Gill Disease

Gill disease is an umbrella diagnosis for any condition that damages the gill filaments and lamellae — the tissue that handles roughly 80-90 percent of both oxygen uptake and ion exchange in fish — so gill pathology impairs respiration and osmoregulation simultaneously. Causes fall into three broad groups: bacterial (flavobacteria such as Flavobacterium branchiophilum causing bacterial gill disease, favored by high organic load and temperatures of 12-20 degrees C), parasitic (gill flukes, amoebic gill disease caused by Neoparamoeba perurans in marine salmonids, attached sea lice), and non-infectious damage from high unionized ammonia, low pH, or suspended solids eroding lamellae. Damaged gills have reduced surface area and thickened epithelium, so oxygen transfer falls and carbon dioxide and acid-base balance are disturbed. The key diagnostic clue is that affected fish show classic hypoxia behavior — piping at the surface, crowding inflow points, lethargy — while dissolved oxygen readings remain normal, directing staff toward gill inspection and wet-mount microscopy rather than aeration troubleshooting. Because the gill is also a major immune organ, chronic gill damage leaves fish permanently vulnerable to secondary infections, making prevention through water quality management the core control strategy.