Population Status of Some Alien Species in Varna Bay, Bulgarian Black Sea Coast (2015-2016)

The introduction of alien species is a severe threat to marine environments. Enclosed or semi-enclosed ecosystems, as the Black Sea, seem particularly sensitive to biological invasions. With the increased shipping traffic, aquaculture and trade, the Black Sea has become a major recipient of alien species. A total of 80 species of different eco-functional groups were listed as alien in the Bulgarian Black Sea coastal waters. The aim of our study was to assess the population status of six alien species (four zooplankton, one benthic invertebrate and one fish species) in Varna Bay, the Bulgarian Black Sea coast, in the period 2015-2016. Two copepod alien species played a significant role in the mesozooplankton community structure, with a maximum share of 62% of the overall abundance in spring-summer for Acartia tonsa and summer-autumn for Oithona davisae. Large aggregates of the ctenophore species Mnemiopsis leidyi formed 'hot spots' along the coast during summer with a maximum of 516 ind. m(-2) in September 2015, followed by high abundance of Beroe ovata (107 ind. m(-2)) in October 2015. The average values of the shell length and total weight of the rapa whelk (Rapana venosa) were the lowest on shallow rocky bottom (5.89 cm and 22.27 g, respectively) and the highest at 21 m depth on silty sediments (6.84 cm and 55.13 g, respectively). The biomass of R. venosa ranged between 2.16 kg km(-2) and 19.28 kg km(-2), being the highest at sandy locations and the lowest at sandy-silty locations, both sampled by dredge. Fulton's condition factor ranged from 2.33 to 9.20. The catches of the alien redlip mullet (Liza haematocheila) in Varna Bay in the studied period were sporadic. According to information about catches for the period 2008-2016 and our observations, the species has become rare along the Bulgarian coast. A drastic reduction in its stock was observed from a maximum catch of 1.923 t in 2009 to 0.075 t in 2016.