It is "Judas Fish" by <PERSON>. I was able to read "Caution! Inflammable!" on archive.org. You just sign up for a free account and read the book online. It's a great resource.
The short story is presented as log entries from <PERSON> (a deep sea station worker). <PERSON>'s job is to harvest schools of fish by creating "Judas Fish" which are fish who lead the other fish back to the station for slaughter. <PERSON> is hindered by large squid who keep showing up at the station and scare the fish away. Eventually he captures one and decides to eat it. It tastes like "oily tuna", and it also gives him really weird dreams. He comes to realize the dreams are genetic memories passed to him by the squid he ate. He realizes that the memories are compelling him to "join" with the squid (who are called the Ilat). The squid begin to offer themselves up for harvest and he sends them to the processing plant. 322
Humans then eat the squid, have the dreams, understand the message and begin to offer themselves back to the squid. The balance is restored. "In the end we will be one, and the old cycle of killing and endless bloodshed, the loss of self will be gone. I will take the servosuit and leave the station. Outside I will join them and we will go to Kuwalua Deep where they wait in their millions to join us. There will be pain, that I am sure, but only for an instant. After that we will make our plans..."
The story ends with a communication between the head psychologist and chief of Pacific foods operations. They talk about <PERSON> and conclude he must have lost his mind down there under the sea. His rants about "the Ilat" are discounted at first. But... The fish harvests are all squid now and afterwards "ten thousand people in Osaka walked down to the coast off Wakayama and walked into the sea"
The letter concludes "I keep thinking <PERSON>; I haven't been to the beach in years. Why not join me this weekend?" 313