Title of the project: Predator-mediated control of an invading species – Cercopagis pengoi (WP 3/4 Invertebrates in AquAlien) Summary to be set out as follows: 1)The project’s role and significance for the programme (cf the programme plan), 2) 2)Environmental relevance and expected significance for environmental policy 3) Objective and hypothesis 4) Methods and implementation 5)How results will be presented in accordance with the programme. This project is proposed within WP 3/4 (Invertebrates) and will assess the natural capacity of Baltic biota to control occurrence and invasion dynamics of an invading species, and to what extent proper management can improve this capacity. The pelagic cladoceran that recently invaded Baltic Sea, Cercopagis pengoi, is available as a prey for fish and mysids. Understanding the interactions between these organisms will provide information for the risk assessment of the invasion (WP 7). Identifying predator species with high controlling capacities as possible means of mitigating risks and ecosystem impacts and incorporating this information into an ecosystem approach for fish management would also contribute to the search for the best management strategy (WP 8). Cercopagis is increasingly recognized as one of the most aggressive invaders with a high potential to affect both plankton biodiversity and fish nutrition. There are also numerous observations that it can clog fishing gears, including trawls, and thus cause problems for fisheries and economic losses. This project will provide knowledge for a strategy to manage and mitigate this particular invasion, but will also explore the concept of ecosystem management as a tool to reduce impacts of invading species. We hypothesize that fish are important predators on Cercopagis and that predation increases with fish size. Fish populations consisting of large individuals thus have a stronger control of the Cercopagis population. As a response to predation, genetic and morphological adaptations may evolve resulting in lower predation rates on such individuals. To test these hypothesized relationships, we will (1) evaluate effects of fish and mysids on abundance, population structure and phenotypic variability of Cercopagis and the potential impact that a change in the fishery management can have, (2) explore variations in abundance among morphotypes of Cercopagis in the areas with different predation pressure and (3) assess genetic and phenotypic variations as potentially beneficial traits for colonization. To fulfil these objectives, standard methods (gut content analysis, abundance and biomass estimates, morphometric analysis, allozyme electrophoresis and PCR techniques) will be employed. Thus, we will deliver: (1) evaluation of main predator species in the Baltic food webs as biological control agents of Cercopagis, focusing on the efficiency of particular size/age classes (input to WP 7 and WP 8), (2) estimates of adaptive changes in morphology and life-history characteristics of Cercopagis at different rates of predation (input to WP7) and (3) assessment of phenotypic plasticity and/or species diversity of C. pengoi complex in the Baltic and prognoses for the further arrival, establishment and spreading in relation to native predator assemblage composition in Swedish coastal and freshwaters (WP 7 and WP 8). Cercopagis rapidly colonizing both estuarine and freshwater ecosystems is an ideal model organism to evaluate the possibility to take an ecosystem approach to mitigation of exotic organisms.