Can fish kills in Izmir bay be explained with satellite image analysis?

Motivated by a significant environmental crisis that emerged, where large numbers of dead fish washed ashore in İzmir Bay in the summer of 2024, this study aims to analyze the spatial and temporal dynamics of water quality in the inner bay prior to this incidence. By calculating indices such as NDCI, SABI, and UWQV, and correlating them with climatic data (air temperature, wind speed and relative humidity), this research seeks to document the occurrence and drivers of algal blooms in the bay using Landsat 8 and Sentinel-2 satellite data from 2017 to 2025. This is the first comprehensive study conducted for İzmir Bay that investigates the relationships between water quality indices and climatic variables. It also incorporates aerial analysis of the inner bay to provide a broader spatial perspective. A customized code using Python is developed for this study to independently download and analyze raw satellite data with respect to defined corrections/masks. The results of eight years of analysis indicated that critical conditions arise every summer with air temperatures reaching 40 degrees in the study area. Estimated aerial averaged NDCI index and Chl-a concentration values show a strong positive correlation with air temperature, particularly in the Spearman’s rank correlation (rs = 0.67 and 0.62 respectively), indicating a significant relationship between these parameters. Aerial distribution of the indices for the selected critical dates also revealed a significant increase in estimated Chl-a levels during the summer months, specifically in the regions determined from the risk maps produced as a result of this study. The areas with the greatest vulnerability coincide where Poligon, Ilıca streams in the south and Bostanlı and Çiğli streams in the north discharging into the bay. It is recommended that any planned external intervention methods for managing algal blooms should start with these highly vulnerable areas as presented by this study. Keywords: NDCI; SABI; UWQV; Chl-a; fish kills