An Ark dev said that these will be tameable in the future!

Tamable using fish baskets from aberration. They seem to flee from the baskets once placed. It is a good strategy to pen then in with fence foundations and walls, or kite them to a pen. Place your basket on the other side of the wall and catch.

You can tame them with a fish basket, catch them with fish basket and release in water and it's tamed

Place a fish basket to tame this.

They can be tamed on Aberration by placing a fishing basket in the water, waiting about 30 seconds, and interacting with the basket when relatively close

Trap them in the shallow water so they cannot go anywhere and just place the bucket on the top of them. Bam. Tamed Sabertooth Salmon

Fish Basket only on Aberration.

What do they eat? Need to know when I tame one.

You can tame these by placing an abberation fish trap next to them, and if they stay within a small area for long enough, they will be tamed and transportable with the fish trap by picking it up.