Spawning aggregations have been traditionally categorized into two main types: transient and resident1. Types are distinguished based on three criteria: frequency of aggregations, longevity of aggregations, and distance traveled by fish to reach the aggregations. Transient aggregations typically involve possible long-distance migrations and a short reproductive season. Resident aggregations form more frequently than transient aggregations over an extended period, and occur close to, or even within, the areas of participating fish. Some species may demonstrate both types of aggregation behavior within their distributional range.
Resident FSAs have the following characteristics:
- The spawning output from a single resident FSA may represent only a small part of the total annual spawning output of that species in the region.
- The spawning site may be reached through a migration of a few hours or even minutes, and often lies within the home range of the aggregating fish.
- Resident FSAs may occur at a specific time of the day and at a specific point in the tidal cycle, and may last for a few hours or less.
- Resident FSAs may occur year-round.
Transient FSAs have the following characteristics:
- Some individuals in transient FSAs may (but also may not) migrate far from their home range to the site. These migrations, when they occur, may take up to several days or longer.
- Spawning aggregation formation typically occurs seasonally, rather than monthly or daily.
- The aggregations typically dissipate between aggregation periods.
- The formation of aggregations and spawning is usually linked to a particular lunar cycle. Environmental aspects such as water temperature can be factors in the timing of aggregations2.
New evidence suggests that the distinction between resident and transient FSAs is not as clear as perhaps once thought. Populations of some species, that were traditionally thought to be transient spawners, actually spawn throughout the year. The squaretail coralgrouper, Plectropomus areolatus, is such an example.
This species was considered a classic transient spawner. For example, in some areas of its distributional range, such as Pohnpei, Micronesia, the species has a relatively brief 4 to 5 month seasonal spawning period. However, in at least some parts of Micronesia and Melanesia, this species aggregates monthly.
The reproductive dynamics of FSA-forming species are highly complex and poorly understood. A study of a Nassau grouper (Epinephelus striatus) FSA in Belize documented a previously unknown behavioral pattern in spawning populations3.
Spawning aggregations have been known to form at Glover’s Reef Atoll for many years, and in 2001 and 2002 Nassau groupers were tagged with acoustic transmitters to track their movements. After spawning in both January and February, all groupers curiously made a synchronized dive (within 1 hour of each other) to a depth of 255 meters, where they remained for three months. The reason for this sudden and consistent change in habitat is unclear; however, this significant finding reveals that we still have much to learn about the reproductive biology of reef fishes, and that MPAs meant to protect these types of FSAs may need to extend into deeper nearshore waters.