"The black bears are eating fish."
Translation:Les ours noirs mangent des poissons.
Fish can be countable or uncountable. This means that you can either count the individual fish (one fish, two fish, etc.) or give an unspecified amount of fish meat (I'm eating fish for dinner).
Another example: chicken. You can certainly go on a farm and count how many chickens they have (countable), or specify what you had for lunch: I had chicken for lunch. (uncountable).
As for the French articles, think about what you're trying to express: countable or uncountable? If you want to count the fish, use "des poissons". If the fish is not countable, use "du poisson". Remember that "des" is the plural of "un", and "du" is the partitive article (used for uncountable objects).
- The bears are eating fish = Les ours mangent des poissons
- The bears are eating fish = Les ours mangent du poisson
The first sentence means the bears are eating more than one individual fish. The second sentence means the bears are eating an unspecified amount of fish meat.
The gender of animals is a different issue, which I explained above.