In the game there are three types/sizes of fish. I wanted them to be based on real fish from the Antarctic that were actually eaten by the seals and penguins. So I grabbed a book out of my book case (Frozen Oceans by David N. Thomas) and found a few possible fishes.
I found that the diversity in Antarctic waters is pretty low, because not many fish can live in sub-zero water. They have to have special mechanisms to keep their blood from freezing: an anti-freeze substance in their blood. But I did find names of several species and google helped me find images.
For the smallest fish I chose the Antarctic Silver Fish (Pleurogramma antarctica), the medium fish is based on the Broadhead fish (Pagothenia borchgrevinki) and the big fish is called the Antarctic Toothfish (Dissostichus mawsoni).
In real life the Toothfish is actually much too big for the Adelie penguins, even for the adults, but there have to be some sacrifices to reality in a game like this.
Speaking of reality, there has been a point during the process that we thought about adding krill to the game as smallest fish, or extra. That is actually what the young penguins eat, so that would have been perfect. But it proved to be confusing for the test players, so we stuck to the fishes.