A great question. When an abalone spawns, either in the wild or in captivity, a small molecule known as prostaglandin (so named because this sort of molecule was first discovered in the human prostate gland) is also released to the external environment. This in turn can stimulate spawning of neighboring abalones in close proximity, increasing the chances of a successful fertilization event occurring. It has also been observed that by adding trace amounts of prostaglandin to the surrounding seawater, both male and female abalone can be induced to spawn, usually within a relatively short period of time after initial introduction. Prostaglandin is, however, an expensive chemical to use in large quantities and so its use at a commercial scale could prove prohibitive for aquaculture purposes. In abalones, prostaglandin is synthesized in the gonads and the first critical reaction in its synthesis is catalyzed by an enzyme, which is in turn regulated by minute concentrations of hydrogen peroxide. By exposing an abalone to low concentrations of hydrogen peroxide, this enzyme is activated, prostaglandin is synthesized, and spawning is induced.

In general, marine invertebrates (animals without a bony spine) usually reproduce by spawning: the males release a bunch of sperm into the water and the females release a bunch of eggs. The idea is that somehow, the sperm will come into contact with the eggs, either through water motion or through random contact. You can see right away that for fertilization to happen, the eggs and sperm would need to be released at the same time and that the males and females would need to be pretty close, since unfertilized eggs and sperm last only a week at most.
In order for spawning to be coordinated in males and females, which ensures that eggs and sperm are released at the same time, there is usually some environmental trigger that will induce spawning. For example, increasing the water temperature from 18C to 22-25C for a sustained period can induce spawning in some species of abalone. In abalone, spawning occurs when an enzyme causes the synthesis of a hormone within the abalone called prostaglandin endoperoxide. This enzyme is activated by hydrogen peroxide. Adding hydrogen peroxide at about 0.25-5 millimoles per liter final concentration in a tank with abalone will induce spawning. This much of an increase in hydrogen peroxide in seawater would never happen in nature, but is a useful trick for people trying to grow abalone for research or commercial purposes. The hydrogen peroxide is probably produced by the abalone itself, in response to an external environmental trigger like an increase in seawater temperature.