What enemy confronted them on their way back from the wreck, and how did Fritz dispose of them?

Fritz and his father are confronted by a shark on their way to the wreck. Fritz shoots the shark.
After the family is marooned on the island, they need to get supplies to stay alive. The wreck has not completely sunk into the sea yet and still has goods they need; one of the most important types of cargo are the live animals on board. They go out to the wreck with all the supplies needed to salvage whatever they can.
Fritz and his father spend a night on the boat, signaling that all is well with lanterns after they receive a similar signal from the shore. 
The next day, they tie the animals with swimming belts and throw them into the sea. However, a shark approaches. The father describes it, saying:

As I was thus engaged, a sudden shout from Fritz surprised me. I glanced up; there stood Fritz with his gun to his shoulder, pointing it at a huge shark; the monster was making for one of the finest sheep; he turned on his side to seize his prey; as the white of his belly appeared Fritz fired. The shot took effect, and our enemy disappeared, leaving a trace of blood on the calm water.

The shark does not return after it is hit by Fritz's bullet. The father praises Fritz for his skills with the gun.