EL5:

Humans are omnivores so while we can live off plants it's much easier to have a healthy diet if you eat some animal protean. Generally people prefer not to eat bugs / 'trash fish' etc, even if we can digest them just fine. So, we generally feed that stuff to farm animals like cows but also farm fish.

The problem is people want more meat than we get just from feeding trash fish to chickens. So, we also use farm grown plants like corn. But, salmon can't eat corn. So we would need another step say corn > bugs > salmon. And it's more efficient to go corn > chickens than corn > bugs > salmon.

So, now we are better off feeding some corn to some chickens. But, we can also feed those 'trash fish' to chickens to speed their growth. And it turns out it's also more efficient to feed chickens a mix of corn + 'trash fish' than it to use that same corn to feed some chickens and all of the 'trash fish' to feed salmon. The reason for this is chickens use much of the protean in their diet to build protean in them, but salmon also use protean in their diet for energy.

I didn't need an ELI5. Your claims are basically a distortion of science which is meaningless.

Think he is referring to trophic levels. There is at least one order of magnitude more energy and resources needed going from one level to the next. So I think he is saying you should eat fish that don't eat other fish mainly.