Fish Spawning Aggregations: A Reef Resilience Toolkit Module

Physical Factors

Physical factors affecting the location of spawning aggregations, and the dispersal of larvae from spawning sites, are the subject of much current research. Although many of the important processes that occur regarding the physical environment remain unknown or understudied, connectivity and ocean current patterns should be taken into account when considering protection of FSAs.

This figure demonstrates the connectivity of reef fish settlement habitat in the Caribbean where the authors found that the typical larval dispersal distances were on the scale of 50 to 100 km. From Cowen et al. 2005

Connectivity

FSAs may attract individuals from a relatively large area to a confined site. The fertilized eggs and larvae produced from FSAs may travel far before settling out of the plankton to mature. The term "connectivity" is used to describe both the migratory connections of adult fish to FSA sites, and the transport of spawned gametes by ocean currents to nursery habitats.

Marine connectivity is one of the most important and least understood concepts relating to management, particularly to FSAs. Spawning fishes release their eggs into the water column, where they are fertilized and then transported by ocean currents. Transport of eggs is passive until the eggs hatch into larvae (about 18-24 hours), and can be estimated using a GPS and simple current drogues. Larvae are active swimmers, and remain in the plankton for anywhere from weeks to months, depending on species and conditions. They may utilize vertical migrations to ride various oceanic and tidal currents to new locations or, alternatively, to stay in a relatively close area to where they were spawned. This important dispersal phase is not well understood and is an active area of research.

The levels of self-recruitment that populations experience is currently a matter of considerable debate, and there is some disagreement among scientists1. Because reef fish most often spawn at the edges of reef habitats, in areas exposed to swift currents and pelagic environments, it had been thought that reef fishes spawn in areas that maximize dispersal of their larvae. However, more recent evidence suggests that larvae do not disperse extensively, and that many return to their natal reefs2. Self-recruitment levels may be influenced by site isolation, coastal complexity and flow variability.3 In some studies of island systems, self-recruitment levels have been estimated at as high as 60%4. The fate of the remaining larvae is unknown.

Although genetic studies help to answer questions about particular locations, they are cost-prohibitive in most places. It can be reasonably assumed that there exists some level of self-recruitment and some level of dispersion with any FSA. In addition, all FSAs are important sources of larvae and thus, it is recommended that all known FSAs be prioritized for protection. Larger FSA reserves (i.e., tens of kilometers) are preferable to avoid overexploitation, and special attention should be given to isolated FSAs, such as those near atolls.