A student of the classical languages would spot the meaning of this word at once, since it derives from Greek ikhthus, a fish, plus phagein, to eat, plus the English adjective ending -ous. Hence, fish-eating.
It has its proper place in biology, where an animal would be so described if fish formed a significant part of its diet. It may with equal seriousness be transferred to human individuals and populations who subsist likewise, though if you’re not careful it comes out sounding irretrievably pompous, as it does in these nineteenth-century examples:
If we give our attention to classed people — classed as to the quality of food they principally subsist on — we find that the ichthyophagous class are especially strong, healthy, and prolific.
The Angler and his Friend, by John Davy, 1855. Or, putting it another way, eating fish is good for you.
Of sprats there are 3,000,000 lbs. weight consumed — and these, with the addition of plaice, are the staple comestibles at the dinners and suppers of the ichthyophagous part of the labouring population of London.
London Labour and the London Poor, by Henry Mayhew, 1851.
We moderns eschew such polysyllabic pomposities in favour of the simple English equivalents of “fish-eating”, so it’s not seen as often as it once was.