Marine Ecology Blog

Biological and behavioural traits of groupers that leave them highly vulnerable to overfishing

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A malabar grouper (Epinephelus malabaricus) perches at the reef edge following a busy night launching stealthy ambush attacks on unsuspecting prey. Traps are commonly used around the world to catch large groupers resting near the reef edge – often a much easier target that other high value fish swimming mid-water. 

The groupers are a large sub-family of fish that make up a significant portion of the Serranidae family, alongside sea basses. They have a wide distribution amongst tropical and subtropical marine environments, and are key members of the ecosystems they belong to. Despite significant size variation amongst species, all groupers have a predatory lifestyle as ambush predators. During the day, a range of species can often be spotted perched between rocks and corals, taking advantage of their excellent crypsis, and feeding opportunistically on prey that comes into range. During the night, many larger species become more active, but remain stealthy as they ‘creep up’ on prey before delivering an ambush attack. Like other types of teleost fish, groupers are able to change the volume inside their mouth (buccal cavity), and in doing so also affect its internal pressure. Extending their mouth outwards causes a reduction in pressure, which they utilise to catch and ‘suck in’ prey.


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A coral grouper (Plectropomus spp.) carefully positions itself against a barrel sponge. Extremely well camouflaged against its background, here it will be able to launch a successful ambush attack on any prey that wander into range. (Photo taken off the west coast of Thailand.) 


Amongst coral reefs, including those of Koh Tao, small grouper species are some of the most abundant small to mid-sized predators with a wide-ranging diet of smaller fish, crustaceans and other invertebrates. Larger grouper species occupy a much higher trophic position, as mesopredators (mid trophic predators) and apex (top) predators. These larger species play a highly important role in promoting biodiversity across the reef through density dependent predation. (Click here to read more about density dependent predation.)

In recent years, the population status of many groupers has become of significant concern, with several species now classified as ‘endangered’ or ‘critically endangered’. Groupers are often heavily targeted by the fishing industry across their entire distribution – involving both local and commercial fishing efforts. Larger species are extremely high value, and with their bolder personalities, are often easy to catch when spearfishing. During the day, groupers spend much of their time on, or close to the edge of the reef, where fish traps are commonly used to catch them. This method has become extremely popular in areas of Asia and Indonesia involved in the live reef fish trade (LRFT), where the price of groupers sold alive for later purchase in restaurants is considerably higher.


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Groupers often have bold personalities and resist fleeing from humans – a trait that often allows for close up photographs, but is also heavily exploited by spearfishing.      


In addition to their relatively high value, there are a number of biological and behavioural traits that leave groupers particularly vulnerable to overfishing. Groupers are protogynous hermaphrodites, meaning that during their life they transition from being a female to a male. This transformation occurs when they are of a certain size, resulting in the larger members of a population typically being made up entirely of males, and smaller individuals being female. In most cases, fishing tends to be selective of larger sized, and therefore more profitable fish. When selective fishing methods are applied to groupers, such as when spearfishing or when rejecting fish below a certain size threshold, populations can be significantly affected as large numbers of males are directly removed. Such disruption to a population often results in sperm limitation, which can have a drastic effect on the population size of future generations – a common factor in unsustainable fishing. 

Another reason why many groupers are particularly vulnerable to overfishing is due to certain reproductive behaviours. A number of species aggregate in vast numbers each year, with spawning aggregations often containing the majority, if not all of a local adult population. Aggregations are formed with precise consistency, regularly synchronising to the exact day (and often hour) each year, and form in the same locations each time. Knowledge of where and when these aggregations form has become of significant value to grouper fisheries. During a spawning event, fishing efforts often become heavily concentrated in the spawning area, resulting in significantly large numbers being caught with relative ease and with much reduced costs and time spent fishing. Removing a large proportion of a population over such a short period of time can cause immediate and catastrophic effects, and has commonly been associated with the collapse of certain grouper populations around the world, as well as many other types of fish that form spawning aggregations.

Groupers are a key member of fish monitoring programs established by the Roctopus ecoTrust. Several local species included in these programs are already labelled as endangered, or critically endangered and their populations are therefore closely monitored. During the COVID pandemic, groupers have become a particularly important group of fish incorporated into other research projects set up by the Roctopus ecoTrust, particularly in areas which have had a significant change in site use amongst common resource users.



-- Click here to see more of Piers' underwater photos of marine organisms. -- 



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