Biology of Aggregating Species

What is a spawning aggregation?

An aggregation is a group of fishes that temporarily assemble at higher than normal densities, maybe for several reasons, including feeding and reproducing.
A spawning aggregation is when fish group together for the sole purpose of spawning (=reproduction), with release of sperm and eggs.
Aggregations should not be mistaken for the common and well-known habit in fish of ‘schooling’, whereby large numbers of fish move, feed and live together for much of their life cycle. Confirming that an aggregation only forms for spawning by direct observation is not always easy since many species only spawn at night, may be disturbed by divers in the water, occur in areas difficult to reach, or only spawn for a very short time each year. In such cases, it may be possible to identify fish about to spawn, or which have recently spawned, by finding ‘hydrated’ eggs (the eggs only hydrate, or fill with water, just before spawning), or postovulatory follicles (microscopically detectable structures in the ovary that are only found after spawning), in females. Other possible indicators of spawning are more indirect but may be used if it is known from other studies, which they only occur around spawning. Indirect indicators include temporary colour changes or courtship behaviour and must be assessed on a species by species basis.

How many fish species aggregate to spawn?

Many of the larger reef fishes show this habit but we do not know the exactly how many.
The groupers (Serranidae), snappers (Lutjanidae), parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) are all well-known for forming spawning aggregations but, even in these families, there are many species that do not aggregate to spawn. Smaller groupers, for example, do not aggregate whereas many larger ones do. Many species of parrotfishes and wrasses (Labridae) have two types of spawning behaviour, one within aggregations, the other does not involve aggregation-spawning. Conversely, we don’t know about the reproductive habits of some common reef fish families that move around in schools, such as the grunts (Haemulidae), or batfish (Ephippidae). It is surprising how much we have yet to learn about the biology of reef fishes, even for those species that are commercially important, or that divers regularly see in large numbers.
SCRFA keeps a global database of all reported spawning aggregations and, as of January
2004, it contained 120 reef fish species from 22 families. Although, SCRFA tends to concentrate on coral reef-associated fishes, there are other types of fish such as croakers (Sciaenidae) that inhabit sandy bottoms or estuaries, and sea breams (Sparidae) on rocky reefs, which also aggregate to spawn.

How long do spawning aggregations last each year?

The duration of aggregation formation varies markedly.
At one extreme are some surgeonfishes and small species of wrasse that aggregate almost daily for much of the year. At the other extreme are certain large groupers such as the Nassau grouper (Epinephelus striatus) of the Caribbean and tropical western Atlantic, or the Indo-Pacific tiger grouper (E. fuscoguttatus), that assemble for about a week in each of a few months of each year. The variation could be between species, within the same species in different regions, and among different species that use the same site (at many sites, several different species may spawn).

How big are spawning aggregations?

C. Lavett-Smith observed a single mass of Nassau groupers that he estimated at between 30,000 – 100,000 fish in the Bahamas in1971.
He was one of the first scientific divers who observed spawning aggregation in the water. People have fished this and many other such sites of the Nassau grouper for decades; when just a couple of fishers were involved the small annual catches could be sustained; however, once the number of fishers increased a little, aggregations were easily fished out. Now, for this species, many aggregations are very small and some no longer form at all.
While other species also spawn in very large groups, not all spawning aggregations are naturally large. The beautiful coral trout, or coral grouper (Plectropomus leopardus) typically forms many small groupings during the reproduction season; these clusters of males and females are also vulnerable to fishing since they form in the same places, and at roughly the same times, each year. Surgeonfishes, rabbitfishes (Siganidae) and emperors (Lethrinidae) are known to gather in their thousands, while many wrasses, such as the humphead wrasse (Cheilinus undulatus), form groups of a few hundred fish at most. The largest of all groupers, the goliath grouper (Epinephelus lanceolatus) is believed to form spawning groups of no more than a few tens of individuals.

Are there physical or oceanographic features consistently associated with spawning aggregations?

Some reef fish aggregation sites are distinctive and predictable; others, to us humans at least, do not show any obvious physical characteristics.

In the Indo-Pacific, several grouper species typically assemble in distinct channels through outer reefs leading to the open ocean; often these channels are associated with periods of rapid water movements. Species like the humphead wrasse, on the other hand, form smaller aggregations at seemingly non-distinct places along, and just out from, outer reef slopes. For some species throughout the tropics, promontories on outer reef slopes, or prominent features along an otherwise featureless shelf edge, may be commonly used. Certain fishes, like rabbitfishes and mullets, instead of going to outer reef areas, spawn inside the outer reef after migrating along the coast, while fishes like the coral trout assemble to one end of their resident reefs. Scientists do not know why there is such a diversity of spawning sites and why different species select one over another, although there are lots of hypotheses to test!

In the Pacific Ocean, several fish species (e.g. Plectropomus areolatus, Epinephelus polyphekadion and E. fuscoguttatus) typically spawn in distinct locations in or near the channels of outer reefs or at the end (promontories) of outer reefs. Other species are not predictable in where they assemble to spawn. In the upper part of the photo it is possible to see distinct channels through the reef leading out to the open sea, also some distinct promontories – knowing that such features are often associated with spawning fishes in the Pacific Ocean, makes their aggregations easy to find.Photo: Yvonne Sadovy

Why do some fish aggregate to spawn?

To ensure that large numbers of individuals are present at once and can encounter mates; effective dispersal of eggs; enhancement of fertilization by concentrating large numbers of eggs and sperm; protecting adults (or eggs) from predation (just as a large flock of birds acts to protect individuals in the flock from predation by shear numbers).
One or a multiple of the reason above explains why fish aggregate to spawn. Worthy to note that there has been much speculation about why different fishes aggregate to spawn at the times, as well as the places, that they do. What we can reasonably assume, however, is that spawning at specific places and times must be biologically significant since so many species have evolved to do it.

How do fish reproduce in spawning aggregations?

Towards the peak time of spawning, often in the afternoon or around sunset, the aggregated fish become more concentrated in space and increasingly active, often with distinct colour changes and behaviours. Pairs (male and female) or groups of fish (usually many males led by a single female) rise rapidly towards the water surface, and away from the rest of the aggregation, in what is known as a ‘spawning rush’. At the top of the rush the females release eggs and the males release sperm that will fertilize the eggs in the water column. The spawners then rapidly move back down to join the rest of the aggregation and the fertilized eggs drift off in massive clouds into the ocean. Although the sperm and eggs are tiny, they may be briefly visible just after release and while still highly concentrated, as a milky cloud. Spawning occurs in groups or in pairs, depending on the species and, in some species, (such as surgeonfishes and wrasses), both ‘pair’ and ‘group’ spawning are found.

Many snappers species spawn in large groups with many males releasing sperm at the same time. The photo is dark because it was taken around sunset, when spawning occurs in many transient aggregations. The pale area at the centre of the photo is a ‘cloud’ of sperm and eggs. Photo: Rachel T. Graham

Spawning behavior in the Nassau grouper,Epinephelus striatus:
(1) fish start to move up nto the water column and an increasing number take on the bicolour phase;
(2) ascent by a small sub-group led by a dark-phase individual followed by bicolours;
(3) release of sperm and eggs;
(4) rapid return of fragmented sub-group to substrate (description from Colin, 1992); figure reproduced from Sadovy (1996), with permission from Chapman & Hall Publishers

The red hind, Epinephelus guttatus, aggregates to spawn but does not spawn in big groups. Spawning takes place in male/female pairs. Here you can see the displaying male (almost upright) being approached by a female. Photo: Charles Arneson

Do species that form spawning aggregations also spawn outside aggregation times and places?

The red hind, Epinephelus guttatus, aggregates to spawn but does not spawn in big groups. Spawning takes place in male/female pairs. Here you can see the displaying male (almost upright) being approached by a female. Photo: Charles Arneson

Do spawning aggregations always occur at the same location?

The basic answer is that typically, yes, they do form in the same places each year.
Some aggregations have been known to occur annually for over 50 years at the same site.
However, there are a few reports of Nassau grouper aggregation sites varying somewhat from year to year in some countries, although the same general area is used. Likewise, the red hind (Epinephelus guttatus) assembles on the shelf edge each year although the exact location varies. The reasons for this, like so many other aspects of aggregating fishes, are not known. Similarly, there are many reports of regular, and locally well-known, spawning spots for groupers in the western Pacific.

How do fish know which aggregation site to go to?

It is suggested that young adultscould learn where the nearest site is by following older fish although little research has been carried out on this topic.
If this is true, oneimplication is that restoration of overfished aggregations by introducing new fish (orrestocking) may not work if the ‘group memory’ of the site has been lost. What we doknow is that once an aggregation has been so overfished that it ceases to form, there is noevidence that it will recover. We also know that, for some species at least, the sameindividual fish return to the same aggregation sites each year, and could depend exclusively on just one site for reproduction. However, we have yet to learn much morebefore being able to answer such questions definitively.

Are there different types of spawning aggregations?

Every species does things a bit differently but, by definition, there appear to be two classes of spawning aggregation, ‘resident’ and ‘transient’. Both occur at predictable and regular sites and times.
In resident aggregations fish only travel short distances to the aggregation sites, and assemble on a regular basis, sometimes almost daily. Such species are generally small, like the six-bar wrasse (Thalassoma hardwicki), which only grows to 20 cm in length, and many species of surgeonfish. Transient aggregations, on the other hand, tend to be 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.

What is the greatest distance fish travel to spawning sites?

The furthest a fish is known to have travelled to an aggregation site is 110 km for a
Nassau grouper (Epinephelus striatus) tagged on a reef in the Bahamas and later recaptured at the aggregation site.
Another Nassau grouper tagged at an aggregation site in Belize was caught 2 years later by a fisherman on a reef in Mexico, 240 kms away. These data mean that fish can live considerable distances from the places where they reproduce, and as a result, that individual spawning aggregations may ‘support’ fish from a very large reef area. This observation has considerable management implications since single aggregations may support fisheries for particular species over a wide area.

How predictable are spawning aggregations?

The timing of aggregation in most species tends to be consistent.
This makes them rather predictable; once they have been discovered, they are easy to find again, and again, at the same time each year and in the same place. This can be a serious problem when fishing is not controlled. As just one example, many spawning aggregations of the Nassau grouper have declined or been eliminated largely, scientists believe, as a result of uncontrolled fishing.

How important are spawning aggregations for the species that form them?

For species that spawn only in aggregations these may well be vital for population persistence.


Conservation and Management

What are the threats to spawning aggregations?

This is particularly a problem for transient aggregations, which tend to be larger and less common than resident aggregations. In transient aggregations, fishes gather from a wide area making it much easier for fishers to catch large numbers of the species than at other times of year. Fishers typically find aggregations before anybody else and, in most countries, there are no regulations to control the number of fish taken. If too many are removed, experience shows that the aggregation can cease to form; there is no evidence that decimated aggregations can reform.
Indeed, unregulated fishing has already resulted in some aggregations disappearing within just a few years of discovery because so many of the fish were caught; one of the earliest examples was in late 1970s in the United Stated Virgin Islands. In another example, from the Bay Islands, Honduras, an aggregation which had approximately 10,000 Nassau grouper in 1988, had dwindled to just 500 by 1991 after heavy fishing. Such declines will greatly reduce the abundance of the species over a wide area, with fewer and fewer juveniles produced, ultimately resulting in population declines, and possible crashes.

What can be done to protect spawning aggregations?

Spawning aggregations should be included in fisheries and conservation management plans, as well as during designation of marine protected areas. Although there is currently little effective management in place, there has been much progress in recent years by governments and conservation groups towards such goals. The primary aim of management is to reduce and control the amount of fishing so that aggregations remain stable over the long-term. In some cases, the removal of small numbers of fish might be sustainable in traditional fisheries but, in most cases, experience has shown clearly that fishing should probably be banned, either at specific times of the year, at key locations, or by some combination of measures. Possible options are to include the aggregation site within a no-take marine reserve, to ban fishing at the aggregation site during the spawning period, or to ban catching the species anywhere during the spawning period.
The last method has the advantage of protecting aggregations that may not even be known to the authorities, but is difficult to enforce unless there is some form of centralized marketing.
For details of the advantages and disadvantages of different management approaches go to the section of Management and Conservation on this website.

What has been done to protect spawning aggregations?

A small number of countries now have regulations to protect spawning aggregations.
Some notable examples are a seasonal ban on fishing all groupers in 3 states in Mexico bordering the Gulf of Mexico and Caribbean, seasonal bans on fishing for 3 species of grouper (black, red and gag groupers) in the United States Gulf of Mexico, and a ban on fishing for groupers at certain times and places in Palau, Micronesia. In the Bahamas, fishing is now banned at one well-known Nassau grouper aggregation site for 14 weeks during the spawning months, while the catching and selling of the species is prohibited throughout the Bahamas for a shorter period. The Cayman Islands also took steps to protect its Nassau grouper stocks by banning fishing at 6 aggregation sites for 8 years, starting Dec. 2003.

Many fish assemble during spawning aggregations and catches can be very high. In some Nassau grouper, Epinephelus striatus, aggregations in the Bahamas, a significant proportion of all the fish present can be taken during a single aggregation. Some of these aggregations have reportedly collapsed.

Should spawning aggregations be fished at all?

In practice, and on balance, the SCRFA believes the best option is not to fish them at all.
Experience to date shows that because spawning aggregations may be crucial for population persistence, care is necessary to avoid overexploitation and the precautionary principle should be applied. It may possible, in some cases, to allow a small catch but such fishing would have to be carefully regulated and monitored.

If a spawning aggregation is completely fished out, will it ever come back?

This is an important question of relevance to management planning and decisions. As far as we know, the answer seems to be ‘no’.
This may not be surprising if individuals learn about aggregation sites by following older fish; clearly, if all fish are removed, none will remain to teach the next generation. However, although not one aggregation had been known to recover following decimation, information is available on too few sites to know whether this represents the typical case. This means that we have to be careful in responding to this question. If it is indeed that case that aggregations cannot reform after disappearing, then it is clearly important that we do not allow them to get fished out in the first place.

Is it OK for recreational divers to dive on spawning aggregations?

At present we know of very few commercial operators specifically offering diving on spawning aggregations and it is far from clear whether this will change in the future. The cash dollars from eco-tourism are increasingly important in conservation by placing an economic advantage to having marine life on the reef, rather than on a dinner plate, but at present there just isn’t enough information on the effects of divers on spawning fishes to say whether recreational diving on aggregations could be used to protect them.
Also, while aggregating fish of some species are almost oblivious to careful divers, others are known to change their behaviour, raising the possibility that groups of tourists could inadvertently stop crucial, once-in-a-year, mass-spawning events. Clearly we need to understand much more about the impacts of dive tourism and other activities on these important biological events.

Do spawning aggregations only form on reefs in the tropics?

No, although the majority of known aggregations of reef fish are from coral reefs, thereare exceptions.
One is the dusky grouper (Epinephelus marginatus) which forms small spawning aggregations on rocky reefs in the Mediterranean Sea. Other fishes that only partly live on reefs but also aggregate to spawn include some of the sea breams (Sparidae).

Do the fish migrating to an aggregation return home immediately after spawning?

Not quite. Courtship always occurs before spawning and this can last for days; may involve colour changes and behaviours only found in aggregations.
However, because spawning itself occurs pretty quickly, resident spawners who use aggregation sites on their own reef can arrive, be involved in courtship, spawn and return to their resident area within a few hours. Transient spawners, on the other hand, may arrive days before the peak spawning period and mill about until courtship and spawning occur. In such cases, numbers of fish build up quite slowly, often the males arriving at the site before the females, and drop quickly following spawning as the fish depart soon after.

If I find a spawning aggregation, what should I do about it?

First and foremost, be very careful whom you tell about it.
There is at least one example of a recreational diver finding an aggregation, telling others in their community, with word getting out resulting in targeting of the aggregation by commercial fishermen and collapse of the aggregation. If the aggregation is in a country that puts a strong emphasis on managing its’ fisheries sustainably, and with good enforcement, you should probably go directly to the relevant authorities so that they can include the aggregation in their management plans. If not, you may wish to keep the knowledge to yourself, but either way SCRFA would like to hear from you. Our mission is to conserve these aggregations so we can provide you with information and relevant government contacts. We would also want to include the information in the database.
If you are lucky enough to come across a spawning aggregation, the normal rules for observing animals apply. Give the fish plenty of space and avoid making any rapid movements. Make notes on the species, time, behaviour, month, location and submit it to our database.

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