I'm not a marine biologist but my understanding is that oceanic food chains are very complex and it's not just orca -> tuna -> sardine -> plankton. Most fish larger than a sardine eat only smaller fish and there's a huge web of various sizes of creature all of which behave in predatory behaviour. Certainly most/all fish large enough to be considered a meal to humans are going to be predators.

I have no idea about biomass ratios, but I'm pretty sure there are more species of fish that eat other fish than not.