distribution of the species. Finally, owing to good long-term relations with local professional fishermen, several occurrence data on the crab were obtained from them. RESULTS AND DISCUSSION The Harris mud crab R. harrisii was first found in the Estonian waters in August 2011. Seven individuals were found in three deployed lines of pike-perch artificial spawning substrata at the northern coast of Parnu Bay (NE Gulf of Riga) (Fig. …