# How to "Poison" an Entire Ocean?

For my story there is a tribe of Orcs trying to get revenge against a fishing-based tribe of Humans, but would like to stay out of blame. So my question is this: What is the most effective way to poison an ocean of about 1,000,000km^3, killing off at least 2/3 of the fish inhabiting said ocean? Or at least to induce a noteworthy famine amongst the humans.

Note 1: 500,000km^3 is a more accurate estimate for the fishing span

The humans normally fish salmon, trout, tuna, and other common fish.

You can change the water volume and provide functions for how the volume (or surface area) affects the "poison" and/or the time it takes to reach the entire span of the ocean if they exist.

This doesn't need to be an actual poison, it just needs to kill all the fish without the Humans believing the Orcs are a part of it.

Assume a Medieval Euro-style time period.

• Not sure I understand the close vote for opinion based...there is nothing opinion-y about this. Feb 17, 2017 at 20:01
• They have access to a powerful, kill-billions-of-creatures poison and they're going to target the fish, not the people? Feb 17, 2017 at 21:07
• @Zxyrra they have to stay covert and it "doesn't need to be an actual poison" I would take answers with ravenous sea creatures or even algal bloom related implications Feb 17, 2017 at 21:51
• 1 million km^3 is an crazy enormous volume of water. Lake Superior has 12,100 km^3 of water. The Black Sea has 547,000 km^3 water. Is there an error of a couple of orders of magnitude in this question? Feb 17, 2017 at 23:43
• @Will I should have looked at the smaller bodies of water, I just knew I wanted about a third of the Mediterranean Sea, but the Black Sea is probably a better estimate for the humans' fishing endeavors. Thanks for that catch. Feb 18, 2017 at 0:09

1 million cubic km isn't exactly a small volume. Even with modern technology it would take months if not years to pollute it sufficiently that most large fish species would die out, or at least no longer visit that part of the ocean, assuming you want to be discreet. Also assuming no water currents which would disperse your pollutant over a wider area.

The most obvious method is to reduce oxygen in the area. It'll kill off most larger species and scare off the rest. This is fairly easy to do: a sewage line into the water, will do exactly that. You would need a sufficiently large orc population, preferably a small city. Given that you mentioned a "tribe", I'm assuming a couple of thousand nomads, which won't be enough.

The next best option is something like water hyacinths, but a version that grows in salt water.It will stay put, stop water travel and kill off most species by removing nutrients and oxygen from the water and adding nitrates. Given the growth rates, a small colony will overrun the local area faster than a medieval society can get rid of it. And unless your orcs are caught red handed, as it were (green/purple-handed?), tossing colonies of the plant into the water, there won't be any proof of their involvement.

If you somehow manage to do this, I don't think it would be very smart for the Orcs. What you want to have done would completely eradicate much of life in the oceans, leading to other intense changes in the biosphere. This would most likely result in a mass extinction, and while that would affect the humans worst, the Orcs wouldn't be much better off.

Of course, there may be ways to kill some fish species off without triggering a mass extinction, but I don't think they would be very easy to do, especially with only medieval technology.

• I'll allow some magical hand waving if need be, and you are correct in how it will affect other ecosystems which I didn't think about. But perhaps like you said if just a few species were killed off or even replaced by non-edible to humans fish that fill the niche that the extinct fish previously filled? Feb 17, 2017 at 5:55
• I don't see how this actually answers the question. Feb 17, 2017 at 17:15

Introduce poison to humans, not to fish. Fish are plenty, killing them off in ocean is like... well killing all the fish in the ocean. That will also lead to catastrophic failure in the food chain, killing your orcs as well. Instead, introduce a human-deadly plant that is safe for the fish. It will stop the fishing in no time. If you insist on killing the fish, introduce an organism that will hunt the fish and non-edible, like bacteria or tiny insectoid creatures. Obviously your local shaman has to do genetic voodoo for these.

To keep one step removed, attack the root of the oceanic food chain. Kill off plankton and the other microorganisms that feed the fish that in turn feed the humans. Populations of the Human food fish will begin to decline and fail weakening coastal human settlements.

You wouldn't really need to do it in the entire ocean either. So long as your plague kills off the sea life in the range that the humans are willing to sail, it could be successful. Given the proposed tech level, Humans would not be willing to sail too far out of the sight of land, except for maybe Vikings. Vikings were the exception, not the rule.

As mentioned before, though, mass ocean life die off would have huge impacts and would probably be successful at killing your Orcs almost as fast as the Humans. Your Orcish mage would have to be insane to do it.

It might just be easier to sneak into town at night and butcher everyone. It seems to be more...orcish to me.

The easiest way is to use something that does this naturally: a pathogen “blooms” and kills most of the fish.

They might determine how a few barrels of the stuff, farmed in the secret weapons lab, can be deployed with a high chance of triggering a bloom if done in the right spot for its microclimate and microbiome, when the weather is right overall.

They may see this phenomenon occur naturally and save some to study as a WMD. Introduced in a different region, the targeted population will not be familiar with it.

Maybe you don't need to poison all the fish. Maybe you just need to poison a few fish in such a horrific fashion that it induces the human villagers to give up fishing. If this world has magic, maybe the fish poison / curse turns individuals who eat the fish into zombies. No magic? A parasite the invades the brain of anyone who eats an infected fish and causes rabies like symptoms (madness, violence) and death. The fish eaten would need to be raw or under-cooked in this case.
Villagers like their fish burned? Then lets make it a fungus on the fish scales that can be transferred to anyone handling the fish prior to cooking. Fungus infects the skin of the hands and spreads. During a three week period, the illness progresses with the skin rashing, bleeding, then completely sloughing off. Sufferers eventually die from the pain and secondary infections.

This approach would be very effective if the fishermen's primary method of harvesting is trapping. Orcs could poison the fish in traps during the night guaranteeing that the payload will be delivered.

Once the villagers figure out that fish are the source of any of these horrible illnesses, no one is going to be fishing anymore, even if the number of infected fish is actually quite low. The terror of infection/poisoning will be completely effective where the poison itself was hardly a risk.