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They are spread over several underwater-forests and live in small tribes under rocks. They eat fish and plants and all live in a big connected Ocean which is about a mile deep. Their level of technology is similar to ours in the Bronze Age.

To "kill them all" as a single person, I thought of creating a virus or a biological weapon, but that doesn't sound very original.

Is there a more original way you can come up with or an example from another fantasy world?

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    $\begingroup$ Ammonia is cheap and very deadly to water organisms. If you can develop such a weapon, you can also just buy a bunch of that stuff $\endgroup$ – Raditz_35 Apr 16 '18 at 7:09
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    $\begingroup$ What is the tech level for the exterminators? HOw big are these underwater forests and holes? Are these forests and holes linked together by rivers, are they in a single large lake, an ocean, everglades, swamps, etc? How many resources can the exterminators put towards the campaign? $\endgroup$ – Dan Clarke Apr 16 '18 at 7:09
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    $\begingroup$ @RonJohn, you're willing to accept mermaids but not that they could have some way of smelting bronze? $\endgroup$ – Separatrix Apr 16 '18 at 8:22
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    $\begingroup$ Welcome to WorldBuilding.SE! Please note that this is a Q&A site, not a discussion forum - questions should be looking for an objective answer, not other users' opinions. Please edit your question to narrow down the criteria, and take the tour and visit the help center to learn more about the site. $\endgroup$ – F1Krazy Apr 16 '18 at 8:33
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    $\begingroup$ @F1Krazy This question has a pretty direct answer -- looking for a specific way to commit underwater genocide that is not a virus spreading through the population. It isn't looking for opinions. I vote to keep the question open. $\endgroup$ – SRM Apr 16 '18 at 10:37
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Suffocate them

This is assuming that the ocean is a small ocean, only a few hundred or a few thousand square miles.

Use excessive quantities of fertilizer and dump phosphates and manure into the major rivers leading into their home.

enter image description here

The low-oxygen water will cause large fish-kills which will lead to even more rotting organic matter which will enlarge the anoxic zone further.

enter image description here

Salting them

One option might be available if there's an inland sea like the Dead Sea with extremely high salt content that's also above sea-level and available to the humans.

They dig a large canal between the inland sea and the ocean and then release the inland sea to flow into the ocean. The ocean floor might end up under a thick layer of brine, note, this might kill everything so better hope the humans don't rely on the ocean at all for food.

An underwater brine pool:

enter image description here

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  • $\begingroup$ The Mississippi has a vast dead zone into the Gulf of Mexico, this could work for very large areas. Alternative options... speaking of the Gulf of Mexico massive oil leaks can also consume deep sea oxygen which may take an unknown period to refresh, but Atlantic size Ocean wide will likely need to get more though. $\endgroup$ – Windlepon Apr 16 '18 at 14:23
  • $\begingroup$ If you dumped a Dead Sea worth of pure salt into the ocean, you'd get...the ocean. The ocean is real big, you aren't going to make it saltier unless you find a salt block the size of a small moon. $\endgroup$ – kingledion Apr 16 '18 at 17:47
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Introduce lionfish to the reefs where these mermaids live. These fish are an invasive species that has been destroying ecologies around the world because they eat a huge range of other species and -- due to their toxic fins -- have almost no predators themselves. They have been devastating the western Atlantic in recent years. If you collapse the small fish and mollusk species on which the mermaids depend, no amount of bronze smithing (however that is achieved underwater) will save them.

Here's a photo of one by Alexander Vasenin - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25512789 enter image description here

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    $\begingroup$ Unfortuantely, if you have a spear to hunt them and a knife to clean off the spines Lionfish are quite tasty so you might just have given the merpeople a new delicacy. $\endgroup$ – Borgh Apr 16 '18 at 11:39
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    $\begingroup$ Thanks, @Borgh , for highlighting that. I thought that the toxin was more body-wide. Turns out to just be the fins. My point stands about finding an invasive species, but perhaps lionfish is not the right choice. $\endgroup$ – SRM Apr 16 '18 at 23:12
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Most strategies fail at the absolute level everywhere, so I expect you need to take several answers and deploy them in an appropriate order to first weaken, then thin out and then finally finish off the populations.

My first to weaken would be incite racial tensions and war between the various mermaid factions and let them do the initial bulk work at low cost per unit.

You will likely need to prod and provoke and breed some really nasty fanatics to get things going to the genocide level, but divide and conquer has long proven success.

I really like Murphy's salt / oxygen starvation approach to finish them off, especially in smaller enclaves that become isolationist in the face of the growing troubles before you get to this final stage.

The middle is more difficult though, most internal strife will be burnt out by then, populations too scattered for disease or high effort localised methods to be effective, and there is plenty of space for them to keep running at this stage.

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How about nuclear or conventional depth charge ?
The shockwave should be enough to kill them and beeing in the water there is nothing they can do to avoid it.

Also, since there is a so big difference in the technological level, you can also use a guided torpedo to deploy the charge in a place as close as possible to your targets without they suspect anything.
Bonus point: you can conceal your torpedo in a big mermaid figure since yo uhave not the problem to make it stealth

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  • $\begingroup$ That would kill mermaids in a specific place, but not throughout all the oceans. $\endgroup$ – Renan Apr 16 '18 at 12:11
  • $\begingroup$ You just need to use more than one. I should have used the plurals $\endgroup$ – Gianluca Apr 16 '18 at 12:29
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Throw trash in the water. People do this all the time. Think about oil, plastic and nuclear dump that's thrown into the ocean. We're threating the whole ecosystem without even trying. I think you should do that.

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All right assuming slightly more advanced tech then todays, there are really just two options.

1) Engineered plague and a clean up squad. Disease won't kill all of them, but as long as it kills 80-90% it will be relatively easy to send in submersibles to harpoon and blow up the rest. Keep the kill squads around for a generation or two and whatever fragments of the Merfolk survived will be too small a population to be really viable.

2) Colony drop. Take several years or decades to maneuver an asteroid to land in the ocean. The shockwave, earthquakes, and decimation of a lot of the sea life will kill many of them. As soon as it's safe for humans, send in the kill squads and have a few nuclear bombs ready to destroy any large cities and pockets that survive.

Since this is a big ocean poison won't work as a realistic option, you'd have to sterilize the sea basically and at that point an asteroid would be cheaper. Nuking and attacking each settlement could work, but too many could escape unless you can cover the entire ocean in drones to track their movements.

It's easier and more cost effective to take the time initially to get them in one big hit.

Also to make the clean up easier, setting up refugee camps where the survivors could get food and resources as the rebuild, would be a good way to get as much of the surviving population in a few places as possible. Once the majority are in these places of safety set off a nuke in each one. That way the kill squads won't have to hunt down nearly as many to finish off the race.

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    $\begingroup$ The nuclear explosions would probably make half the ocean radioactive, and nobody could go near for months or even years. $\endgroup$ – Redwolf Programs Apr 16 '18 at 15:55
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    $\begingroup$ @RedwolfPrograms If detonated underwater, that wouldn't be too much of a problem. $\endgroup$ – wizzwizz4 Apr 16 '18 at 17:13
  • $\begingroup$ @RedwolfPrograms, nuclear tests on earth have seen the areas recover quickly from radiation. Why would this ocean be different? $\endgroup$ – Dan Clarke Apr 16 '18 at 17:35
  • $\begingroup$ @DanClarke true, true. But if it blew up nearby areas accidentally, or the fallout blew somewhere else, it would be a problem $\endgroup$ – Redwolf Programs Apr 17 '18 at 13:36
  • $\begingroup$ Bikini Atol saw 23 nuclear tests over several years. While the ecosystem was irradiated and is still unsafe for humans to eat, the plants and animals are remarkably healthy on the island and in the shallow waters. In deeper nearby waters the radiation level is no different from the rest of the ocean. If nuclear explosions would sterilize half the ocean we would have done it to ourselves with the 1000+ uNderwater nuclear tests done so far. $\endgroup$ – Dan Clarke Apr 18 '18 at 1:59
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It’s very easy, poor mermaids.

Acidification

Search for all the trapped carbon on earth and below oceans, mostly in the form of petrol. Pump it, refine it and give it to everyone so that they can burn it to get energy.

By doing so, they will release industrial level of CO2 in the atmosphere, thus leading to an acidification of the oceans. If the mermaids could survive it per se, it’s likely that their coral and forest homes will die, bleached out. Their food will disappear. They will get sick, starve and reproduce less. Eventually they’ll die out.

Trawling

You could also encourage heavy fishing with destructive means such as bottom trawling to accelerate the phenomena.

Bonus

As a side bonus, your civilization will prosper happily, with a nearly free source of energy and food (as long as they have another planet to go to if something unplanned goes wrong with the aforementioned plan).

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