There are two classes of spawning aggregation, “Resident” and “Transient”. Both occur at predictable and regular sites and times.
Resident aggregations are formed by fish that only travel short distances to the aggregation sites, and assemble on a regular basis, sometimes almost daily and for extended periods. Such species are generally small in size.
Transient aggregations are formed by larger species physically able to travel greater distances. Transient aggregations usually form for just a few months each year, often for a week or two at a time. As a general rule, transient aggregations are larger, of shorter duration and less common than resident aggregations.
The best-known examples are species of grouper and snapper, but many surgeonfish, rabbitfish, parrotfish, wrasse also aggregate to spawn. There is a great deal of variability among different species in the dynamics of aggregation formation. For instance, spawning aggregations of some small wrasses may consist of just ten individuals spawning close to their normal home range, while those of some large groupers consist of tens of thousands of fish that may travel over one hundred kilometres to an aggregation site on a particular reef.
SCRFA recently produced a two-minute film is about the importance of spawning aggregations and the need to manage them.