Disentangling the potential effects of four non-indigenous species on commercially and recreationally used fish stocks in the Baltic Sea-a review

Non-indigenous species (NIS) are on a rise globally. They can pose strong impacts on ecosystems in their non-native range and can therefore be a serious threat to biodiversity. Here, we compile the existing information available regarding the extent to which commercially and recreationally used fish stocks (exploited fish species) are affected by NIS. To do so, we conducted a literature review to summarize the known and presumed impacts of four case study NIS with already known strong effects on the Baltic Sea ecosystem: round goby, Neogobius melanostomus, sea walnut, Mnemiopsis leidyi, mud crab, Rhithropanopeus harrisii, and fishhook water flea, Cercopagis pengoi. We found that round goby, mud crab and fishhook water flea are documented to serve as a new food source for native fish species, while sea walnut and fishhook water flea are supposedly affecting planktivorous fish through resource competition. Round goby is very likely a strong competitor for the benthivore fish community. There are also indications that it feeds on juvenile fish and fish eggs. Generally, our results show that large knowledge gaps exist, while the published impacts on exploited fish species are often solely based on correlations (e.g. decreasing abundances of native species with cooccurring increasing abundances of NIS), regionally restricted studies or expert judgements. In addition, many studies are older and the current population size of the NIS, which is obviously associated with their impact, is unknown. Thus, the majority of described impacts of NIS on commercially and recreationally used fish stocks seems to stem from assumptions. Therefore, more field observations and experimental studies are needed to be able to scientifically evaluate the impact of NIS. Nevertheless, in this review, the available information was summarized, even if they are speculative, and specific knowledge gaps were identified. Moreover, we outline further investigations that are needed to advance our mechanistic understanding of the interactions between NIS and exploited fish species in the Baltic Sea. This knowledge is essential for the sustainable management of aquatic resources and management of NIS of the Baltic Sea.