[Click on the images above for videos]

 

What are spawning aggregations?

The short answer: a spawning aggregation is formed when individuals migrate to a location to spawn together at the same time. A more comprehensive answer is actually quite difficult to provide. How many individuals are required to form an aggregation? Do haremic species form aggregations? What about fish that normally live in schools? How far do they need to migrate? Spawning aggregation formation falls at the end of a series of scales of group behaviours with no clear delineation.

Spawning aggregations can be spectacular phenomena (see video links above, and some amazing photos by Tony Wu). Thousands of large fish can gather together, only once a year, with some fish migrating over 100 km. But they can also be rather less impressive: a few small fish swimming short distances to meet for minutes each day to spawn.

The reasons why fish choose the sites and times that they use for spawning are somewhat unknown. We used to think that it was to coincide with favourable currents that help the eggs and larvae disperse, but recent studies suggest that the site and time might actually help to retain eggs and larvae close to where they were released.

 

Fishing spawning aggregations

Regardless of the underlying reasons, spawning aggregations can be very predictable: they reoccur at the same site and same time. From the perspective of fisheries, a spawning aggregation represents an unusually high density of individuals and a better place to fish (i.e. higher catch per unit effort).  Once a spawning aggregation has been located by fishers, continued targeting of the site can have a devastating effect on the local population of fish that migrates to spawn there. The plight of the Nassau grouper in the Wider Caribbean Region is the best documented example of a species’ decline primarily because of spawning aggregation fishing. From being one of the most important fisheries species in the region, Nassau grouper is now endangered (IUCN Red List) and commercially extinct in much of its historical range.

 

Protection of spawning aggregations

Because of their vulnerability, there has been a global effort to provide greater protection to spawning aggregations, in particular for groupers and snappers. This has been achieved through establishing no-take zones around spawning aggregation sites (e.g. Belize, Mexico, Palau, Puerto Rico, Turks and Caicos Islands, US Virgin Islands) and by closing fishing for species around their spawning seasons (e.g. for Nassau grouper in Bahamas, Belize, Cayman Islands, Dominican Republic, Mexico, Turks and Caicos Islands, and for other groupers in Fiji). On the Great Barrier Reef there are three short (2-day) closures of the coral trout fishery around the new moons of consecutive months when the species aggregates to spawn.

 

Organisations working on spawning aggregations