Groupers are reef fish, so like most other reef fishes, their spawning occurs offshore on the shelf and shelf-edge reefs, because they are primarily bottom feeders. The spawning process of groupers of different species may not be the same, but some similarities are fascinating phenomena. Groups of groupers spawn together.
How do groupers spawn?
Most groupers spawn in aggregation, which means that when the spawning season starts, typically for the few days around the full moons of December and January, several groupers of a specific species will gather at a particular spot to spawn together. Usually, a minimum of 100 groupers come together at the spot to spawn in an aggregation.
The spot will be the most favorable for spawning. Some groupers will swim from miles away to reach that spot when it’s time to spawn. The number, size, and location of spawning aggregations can differ significantly depending on the species and the habitat, but they are on a more or less continuous spectrum.
Take the Nassau grouper, for example. They have spectacularly large aggregations that typically occur during the few days around the full moons of December and January. On the other hand, the gag groupers and scamp groupers form aggregations that last for two to three months at a stretch. The area they aggregate in is pretty large as well.
Again, the Goliath grouper aggregates between the late summer and early fall, as in August to October, in aggregations of 100 individuals. Moreover, there are some exceptions to the rule as well. Some groupers don’t spawn in aggregations in the first place. The best example is the red grouper. This shows that the aggregation behavior of each species differs from various factors.
The spawning process using which groupers reproduce is known as broadcast spawning. In this process, females first release the eggs they produce. Then the male groupers release their sperm into the water column above deep reefs. The sperm cells then penetrate the floating eggs at random and fertilize them.
Despite sounding rather absurd, this method increases the likelihood of the eggs becoming successfully fertilized to a great extent. It also ensures that egg predators on the reef surface will not eat the fertilized eggs.
Why does the spawning method of groupers make them vulnerable to overfishing?
Due to spawning in aggregations, groupers are especially susceptible to overfishing. The reason is apparent. That location becomes very obvious when many groupers assemble at the exact spot during every spawning season. During this time, if the bait is thrown and the groupers are targeted for capture, then it becomes extremely easy to get a catch quickly.
For gag groupers, this particular feat becomes even a bigger problem. This is because the male groupers become more vulnerable to being overfished than the females because of selective fishing. They stay at the aggregation sites for the whole year while the females move in and out seasonally.
So not only do they get captured during the spawning season but also the other times of the year. This results in the number of males becoming significantly lower than that of females. As a result, there isn’t enough sperm to fertilize the eggs, and spawning opportunities for females are reduced.
Luckily, fishing for groupers has become regulated in most locations by the government to make sure none of the species go instinct since groupers play such a key role in their respective ecosystems. Grouper fishing is forbidden during the entire spawning season to avoid mass-fishing of aggregated groupers. The restriction stretches to a few months after the season ends to let the baby groupers grow.
Where do groupers live?
Groupers live in tropical ecosystems, where water temperatures are moderately high and not too cold. Since they are bottom feeders, they are usually found in the deep-water region near the bottom. They are also seen in shallow hard bottom reefs, following the Pleistocene shorelines of the continental shelf and shelf edge.
The habitat range of groupers includes the Gulf of Mexico and the Florida Keys in the United States. They are further found in the Bahamas, the Caribbean, and the Brazilian coast. On some occasions, goliath groupers have been caught off the coast of New England in Massachusetts and Maine. They are commonly spotted close to caves, rocky ledges, and shipwrecks.
Do groupers change their sex?
So what happens when the number of males becomes significantly lower than that of females? The root cause of the decline in the number of males is the disruption of the social system of aggregations. When there is a disruption in the social situation, female groupers get the cue for sex change. That’s why the percentage of transitioning is highest during the immediate post-aggregation period.
In many grouper species, all groupers are born female. After gaining sexual maturity, some groupers will change their sex to male depending on their social situation. This can happen because groupers are born with both testis and ovaries, so they can do it. After maturing, the female grouper stays as a functional female for a certain period and can transition into a male if the need arises afterward.
However, though all groupers are typically born as females, several groupers are capable of maturing into males directly. In other words, they don’t have to undergo a female life phase. This type of reproductive development in an animal is called diandric protogynous hermaphroditism.
Why do groupers change their sexes?
To maintain the male-female ratio in the population and keep it under the balance, female groupers often change their sex to male. However, why exactly groupers are physiologically equipped with this unique reproductive trait is not clearly known.
Scientists guess that the reason might be to provide the fish with an ecological advantage. Because females mature into males, the entire male population is older. As a result, they are more capable of defending their territories. If the male grouper dies, the female grouper does not have to look for a new mate. She can simply change her sex to male and find a female mate, which is easier.