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.