Invasive species in the Baltic Sea and their impact on commercial fish stocks

The brackish, semi-enclosed Baltic Sea is especially vulnerable to the possible negative impacts of invasive species. In the study, scientists examine the effects on commercial fish stocks of four species established in the larger areas of the Baltic Sea:

• the round goby, a fish that has become an important prey for many predatory fish and birds, but it can also compete successfully with native fish for prey. 
• the sea walnut, a ctenophore jellyfish that has invaded the southern part of the Baltic Sea where it preys heavily on zooplankton, thereby potentially outcompeting planktivorous fish such as herring and sprat. Initial concerns that the sea walnut may prey on fish egg and larvae were put to rest after studies showed no such effects. 
• the mud crab that has become a stable component of benthic communities in most of the Baltic Sea. No negative effects of this invader have been reported; however, mud crabs have become an important prey for benthic fish in many areas.
• the fishhook water flea, a small planktonic crustacean that has invaded the Central Baltic Sea, and its role has become double-edged. It contributes significantly to the diet of herring, sprat and smelt in the Baltic Sea, while also competing with these other fish species for zooplankton as prey.