Initial and long-term consequences of attempts to improve fish-food resources in Lithuanian waters by introducing alien peracaridan species: a retrospective overview By K. Arbacˇiauskas, V. Rakauskas and T. Virbickas Nature Research Centre, Vilnius, Lithuania Summary In this overview, we consider the rationale and practice of invertebrate introductions into Lithuanian waters, the current distribution of introduced species, test the theoretical expec- tations of introduction of peracaridan species into lakes, and summarize the large-scale consequences of these transloca- tions. After some early attempts to transfer locally available species within the country, peracaridan species from the Ponto-Caspian region were introduced into Lithuanian waters during the 1960s (Pontogammarus robustoides, Obesogamma- rus crassus, Chaetogammarus warpachowskyi, Paramysis lacus- tris, Limnomysis benedeni and Hemimysis anomala). After their establishment at the site of first introduction in a newly constructed water reservoir on the Nemunas River, further deliberate introductions into Lithuanian lakes and water reservoirs, as well as into more distant areas, were undertaken. These introduced species soon contributed to fish diet and a subsequent increase in the production of commercially impor- tant fish was envisaged. However, our collation of available data does not support the enhancement of fish production in Lithuanian lakes. Although perch (selected as a model fish species) assimilated the introduced species into diet and sometimes in large quantities, there was no subsequent influence on somatic growth rates when compared with perch from lakes devoid of alien peracaridans. Comparisons of littoral fish communities of different lakes, and commercial catches in the lake with the most numerous populations of introduced species also indicate no significant effect on fish production. Negative consequences of introduction are more than obvious in contrast. To date, most of the introduced Ponto-Caspian amphipod and mysid species occur in different sites of the Baltic Sea basin outside Lithuania, and their further dispersal may be expected. These species, especially amphipods, have proved to be highly ecologically aggressive, inducing change in resident macroinvertebrate communities and causing increase of biological contamination (i.e. detract from naturalness). Thus, the introduction of alien fodder species was ill advised from an economic, and even adverse from an environmental perspective. Introduction In the middle of the 20th century, it was common practice to introduce aquatic invertebrates in an attempt to improve fish food resources and consequently to enhance the production of commercially important fish, especially in the former Soviet Union (Karpevich, 1975). Peracaridan species, especially those of Ponto-Caspian origin, were among the most popular invertebrates used for introductions. It is noteworthy that peracaridan species of glacial origin, and primarily the mysid Mysis relicta, were also widely used for introductions to North American and Scandinavian lakes, and the controversial results of these translocations have been summarized by Lasenby et al. (1986). Lithuanian waters were the first place Ponto-Caspian peracaridans, mysids and amphipods in particular, were deliberately introduced in the Baltic Sea basin. Following the establishment of populations of the introduced species and their appearance in fish diet, it was suggested that a 20% enhancement of fish production had occurred from fishery reports; however, this statement did not receive a scientific substantiation. The purpose of the current study was to review the rationale and practice of invertebrate introductions into Lithuanian waters, determine the current distribution of introduced species, test the theoretical expectations of intro- duction of peracaridan species into lakes by applying original and published data, and summarize the large-scale and long- term consequences of these translocations. Rationale for biological introductions The intentional acclimatization of potential fodder species of invertebrates obviously served to enhance the biological production of commercially important fish species. Introduced species were expected to increase the channelling of primary production toward these fish species. Theoretically, five types of acclimatization of aquatic organisms have been distin- guished (Karpevich, 1975), which may be unionized into two general cases (Zenkevich, 1940): acclimatization as insertion of target species into a community; and acclimatization to replace Ôlow-valueÕ native species with Ôhigh-valueÕ target species. When considering invertebrate introductions, the first accli- matization case was thought to have more sound biologically reasoning and economic purpose. A major assumption of acclimatization as insertion was that there should be a free niche space. That was the case for introduction of alien peracaridan species to Lithuanian waters which were consid- ered to be relatively ÔpoorÕ in higher crustaceans due to geographical isolation (Gasi unas, 1963). It was hypothesized that detritivorous and herbivorous alien mysids and amphi- pods would obtain sufficient nutrition to develop high biomass and thus, supplement the local invertebrate communities with high-value fish-food items. Assimilation of the introduced species and subsequent enhancement of fish production was expected based upon three further assumptions. Firstly, introduced peracaridan species would provide high quality (high-calorie) and easily accessible food and hence, fish nutrition and subsequent growth would improve, and standing J. Appl. Ichthyol. 26 (Suppl. 2) (2010), 28–37 doi: 10.1111/j.1439-0426.2010.01492.x Ó 2010 Blackwell Verlag, Berlin ISSN 0175–8659 Applied Ichthyology Journal of