"J'aime le poisson."
Translation:I like fish.
Yes, it is now possible for a context like: "I like the fish but the vegetables are overcooked".
If you like fish, you like it as a whole category of food, not "some fish".
With appreciation verbs (aimer, aimer bien, adorer, apprécier, préférer, détester, haïr, respecter, admirer), the direct object gets a definite article because the object is general by nature.