Most fish are predators because there isn't much underwater vegetation to graze on...

If you are counting individual fish (independent of size) and you count all sea creatures, I think this is false. The "vegetation" are small creatures like phytoplankton, and the fish that eat them probably vastly outnumber the fish that eat fish. Each stage of the food change contains about an order of magnitude less fish than the level below.

Most small fish eat small crustaceans and other small animals, not algae.

Most phytoplankton, i.e. unicellular algae, is eaten by zooplankton, i.e. mostly by small crustaceans, like copepods.

The fish that eat other animals vastly outnumber the fish that eat algae or aquatic plants.

For reference, the copepod, the main ingredient of zooplankton, has a typical size of 1 mm to 2 mm.

This is a good comment that I mostly agree with.

> The fish that eat other animals vastly outnumber the fish that eat algae or aquatic plants.

This is why I said "if you count all sea creatures". The original comment said "most fish are predators because there isn't much vegetation". As you say, there is quite a lot of vegetation, and there are many more sea creatures in that category than there are predators.

So, most fish are predators, but not at all for the reason in the original comment. It remains true that population and total biomass drop off exponentially as you go up in the food chain (which is really the only point I was trying to make).

OK, I agree with what you say.

The main reason while the majority of big aquatic animals are eating other animals, while the majority of the big terrestrial animals are eating plants, is the disparity in size between big animals and most aquatic primary producers, which requires at least one intermediate layer of small animals, who gather efficiently the small algae, packaging the nutrients and energy into bigger bodies, which can be gathered efficiently by the big animals.

Ok but at the point where you're defining "fish" as "all aquatic life" and "vegetation" as "plankton" (is yeast vegetation too?) you've kind of gotten away from what I meant.

Cows can get big by eating grass because most of the land is covered in plants, but tuna can't get big by eating seaweed because most of the sea is abyss.

There is a lot of phytoplankton, plant-like algae. Though apparently most fish don't eat it directly. Zooplankton (which consumes phytoplankton) seems to be a more common food source for fish. You could call a fish eating zooplankton a "predator" because zooplankton are non-plant algae, but that stretches the definition.

Anyway, tuna don't just eat zooplankton, they eat smaller fish.

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.

Fish mainly eat copepods (a type of zooplankton), which are tiny crustaceans (1-2 mm) carried around by the sea current like algae. Not other fish. Tuna do mainly eat other fish.

There are risks of disease and parasites from eating mammalian predators that are not a risk with fish.

Predators are also a lot more expensive to raise as domestic animals. Pigs are omnivores, and chickens will eat some small animals. We feed them what is cheap. Animal feeds do sometimes contain waste meat or fish.

People do eat reptilian predators. I have eaten crocodile and monitor lizard, a number of others are eaten.

Chickens kill (and usually eat) all kinds of small living things, including mice.