Biology of Aggregating Species

What is a spawning aggregation?

A spawning aggregation is when fish group together for the sole purpose of spawning (=reproduction), with release of sperm and eggs. Learn about the types of spawning aggregations here

Spawning aggregations should not confused with the better-known habit in fish of ‘schooling’, whereby large numbers of fish move, feed and live together for much of their life cycle (they may also reproduce in these groupings but since schools are not only for mating they are not considered to be ‘spawning aggregation’.

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, aggregate in remote or deep areas, or only spawn for a very short time (less than an hour) each month or year.

Indirect indicators include temporary colour changes, known only to occur at spawning, courtship behaviour, running ripe gonads (reproductive organs which exude eggs or sperm) and also seasonally very high catches of fish with ripe gonads. With a microscope it may be possible to identify fish about to spawn, or very recently spawned, by finding ‘hydrated’ eggs (eggs only enlarge and hydrate ‘=fill with water’, just before spawning), or postovulatory follicles (tiny structures in the ovary that collapse after egg release and only found after spawning), in females.

How many fish species aggregate to spawn?

Many fishes show this habit. It is not necessarily characteristic of a particular family as some species may aggregate while their close relatives do not. What we do know is that well over 100 species have this habit and that they tend to be medium to larger sized species because these are more physically able to travel longer distances. Those that form transient aggregations and can travel tens of hundreds of kilometres from how reefs to aggregation sites tend to be the biggest, most mobile, species. Resident aggregations are more common among medium to smaller species.

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, from almost daily for much of the year (resident aggregators) to just a week or so once or twice a year. Duration can vary within a species across different regions, or can vary slightly from one year to the next. So location-specific and longer- term studies are useful if management is to be seasonally based.

How big are spawning aggregations?

Estimating the number of fish in an aggregation can be very difficult. If from fishing, it is only possible to know the minimum number present (from the total catches). A visual estimate can be reasonably made if a few thousand fish are present but becomes ever more challenging the larger the aggregation is. The largest known spawning aggregations were estimated by a diver to be somewhere between 30,000 and 100,000 fish (Nassau grouper, Epinephelus striatus, in 1972) and certainly several other aggregations of this species are known to have once contained 10,000 fish are more. The camouflage grouper, Epinephelus polyphekadion, also has large aggregations, the biggest one with approximately 17,000 fish (2015). Many aggregating temperate species also have or once had large aggregations, ranging from the Atlantic cod, Gadus morhua, to the Alaska pollock, Gadus chalcogrammus to populations of herring, Clupea harengus. These species and many others have yielded massive catches from their aggregations although total fish numbers were unknown. Other species naturally have much smaller aggregations of a few tens or hundreds of fish, and these are probably best known for coral reef fishes such as the leopard coral trout (Plectropomus leopardus), and the Napoleon or humphead wrasse, Cheilinus undulatus, amongst many others. Learn more about aggregating species here

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. Some aggregation sites are not as distinctive although throughout the tropics, promontories on outer reef slopes, or prominent features along an otherwise featureless shelf edge, may be commonly used for transient aggregations. Resident aggregations tend to be inshore, while for certain fishes, like rabbitfishes and mullets, instead of going to outer reef areas, migrate along the coastline. Many temperate species have specific spawning grounds that are well-known fishing areas although what aspects of the habitat might be important is not known. Several types of fish, such as croakers, move to estuarine areas to spawn.

Photo: Yvonne Sadovy

Why do some fish aggregate to spawn?

We cannot know why some fishes have adopted the habit of aggregating to spawn. However, there are several possibilities and hypotheses and the reason is most likely linked to increasing the probability of mating or successfully producing young. Possible reasons include ensuring that mates encounter each other or that eggs are fertilized and either disperse or are retained in ways that best enable the progeny to survive. The production of large numbers of eggs by large numbers of adults may reduce the probability of predation on young or the adults themselves. Having a small location for many adults to converge on may be linked to sexual selection or fertilization success.

How do fish reproduce in spawning aggregations?

We mainly understand spawning behaviour from direct observations on reef fishes but in colder or deeper waters, cameras have been deployed that have begun to open up the secrets of spawning behaviour. Typically, fish start to assemble a number of days before spawning actually occurs. The males may come in first and set up territories, followed by the females but that is not always the case. Courting may occur, often associated with colour changes, and spawning can occur in pairs (male and female) or in groups of fish (usually multiple males led by a single female). Spawning usually occurs by the pair or group rising up into the water column in a ‘spawning rush’, at the apex of which female(s) release(s) eggs and the male(s) release sperm. Fertilization probably take place within a minute or so and the fertilized eggs will then move away with the currents. Some species, like triggerfishes, gather in groups and attract females to their nests which they look after until the eggs hatch and the larvae move away SEE CASES STUDIES.

Spawning behavior in the Nassau grouper, Epinephelus striatus:

(1) fish start to move up into 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 (usually a female) followed by bicoloured fish (usually males);
(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 .

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

Some surgeonfishes, wrasses and parrotfishes spawn both in aggregations and in pairs outside of aggregations, while most species that aggregate are only known to spawning in these aggregations. However, more research is needed before this question can be fully answered.

Do spawning aggregations always occur at the same location?

Generally, for many demersal species the answer appears to be ‘YES’ although some sites have been known to shift a little between years. Some aggregations have formed at the same site annually for over 50 years. This is why the protection of spawning sites can be a powerful management or conservation measure. On the other hand, spawning areas for some pelagic species, such as bluefin tuna, can vary somewhat between years.

How do fish know which aggregation site to go to?

It is suggested that young adults learn where aggregation sites are by following older fish, although little research has been carried out to verify this idea. Suggestive of this possibility are observations of juvenile fish at aggregation sites where they are not normally present. If true, one implication is that restoration of overfished aggregations by introducing new fish (or restocking) may not work if the ‘group memory’ or ‘traditional knowledge’ of the site has been lost. We also know that in some species the same individual (tagged externally or acoustically) fish return to the same aggregation sites each year and, hence, show evidence of site fidelity.

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. As a general rule, transient aggregations are larger, of shorter duration and less common than resident aggregations. Learn more about the types of spawning aggregations here

How far do adults travel to and from spawning sites?

Little appears to be known about temperate species’ movements to and from aggregations but tagging studies on coral reef fishes have shown that individuals can travel over 200 km between their home reef and aggregation site (Nassau grouper, Epinephelus striatus). 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 (and fisheries) 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. For other species that have been tagged, distances to transient aggregations can range from a few km to many tens of km.

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. For this reason for exploited this reproductive event must be safeguarded, by seasonal and/or spatial protection, to ensure that enough adults remain in the water to replenish fished population each year.


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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