Spawning aggregations of coral reef fishes: Characteristics, hypotheses, threats and management

Many coral reef fishes migrate to form short-lived aggregations at predictable sites and times in order to spawn. For the purposes of this review, such spawning aggregations are defined as any temporary aggregations formed by fishes that have migrated for the specific purpose of spawning. Spawning aggregations are known to be formed by 164 species from 26 families of coral reef fishes, but the actual number is likely to be much higher. Aggregative spawners share a number of common features. (1) All except one species release pelagic eggs. (2) They tend to have large body sizes. (3) They are more abundant in some phylogenetic groups, such as the Labridae, Scaridae, Serranidae, Acanthuridae, and Lutjanidae, although they are relatively uncommon in all but the least speciose families of Albulidae, Chanidae, Gerreidae, and Scombridae. (4) They are more likely to come from large populations with high densities. However, these features are not independent and their relative importance is not easily assessed. Known spawning aggregations form at the same sites over successive, predictable spawning seasons. However, from the limited data presently available, spawning aggregations do not appear to form consistently on predictable reef structures. The periodicity of spawning aggregations can differ greatly for the same species with relatively small degrees of spatial separation. A number of hypotheses have been proposed to explain why, when, and where spawning aggregations are formed. These include those that predict that the phenomenon of aggregative spawning (1) reduces predation on spawning adults and their eggs (the predator satiation hypothesis), (2) increases the degree of mate selectivity, and (3) allows individuals to assess sex ratios of populations and make decisions on sex change accordingly. Other hypotheses predict that the location and timing of spawning aggregations (1) reduce predation on both eggs (the egg predation hypothesis) and spawning adults (the predator evasion hypothesis), (2) increase the probability that larvae will settle on reefs (the egg dispersal hypothesis and the larval retention hypothesis), and (3) enhance the survival of larvae during their pelagic phase (the pelagic survival hypothesis). However, very little quantitative research addressed at an appropriate scale has been conducted to distinguish among these hypotheses, many of which make common predictions. Spawning aggregations of commercially important coral reef fishes have been lost in many locations throughout the tropics because unsustainable fishing targets the spawning aggregations themselves. The live reef food-fish trade has proven to be unsustainable in almost all locations in which it has operated, leading to widespread impoverishment and eradication of spawning aggregations. Appropriate management, legislation, and enforcement are essential to protect the stocks of commercially important aggregative spawners, as is a more comprehensive understanding of the dynamics of spawning aggregations.