The main threats to coral reef ecosystems are human contact through boating and anchoring, which can severely damage the fragile reefs over time. Runoff water carrying silt from eroded soil can block sunlight to the reefs, preventing photosynthesis and killing the reefs gradually. Untreated or improperly treated sewage also promotes harmful algae growth. Cyanide fishing involves using cyanide to stun fish, which poisons and kills the coral polyps and organisms necessary for healthy reefs. Fertilizer runoff, pesticides, and other chemicals from pollution also endanger reefs, as can the shock waves from blast fishing.