By-catch is fish or other marine life that were caught <em>incidentally or unintentionally</em>. 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).
Nucleoside analogs are the mutagenic chemicals that have a structure that is similar to the normal bases of the nucleotide and thus can be incorporated in the DNA during the process of replication, however as it has different base-pairing rules than the normal nucleotide base it can lead to mutations.
An example of a nucleoside analog is acyclovir which an analog of guanosine.
