Assessment of the Water Quality Index of Aquaculture Areas around the Korean Coastal Waters

This study evaluated seawater quality in the coastal aquaculture grounds offshore Korea using monitoring data from 2011–2020. Over the study period, 76.4% of stations recorded grades I–II, whereas grades IV–V (poor) occurred in 4%, although in the Korea Strait, this number reached 8.5%, and it was only 1.3% in the southwest. Poor quality persisted in the shellfish and seaweed farms of the Korea Strait, dense farms such as Jinhae Bay, and river estuaries during the month of August. However, several key parameters varied: transparency (Secchi Depth, SD), dissolved oxygen (DO) saturation in bottom waters (East Sea); dissolved inorganic nitrogen (DIN) and chlorophyll-a (Chl-a), DO, and SD (Korea Strait); Chl-a, DIN, dissolved inorganic phosphorus (DIP) (southwest and central West); and Chl-a (Jeju). In all ecological zones, the index values of water quality items were high in the waters around shellfish and estuary fishing grounds. This is thought to reflect the influence of topographical characteristics and self-pollution in the fishing grounds. These results highlight that both anthropogenic and natural factors shape water quality, indicating the need for region-specific assessment and policy rather than uniform grading.