The Adriatic Sea has been exposed to multiple stressors in recent decades – from climate change and excessive navigation to the impacts of commercial fishing. Sea warming and certain fishing techniques have been shown to have negative consequences on the composition and structure of marine living communities. Therefore, in order to better understand the spatial and temporal dynamics and monitor the biodiversity of marine species, as well as establish a sustainable strategy for the management of marine resources, it is important to include information obtained directly from fishermen, who often have useful local knowledge. The importance of such a recent interdisciplinary approach has been widely recognized by the scientific community, so the concept of Fisheries Local Ecological Knowledge (abbreviated FLEK) has been introduced. Given that the Zadar County fishing fleet is at the forefront of catching the most economically important species, sardine in particular and is traditionally primarily focused on catching small pelagic fish, it is particularly interesting from the point of view of sustainable fisheries management based on fishermen’s knowledge as an essential component. This paper used interviews with fishermen to investigate their knowledge regarding purse seine catching techniques in fishing zones of the Zadar County (zones B, E, and F), target fish species and by-catches, as well as the frequency of allochthonous species appearing in the catch in this part of the Adriatic. The purpose of the paper was to obtain an overview of potential quantitative and qualitative changes in marine species caught by purse seines over the past few decades, as well as the frequency of invasive species appearing in the catch, namely based on the data collected from fishermen. The findings have indicated qualitative and quantitative changes in the caught fish communities. It was noticed that in certain time periods the total catch and size of target species as well as by-catches decreased. The new fish species have appeared in the catch in the observed area in the last twenty years. To achieve sustainable fishing in the Adriatic Sea, fishermen proposed spatial and temporal regulation of fishing, modification of certain types of fishing gear, prevention of illegal fishing and the establishment of a wholesale fish market and leaving key issues related to fisheries to the educated profession.