Some fish species change sex during their lives.
They may live as females and later become males, or go in the opposite direction. They do not change at just any time; they do so once they have grown to a certain size.
But observations have found that when people selectively catch only large individuals, that timing moves earlier.
3-line summary
1. Fish do not necessarily live their entire lives as the sex they were born with.
2. They changed at age four in protected areas and at age two in fishing areas.
3. Selectively catching large individuals changes the structure of the group.
Two groups were measured side by side in the Mediterranean
Off Spain’s Balearic Islands lives a fish called the pearl-scaled fish. It is a palm-sized species in the wrasse family.
Researchers compared fish from two places: a marine protected area where fishing was prohibited and an area where fishing took place. Here is what they found.
Fish in the protected area averaged 17.4 cm in body length and four years old. Fish in the fishing area were 14.9 cm and two years old. Those ages were also the timing of sex change. In the fishing area, they changed from female to male at age two; in the protected area, they did so at age four.
A full two years disappeared. The time spent as females.
Being large is advantageous for females
Why does size determine sex?
Making eggs requires space. A larger body can hold many eggs in its abdomen. Sperm, on the other hand, is not closely tied to body size. Even small males make enough sperm.
That is why, for such fish, strategies of being male when small and female when large, or the reverse, can work. Species that lead groups, such as the pearl-scaled fish, follow the latter strategy. Because one large male mates with several females, they lay eggs as females while small, then seek the male role after growing sufficiently large.
When the largest individual in a group disappears, the next in rank changes sex to fill the position. Clownfish—the orange fish with white stripes—go in the opposite direction: when the dominant female disappears, a male becomes female.
In other words, each species has its own way of filling a vacancy.
Fishing takes the large ones
This is where the problem arises.
Whether by angling or nets, the fish people take are generally large individuals. But in this species, large individuals are males. If males continue to be removed, the male position in the group remains vacant.
Then females that are still young move up to fill it. They become males at age two.
A researcher said: “Living longer as a female and growing larger can allow them to leave more offspring, but changing sex early removes the opportunity to lay eggs.”
This also means that releasing small fish alone does not solve the problem. The individuals being removed are at the other end.
This loss does not show up if one only counts the number of fish caught. Even if the same 100 fish are caught, a different age and sex composition among the fish left in the group changes the number of eggs produced the following year. Counting fish and looking at the group are different things.
The genes were the same, but the markings differed
There is another aspect of this study that stands out.
Researchers extracted DNA from the fins of 120 fish and compared it. The DNA sequences themselves were virtually the same in fish from the two areas. Naturally, they had not become different species.
Instead, chemical markers attached to DNA differed at 291 sites. These markers act as switches that turn genes on and off. It is like having the same text but different highlighter marks showing which parts to read.
Differences reportedly appeared in genes involved in energy use, neurons, and immune regulation. The paper was published July 23 in the Royal Society journal *Philosophical Transactions B*.
This distinction matters because the speeds are different. Changes in DNA sequences take a very long time. These highlighter marks, by contrast, can change over much shorter periods. If the effects of several decades of fishing can remain in fish bodies, this rather than the sequence is more likely to be the pathway.
Where the researchers were cautious
It is best to present this point as it is.
The researchers stated that this was a strong association, not proof of causation. Other explanations are possible, including population density, predators, food conditions, and disruption of social structure.
The timing of fish sex changes is inherently a value that shifts according to surrounding conditions. It is therefore difficult to isolate fishing pressure and pin it down as the cause.
Still, the protected-area population was clearly preserving genetic diversity much better.
Fish that change sex also live in Korean waters
This is not an unfamiliar story. Black porgy grow up as males, then most change to females at around four or five years old. Groups of seven-band grouper also generally change from female to male.
The individual anglers call “a good-sized one” may, depending on the species, be the fish that lays the most eggs in the group or its only male.
There is also something to check before heading out to sea. The Fisheries Resources Management Act specifies, for each species, periods when catching is prohibited (closed seasons) and sizes that may not be caught (minimum legal sizes), and the Ministry of Oceans and Fisheries publishes the full standards on one page — Closed Seasons and Minimum Legal Sizes by Species. Before a fishing trip, simply look up the line for the target species.
One large fish is not the sum of several smaller fish
One large fish is not the sum of several smaller fish.
It has a role within the group, and when that role is vacant, another individual fills it even by changing its body. The two-year difference measured in the Mediterranean is that cost, recorded in numbers.
References
- Hankyoreh As fishing takes away ‘dominant males,’ young females undergo premature sex change… “Opportunity to lay eggs disappears”
- Phys.org How fishing may be changing one fish species at the molecular level
- Philosophical Transactions of the Royal Society B Fishing pressure induces changes in DNA methylation in genetically homogeneous marine metapopulations
- Asia Economy [[Reading Science] Female? Male? Marine life that changes sex](https://view.asiae.co.kr/article/2019081615010932017)
- Hansan Newspaper Why black porgy change sex
The figures follow the paper and news reports above. The researchers’ own statement that this is not confirmation of a causal relationship has also been included.