In the center of Stockholm, a lock by the name Victoriaslussen is being renovated, where in conjunction with this, a fishway is being designed. The purpose of the fishway is to facilitate migration of weak-swimming fish between the Lake Mälaren and the Salt Lake. Perch (Perca fluviatilis) is counted among these weak swimming species. The aim of the study is to, based on research results, reports, and interviews, create an analysis of the challenges the perch face today, and in the future, as well as assessing the need of a fishway in this location. Data show that the perch population in Sweden is in a steady state today, although a reduction in catches in the study area has occurred during the last 25 years. Based on the accessible data it was stated that there is sufficient scientific evidence to draw conclusions about the sustainability of the perch’s stock in lake Mälaren today. The study shows that the decrease in stock can be attributed to human impact on the environment in the form of pollution of the water as well as to climate change. Furthermore, the impact of climate change on the lake’s ecological status, for instance, through an increase in temperature, resulting in eutrophication and descending oxygen levels, will amount to consequences for the perch such as changes in metabolism, size structure in the stock (population and body size). The fishway system created is a circuit (In Swedish: omlöp), which is the most natural and gentle system for a wide diversity of weak-swimming fish. The study concluded that the perch would benefit from a fishway in Victoriaslussen. It would improve the perch’s situation in Mälaren. Although the stock is in a steady state for the time being, climate change in the future will bring new challenges which would make the fishway useful solution.