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

Salinity

Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

, and the abundance of food. Catfish of the genus Corydoras

Corydoras

Members of the South American Corydoras genus are freshwater temperate and tropical catfish in the armored catfish family , and are commonly referred to as corydorases, cories, or cory catfish.-Taxonomy:...

, 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

Mosquito

Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...

 larvae.

Spawning triggers allow a species of fish to synchronise their breeding, making it more probable that individual fish will find a mate

Mating

In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

. 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

Grunion

Grunion are two fish species of the genus Leuresthes: the California grunion, L. tenuis, and the Gulf grunion L. sardinas...

); or only at a certain point in their life cycle (e.g., eel

Eel

Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

s).