By-catch is a term for that part of the catch a. that is used to feed the ship’s crew b. caught at the lowest depths c. that was actually being fished for d. that was not actually being fished for

The answer is:

D. that was not actually being fished.

Here's what it is:

By-catch is fish or other marine life that were caught incidentally or unintentionally. They could be dolphins, turtles and the like. As long as it is not the intended target fish, everything else is considered as by-catch. This happens when other fish are attracted to the bait or they get hooked accidentally as they pass by. It also happens when large nets are used intended for smaller fishes, but because the larger marine animals get in the way, they are caught along with it.

Many might say that this is not a big problem, but it is! It can pose a great threat to the different marine species. There was an incident where thousands of dolphins died when they were caught along with tuna (which was the intended target).