How to sneakily erradicate thousands of humanoid tribes that live in the sea? [closed]

Question

This question isn't about structuring the plot of a story. It has nothing to do with any element that isn't listed in this post. It's about what technical solutions would be best to solve a really hard task in the most safe and time efficient way.

• You have two years time and quicker is always better.
• You have as much cash as you want
• There are lots of scientists from all fields that would do anything for money and it's easy to get in touch with them.
• Most companies don't care about what you're doing with their products.
• You have lots of smaller bodies of water where you could test your plans.
• The sea in which the tribes live has about the size of the pacific ocean.
• The pressure rises 5 times as quickly as on earth as you go deeper.
• The tribes live between -50 and -350 meters below sea level.
• They can dig themselves into sand and sleep there for many lifetimes when they are getting scared.
• You're pretty much the only one who hates the sea people.
• Multiple governments and activists would want to protect the tribes if they found out what you're trying to do.
• Your tech level is about where we are on earth.

If you need any other information, just assume that it's planet earth we're talking about and go from there.

Thanks and good luck.

Answer 1

Asteroid:

-Given a small support base, genocidal intent, and multiple states opposing you efforts (reversing anything slow you might do) you need to kill not just the sea people in the water, but any that might already be hibernating in the lower levels. The unlimited budget helps. I assume a large tech base for your mass murderer, or at least access to space (multiple species on a planet and enough sophistication for surface-dwellers to be involved in the deep ocean).

-You need to kill them all, very quickly, without reasonable chance of recovery. I'm guessing you don't want to render the entire planet uninhabitable, but given the handwashing level of evil I could be wrong. Drop an asteroid into the ocean and you'll disrupt the planet but most critically the ocean. Massive basin-wide disruptions from the huge shockwaves may not kill all of them, but will so disrupt the population and supporting ecosystems that extinction would be likely, and massive die-off and genetic bottlenecking is almost guaranteed.

• Control the narrative. "Oops, I did this terrible thing. What a disastrous accident!" Try to make your villain out as a regretful humanitarian trying to make things right and save the sea-folk.
• Set up a program to "help" the sea folk, moving any remaining individuals into "breeding programs" aka concentration camps where you can eliminate the survivors at your evil leisure. Get rid of them via mistreatment, massacre, or mishandled environmental remediation. Think native Americans on reservations where the person in charge isn't just incompetent or lazy, but genuinely malicious.
• That isn't enough, so transform the ocean with some sort of invasive algae that destroys the remaining ecosystem, choking out native plants and that can't be digested by the organisms that the sea-folk depend on for survival. This will tip your villain's hand once people realize what you unleashed.

Answer 2

Nanobots. Given infinite money, presumably a Secret Lab, and some Mad Genius, the best way to secretly annihilate such a species would be self-replicating nanobots. You design them to be able to identify and attack the target species, preferably in a manner that mimics a common disease but cranked up to 11. So say, the nanobots destroy the target's gills and they suffocate, or destroys their nervous system so they die from the various ways that can kill you. Pick any of them. The mimicry is only to throw land governments off the scent until the Deed is Done. You also need to have VERY FIRM parameters for these things. make sure they can only function in water, make sure they won't attack every fish in the sea, make sure they won't replicate themselves into a planet or ocean-wide grey goo. My first thought was to have them only replicate when either A: they are actively attacking a Merperson (hereafter Merson) because they'll probably have to deal with the Mersons' natural defenses like white blood cells. or B: when there are fewer than X number of nanobots per cubit meter of water.

Once the design is perfected you release a vial of nanobots into the ocean. Then you wait. You've done the math, you've calculated replication rates and ocean tides and all the rest of it. You wait until you are 100% certain there is a hundred or thousand or however-many-you've-mathed-out-is-required nanobots per square meter of ocean. Then you hit the ON button. Suddenly every merson in the ocean starts bleeding from the gills and is dead within minutes. Once they've made a kill the nanobots disperse back into the sea. If you didn't mind a bit of lag between pushing the button and Final Victory you could even design the button to spread at a slower rate (maybe triggering every 'bot within 100 miles and then expanding 100 miles a day thereafter) so it looks more like a natural plague to the dirtsiders. That way it'd be REAL hard to figure out exactly what killed the mersons. Even if the governments of the world realized it was nanobots, they'd have a helluva time finding them and purging the entire ocean. Remember you've waited long enough for the whole ocean to be tainted, and they do nothing to fish, people, etc.

Some hibernating mersons may survive this initial purge, but in the long term they're doomed too. The nanobots are in the ocean, dormant unless they either fall beneath the bots-per-gallon level or contact a merson. So every time a merson wakes up from hibernation it breathes its first breath(?) of water, swallows nanobots, and dies within minutes.

And don't forget the final step, popping the cork on some Evil Champaign and kicking your henchman in celebration! Congratulations, you have successfully committed Xenocide!