Fish Spawning Aggregations: A Reef Resilience Toolkit Module

Threats to FSAs

While there are numerous threats to reef fish populations in general, there are also many direct threats to the existence of their spawning aggregations. These threats may include:

Fish that are captured for the live reef fish trade are often taken from spawning aggregations. By removing these fish, and therefore their reproductive output, the reproductive success of the entire population is impacted. Photo © Will Heyman/TNC

• FSA fishing
• Loss of socially mediated reproductive cues
• Tourism
• Poor understanding of FSAs and FSA-forming species
• Overfishing of other life history stages

Factors Influencing FSA Overfishing

All FSAs are sensitive to overfishing, given their vulnerable nature and attraction for short-term economic gain. Some inherent and external factors can increase their vulnerability:

• Fishing method
• Size of the FSA
• Intensity of fishing pressure
• Spawning (operational) sex ratio
• Spawning method (e.g., pair-spawning or group spawning)
• Sexual pattern (gonochoristic or hermaphroditic)
• Variations in sex-specific patterns of movement
• Length of the reproductive season
• Ease of access to the spawning aggregation
• Other reproductive behaviors (e.g., spawning stupor)
• Use of migratory corridors or other areas that concentrate reproductive adults

Fishing Method

Spearfishing can be a very destructive form of fishing because of targeting the larger breeding individuals. Photo © Great Barrier Reef Marine Park Authority

Destructive fishing using cyanide has been implicated in a number of cases of FSA overfishing, primarily by the Southeast Asia-based, live reef, food fish trade1. Using this method, fishers can extirpate even large aggregations within brief time periods. Spearfishing, often used at night, has been implicated in FSA overfishing, as has hook-and-line fishing2, which is often characterized as a non-selective and more sustainable fishing method3.

Size of the FSA

Smaller FSA size can increase the vulnerability of overfishing and loss. Clearly, small FSAs can be more readily fished out than larger FSAs.

Intensity of Fishing

It has been suggested that anything greater than light fishing pressure, such as that characteristic of subsistence catch, can promote overfishing of FSAs. However, FSA loss has been recorded even from subsistence fishing. Given the vulnerability and complexity of FSAs, and FSA-forming species, no level of fishing is currently recommended for FSAs.

Spawning (Operational) Sex Ratio

For many reef fishes, there is an operational sex ratio that creates function within the reproductive population, and maximizes reproductive success. Alterations to this sex ratio from any means can increase the vulnerability of aggregation-forming populations, by reducing reproductive output, which may in turn endanger population persistence.

Spawning Method

Similar to spawning sex ratio, individual species may have a ‘preferred’ functional spawning method. For group spawners, for example, changes in sex ratio can affect reproductive output by altering the number of available members of one sex over the other, thereby leaving eggs unfertilized. Similarly, in pair spawning, a reduction in one sex can alter reproductive success and output.

Sexual Pattern

Sexual pattern can play a role in the vulnerability and persistence of FSAs. Gonochores are often more resilient to fishing than hermaphrodites4, and this has been demonstrated for several species in the wild. This should not be taken to mean that fishing FSAs of gonochoristic species may not lead to FSA loss, or to changes to reproductive output.

Variations in Sex-specific Patterns of Movement

For many FSA-forming species, variations in sex-specific movement and residency times can increase vulnerability to fishing, that in turn reduces reproductive success and output. Several recent examples show that males arrive at the FSA prior to females, and stay at the site longer, both within and during an aggregation month5. These differences can influence the number of males taken by the fishery, which in turn alters the sex ratio.

Epinephelus striatus—Nassau grouper. Photo © Brian Silliman

Length of the Reproductive Season

The length of the reproductive season may influence the vulnerability of FSAs to fishing. Fish forming spawning aggregations over a greater number of months have more opportunities to spawn and maintain populations. In contrast, the same level of fishing can greatly impact fishes spawning over shorter time frames, e.g., 1-2 months a year, since the number of annual spawning events, i.e., opportunities to replenish the population, is diminished.

Ease of Access

Where FSAs form in nearshore, shallow areas free of physical or environmental constraints for fishers, their ease of access to fishing increases, and therefore the chance of extirpation increases. Species that form FSAs in areas difficult to access may have a greater chance of survival, assuming the larval supply to the population is not dependent on an outside, easily accessed FSA.

Other Changes in Behavior

Some spawning fish exhibit a behavior that has been termed “spawning stupor6.” While fish are still aware of their surroundings, it has been noted that they may allow divers to approach with less concern than during other times. This behavior may allow fishers to come into close range, thus making them easy targets.

Problems with Fishing Migratory Corridors

A study of a Nassau grouper spawning aggregation in Mexico suggests that exploitation of migratory corridors may have ultimately caused the disappearance of the FSA.

It is reported that despite relatively heavy exploitation of the aggregation for over 50 years, mean fish sizes and sex ratios remained stable. During this time, fishers used only hook-and-line gear, and reported annual landings of up to 24 tons.

However, following the introduction of gillnets and spearguns in the early 1990’s, the mean sizes and length frequency distribution of groupers began to decline; and in 1996, the aggregation, along with smaller aggregations that had appeared nearby, disappeared. It is suspected that the efficient capture of migrating Nassau groupers using gillnets, as shown below, catalyzed the disappearance of the FSA.

View of the Mahahual coast showing coral reef morphology, the area of the traditional aggregation site and areas where fishermen deploy their gill nets to catch the migrating Nassau grouper. The reef border is followed northwards by the fish. From Aguilar-Perera and Aguilar-Dávila 1996.

Use of Migratory Corridors

There is evidence that fishers may target not only FSAs, but also the migratory corridors of spawning fish7. Like FSAs, migratory corridors concentrate fish in a small area in high density, making them attractive to fishers. Fishers who know the location of FSAs are also likely to know the location of migratory corridors. For this reason, it is imperative to identify and incorporate migratory corridors into management plans, such as MPAs.

Loss of Socially Mediated Reproductive Cues

The mechanisms by which migrating fish identify proper or historical spawning sites, and identify the same sites year after year, are not understood, but thought to be socially mediated. It has been hypothesized that ‘knowledge’ is constantly transferred from older fish to younger fish, and therefore that FSAs may consist of distinct social units8. In the Bahamas, it has been shown that fish are drawn to specific FSAs, and are capable of ‘learning’ precise migration routes9. This may occur for fishes completing long-distance migrations, as well as resident-type migrations. In a study of the bluehead wrasse (Thalassoma bifasciatum), the experimental replacement of entire local populations resulted in these populations using entirely new spawning sites, suggesting that FSA site selection and location is more of a socially transmitted process than that of an individual ‘decision’ process, based on its assessment of the local resource conditions.

Caution should be exercised in allowing fishers to remove the largest members of a population. If the above hypothesis for FSA site selection is true, then the removal of fish that have previously spawned at a FSA site, i.e., are socially conditioned to re-locate certain sites, may prevent future FSA formation at that particular locale. If certain FSA sites represent locations that promote population persistence, and create evolutionary advantages for the species or population(s), the loss of these socially mediated cues may threaten local or even regional population persistence.

Tourism

Tourism may have negative effects on spawning aggregations, via indirect effects of coastal development on essential fish habitat; and snorkelers and divers may impact reproductive behavior at FSAs. Boats and humans may disrupt not only spawning activities, but also fish that may be feeding on these aggregations. There has been little work done to document the effect of such activities on FSAs, so it is recommended that such activities be discouraged, and when allowed, closely monitored. Particularly threatened species should be protected from all possible disruptions during reproductive periods10.

Poor Understanding of FSAs and FSA-forming Species

Ultimately, an incomplete understanding of FSAs can be a threat to proper management. Many FSAs remain undiscovered, and the list of species that form FSAs is constantly being expanded, especially in areas outside the Caribbean and Indo-Pacific11. Other gaps in knowledge include an understanding of connectivity between FSAs, and understanding of the impacts of a loss of an FSA on a local fishery or fish population. For this reason, a “data-less management” approach12 has increasingly been embraced. The theory behind this approach is that the pace of our understanding of human effects on FSA is considerably slower than the rate of disturbance. Scientific understanding of FSA and FSA-forming species typically entails costly, lengthy investigations, that may take years to complete. By comparison, intense fishing can significantly deplete an FSA in days or weeks. Given the well-established importance of FSAs, and their relative vulnerability, drastic measures such as full closures of FSAs are justified, even in the complete absence of scientific data to support these decisions.

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