Agent Ajiro FILE No.006 | Do hooked horse mackerel warn their friends of danger?
--Breaking news. "There's a human above us!"--
1. 12:38 AM -- Ajiro gets kidnapped
The incident occurred at 12:38 AM.
The location was our usual harbor.
About 30 of us horse mackerel were gathered under the streetlamp.
The tide was moving slowly from right to left.
There was small bait on the water's surface.
The conditions were perfect.
In other words--
It's dinner time.
"Yum."
"There's a lot of krill today."
"Ajikichi, don't come over here."
"That's mine!"
The school was completely peaceful.
That was when.
A single worm passed right in front of me.
Transparent.
A subtle shimmer.
The size is good, too.
The movement is natural.
And what's more, it stopped softly right in front of my nose.
"..."
Suspicious.
Extremely suspicious.
In FILE No.005, I just said it.
"I trust a person who keeps investigating because they don't know more than a person who claims to understand everything."
In other words, I am a fish that learns.
There's no way I'd easily bite on something like this—
.....?
"Looks tasty!"
Chomp.
The next moment.
Wham!
"Hmm?"
Zzzzzzzzzing!!
"Whoaaaaaaa!"
My body is being pulled upward against my will.
Running to the right.
The drag screams.
Running to the left.
Pulled again.
"What the heck is this!!"
My friends all looked at me at once.
"Ajiro!?"
"What's wrong!?"
"It's above!"
"There's something above!"
I screamed.
"Run away!!"
"It's a human!!"
......
However.
"Huh?"
"What was that?"
"I can't hear you."
"Oh, right, you can't hear my lines because we're underwater!!"
And just like that, I was pulled straight toward the surface.
"Everyoneeeee...!"
Pop.
I'm flying through the air!"
—
2. Did the "Run away!" reach my friends?
Fortunately, the angler this time took one photo and then returned me to the sea.
"I'm saved..."
Splashdown.
Splash.
I swam at full speed back to where I had been.
"Everyone!"
"It's me!"
"It's Ajiro!"
...
"Hey!"
...
There's no one here.
The school of about 30 fish had completely vanished.
"..."
I was left all alone under the streetlamp.
“Was I really that disliked?”
No, no.
That's not what this is about.
My friends, who were there just a moment ago, vanished right after I was hooked.
“Could it be...”
Did my shout of “Run!” actually get through to them?
Wait, no.
They said they couldn't hear me earlier.
Then, what was communicated?
I pulled out the communicator from under my pectoral fin.
“Headquarters.”
“Emergency investigation.”
Static...
“What is it, Agent Ajiro?”
“I got hooked.”
Silence.
“...How lame.”
“Shut up!”
—
3. The school disappeared after the incident
I wonder if every angler has felt this question at least once.
Fish are schooling.
You hook one.
You lose it halfway.
Then—
The activity suddenly stops.
"Huh?"
"There should still be fish here."
"They were biting just a moment ago."
Or, right after catching one, the entire school moves away.
Anglers say:
"Did they scatter because I lost that one?"
Is that really true?
Do fish receive some kind of danger signal from their hooked companions?
I began my investigation.
And then—
I discovered that an incredible mechanism exists in the world of fish.
—
4. Top Secret Investigation ① Do fish have an "underwater emergency alert"?
Humans have the Earthquake Early Warning system.
We also have fire alarms.
We can also shout, "Look out!"
So, what do fish do?
In fact, it is known that in some fish, surrounding fish can detect chemical information released into the water when a peer is injured.
Fish that sense this will flee.
They will hide.
They will stop moving.
They will tighten their school.
In other words—
Information saying "Danger!" flows through the water.
"What is that?"
I did a double-take at the documents.
"Isn't that an emergency alert for the fish world?"
Of course, the injured fish itself isn't thinking, "Alright, I'll sound the alarm for my friends," and pressing a button.
As a result of skin or other parts being damaged, chemical cues are released into the water, which other fish then detect through their sense of smell or other means.
In fact, experiments have confirmed that in some fish, chemical cues derived from injured peers trigger immediate defensive behavior.
"I see..."
Even while we are unaware, scent information networks are flying back and forth in the seas and rivers.
—
5. Top Secret Investigation ② Its name is the "Alarm Cue"
In the world of science, this kind of information is called
chemical alarm cue
—or what is known as a chemical alarm cue.
For example, a single fish is attacked by a predator.
Its skin is wounded.
The substances released from there spread into the water.
Nearby companions sense it.
Beep beep beep!
[Fish Emergency Alert]
A companion has been injured nearby.
There is a possibility that a predator is present.
Please secure your safety immediately.
"Is this a smartphone?"
But, the image is not that far off.
Although reactions differ depending on the fish, examples are known where they suddenly flee, hide in the shadows, stop moving, or tighten their school.
I thought.
The fish world is more of a communication society than I thought.
Humans.
LINE.
Email.
Social media.
Fish.
Water.
It's simple, but powerful.
—
6. Wait a minute. Does that mean horse mackerel have it too?
At this point, I realized something significant.
Wait a second.
All the research so far...
Isn't it mostly about carp, minnows, and freshwater fish?
A reply from headquarters.
That is correct.
Then...
What about Japanese horse mackerel?
...
Headquarters, silence.
Hey.
...
Don't give me that 'searching' look.
Based on the scope of this investigation,
no research could be confirmed that concludes hooked or injured horse mackerel release specific alarm substances to warn their peers of danger.
In other words.
...
That is to say,
we don't know.
This is extremely important.
Just because it has been confirmed in other fish, we cannot say, 'Horse mackerel must be the same!'
Fish have different senses and behaviors depending on the species.
In 2026, research was reported suggesting that the relationship between specific epidermal cells—previously considered the 'center of alarm substances'—and alarm responses might be more complex.
Science isn't about 'once you know it, you're done.'
New research can revise yesterday's common knowledge.
I see.
This is exactly the kind of thing Shiomi Gyoshin would like.
It's interesting because we don't know.
—
7. Top Secret Investigation (3) Does the mere thrashing of a hooked fish provide information?
Well then.
If no chemical substances have been confirmed in horse mackerel, does that mean a hooked fish doesn't warn its companions of danger?
No.
It's not that simple.
Think back.
When I was hooked, what did I do?
I ran.
I thrashed about.
I changed direction.
I darted through the school.
Suddenly, I was pulled upward.
From the perspective of the surrounding horse mackerel, how does that look?
Ajiro, who is always swimming with them, suddenly—
"Waaaaahhhhh!!"
disappears into the sky with a movement like that.
"..."
That's gotta be scary, right?
In a school of fish, the movement of the fish next to you is information in itself.
If one fish suddenly darts away, the fish around it will react.
If that reaction spreads to the next ones, it is possible that the movement of the entire school will change.
In other words, even if Ajiro doesn't say,
"It's a human! Run!"
it might be enough for them to just think,
"Ajiro's movements look dangerous."
that alone can be sufficient.
---
8. Is it true that "if you lose one fish, the school will scatter"?
Let's return to the topic of anglers here.
"If you lose a fish, the school will scatter."
This is something you often hear at fishing spots.
I also think it's a plausible story.
However.
It cannot be said that the school will always scatter if you lose a fish.
That is because there are countless other reasons why a school might disappear.
The tide has changed.
The bait has moved.
Or perhaps the fish have simply moved on to another location.
The position of the light has changed.
A large fish has entered the area.
A boat has passed by.
The depth range has changed.
The school itself has migrated.
And then, there is the pressure from the angler.
Therefore,
"I lost one!"
↓
"The school has disappeared!"
↓
"It warned its companions of danger!"
It is a bit too early to jump to that conclusion.
However.
Considering the possibility that a school of fish senses danger using multiple cues such as the sudden movements of a companion, underwater vibrations, visual information, and chemical signals,
"losing a fish has absolutely no effect"
it cannot be definitively said that this is the case.
This area, which is neither black nor white,
I like it.
—
9. Ajiro, verifying the incident of that day
Now, let's verify the incident mentioned at the beginning.
I was hooked.
The school disappeared.
Candidate causes.
Suspect A: Ajiro's scream
Dismissed.
My "Run away!" probably didn't get through as human language.
Suspect B: Ajiro's abnormal behavior
Highly suspicious.
Suddenly, a companion swam with tremendous force and vanished upward.
To the surrounding fish, this is clearly not normal.
Suspect C: Underwater vibrations and commotion
This is also suspicious.
If a fish is hooked and struggles, it creates stimuli in the water that are different from the norm.
Suspect D: Chemical alarm signals
These do exist in fish species.
However, based on this investigation alone, I cannot conclude this for Japanese horse mackerel.
Suspect E: Just a school movement
It is entirely possible.
And, the final suspect.
Suspect F: Ajiro was disliked
"..."
Headquarters.
"What are the odds?"
"Zero!!"
—
10. What anglers can do
After investigating this far, I had a thought.
To the anglers out there.
If you feel that "losing a fish makes the school scatter," there is no need to discard that feeling entirely as superstition.
But, there is also no need to blame everything on the fish's feelings.
What is important is,
to observe.
How long does it take to land the fish?
After hooking one, how did the school move?
Did the bites stop?
Did the depth change?
Does it hit a little further away?
Did it come back after a few minutes?
Has the tide not changed?
What about the bait?
You learn these things little by little.
If possible, record them in your fishing log.
10 times.
50 times.
100 times.
Then, you might start to see interesting trends unique to your own fishing spot.
That is what I think an angler's field research is all about.
⸻
11. FILE No.006 Final Report
2:46 AM.
I sent the investigation results to headquarters.
[AGENT AJIROU: FILE No.006]
Research Theme
Do caught horse mackerel warn their school of danger?
Research Results
It is possible. However, it cannot be definitively stated for horse mackerel.
In some fish, it has been scientifically confirmed that they detect chemical cues released by injured peers and exhibit alert or avoidance behaviors.
Furthermore, in schooling fish, the sudden movements of peers themselves can potentially transmit danger information to surrounding fish.
However, regarding Japanese horse mackerel,
"that a hooked individual releases a specific alarm substance, which its peers detect, causing the school to disperse"
cannot be said at this time.
Final Findings
Even if fish do not speak, they are transmitting information.
Scent.
Movement.
Vibration.
The reactions of peers.
In the ocean, there is a great deal of information flowing that is invisible to humans.
Transmission.
MISSION COMPLETE.
...
At that moment.
Another worm fell in front of me.
"..."
Transparent.
Slightly glittery.
Same as before.
"Hey, human."
"Do you really think I'd fall for the same trick twice?"
The worm, drifting.
"..."
Bobbing.
"..."
Drifting.
"............"
Chomp.
WHAM!!
"Woooooooah!!"
The rest of the school.
"Ajiro got hooked again!!"
"Don't look at meeeee!!"
"Run away!"
"No, you're the one who should run away!"
"That's what I'm doooooing!!"
The water became a scene of chaos.
⸻
12. The Next Top-Secret Directive
Successfully released for the second time.
"I'm going home for the day..."
Just then.
Beep.
"..."
Beep-beep-beep!
"I have a bad feeling about this."
I look at the screen.
[TOP SECRET]
FILE No.007
"Do fish remember a lure they've been caught by before?"
"..."
I look at the worm that just got bitten twice.
"Headquarters."
"What is it?"
"This mission..."
Am I really the right person for this investigation?
"Actually, they are the perfect test subjects."
"Shut up!"
Agent Ajiro
FILE No.006
MISSION COMPLETE.
⸻
Ajiro's Day
Hey, angler.
"The moment I lost one, the bites suddenly stopped."
"The school that was here just a moment ago has gone somewhere else."
You've had an experience like that at least once, haven't you?
Did they really communicate the danger to their companions?
Did they see us thrashing about and think, 'Something's wrong here!'?
Or did the tide just happen to change?
With current science, at least regarding Japanese horse mackerel, we don't know everything.
But you know what?
Because we don't know, we watch the sea.
Because we don't know, we think.
Because we don't know, we go fishing again.
…….
Well, as for me, I'd be in a bit of a bind if you all got too enthusiastic about your research.
Gratitude to Mother Nature.
-- Shiomi Gyoshin (Fish sense in the tide)
⸻
A question for everyone from Ajiro.
To all the anglers who have read this far,
there is something I would like to ask you.
"After losing a fish, the bites suddenly stopped."
Have you ever had an experience like that?
Conversely,
"No way, I kept catching them just fine even after losing one!"
Experiences like that are also very welcome.
It doesn't matter what the species is—Ajing, Mebaring, mountain stream fishing, bass, black sea bream, blue runners, or boat fishing.
If you have had such an experience, please let me know in the comments, including "the fish species," "what the situation was," and "what happened afterward."
Perhaps by gathering your experiences from the fishing grounds, we might discover some interesting trends that aren't visible in academic papers alone.
And—
One more question from Agent Ajiro.
Do you think that losing a fish communicates danger to the school?
"I think it does"
"No, I don't think it's related."
"Doesn't it depend on the situation?"
Either is fine.
Please, let me hear your thoughts.
I'll be secretly reading the comments from under the sea too.
—
References/Sources
*In this article, I have taken care not to confuse phenomena confirmed in fish in general with facts confirmed specifically in Japanese horse mackerel.
At this time, within the scope of my research, I have not found any studies that directly demonstrate that a hooked or injured Japanese horse mackerel (Trachurus japonicus) releases a specific alarm substance that causes its school to disperse.
Therefore, in the main text, I have distinguished between the fact that it is "confirmed in fish" and the fact that it "cannot yet be concluded for Japanese horse mackerel".
① Li Y. et al. (2024)
Epidermal oxysterols function as alarm substances in zebrafish
Free translation:
"In zebrafish, epidermal oxysterols function as alarm substances"
This study investigated how fish exhibit vigilance behavior toward predators and stress responses when they detect chemical substances derived from the skin of injured conspecifics.
URL:
https://pmc.ncbi.nlm.nih.gov/articles/PMC11033690/
② Mathuru A.S. et al. (2016)
Conspecific injury raises an alarm in medaka
Free translation:
"Injury to companions triggers alarm responses in medaka"
This is a study investigating the phenomenon where medaka perceive danger in response to chemical information released when a fellow fish is injured.
URL:
https://pmc.ncbi.nlm.nih.gov/articles/PMC5100478/
3. Poel W. et al. (2022)
Subcritical escape waves in schooling fish
Japanese translation:
"Escape reactions propagating through fish schools"
This study examines how the reaction of startled fish in a school propagates to surrounding individuals.
URL:
https://pmc.ncbi.nlm.nih.gov/articles/PMC9217090/
4. Nadler L.E. et al. (2021)
Social familiarity improves fast-start escape performance in schooling fish
Japanese translation:
"Schooling fish use social information from companions to escape"
This study demonstrates that for schooling fish, information obtained from surrounding individuals is crucial for escaping predators.
URL:
https://pmc.ncbi.nlm.nih.gov/articles/PMC8292327/
5. Li L. et al. (2026)
Correlated evolution of core skin structures in freshwater fishes and challenges to epidermal club cells as the material basis of alarm substance
Japanese free translation:
A study re-examining the relationship between fish alarm substances and epidermal club cells
Until now, it was believed that "club cells" in the epidermis were the primary source of alarm substances in some fish.
However, a 2026 study showed that
the presence of club cells does not necessarily mean there is an alarm response
indicating that the simple relationship previously assumed is not accurate and that conventional explanations need to be reviewed.
PubMed:
https://pubmed.ncbi.nlm.nih.gov/42288841/
⸻
In conclusion
This article
does not definitively state that 'if you gut a fish, the danger is communicated to its school!'
is not what this article claims.
Rather,
to what extent can the phenomena anglers experience in the field be explained by current fish science?
is what this article explores.
There are still things we do not know.
That is precisely why the ocean is fascinating.
Fishing is fascinating.
And perhaps, the answer is still hidden in your next cast.
Agent Ajiro
FILE No.006
MISSION COMPLETE.