Spawning triggers are environmental cues that cause fish to breed. Most commonly they involve sudden changes in the environment, such as changes in temperature, salinity, and the abundance of food. Catfish of the genus Corydoras, for example, spawn immediately after heavy rain, the specific cues being an increase in water level and a decrease in temperature. Discus, on the other hand, will breed when temperature goes up and there is an overabundance of prey such as mosquito larvae.
Spawning triggers allow a species of fish to synchronise their breeding, making it more probable that individual fish will find a mate. However, many fish do not respond to specific spawning triggers and will breed either constantly (e.g., guppies); at specific times of the year (e.g., grunion); or only at a certain point in their life cycle (e.g., eels).
Aquarists can sometimes trigger spawning by duplicating the natural conditions where fish would breed. Raising the water temperature, lowering the water level, and feeding mosquito larave are examples of things that work for certain species.