Why is fishing ok, considering it's the removal of a fully grown animal which is far less common than its juvenile conspecifics, but collecting a small fish for an aquarium where it may, statistically, outlive its brethren, is not?
I didn't say "fishing was OK". The OP specifically was asking about taking objects -- a shell and a jaw. The discussion bloomed from there to include all sorts of other collecting, I suspect because those that like to collect find comfort in the fact that fishing is a much larger issue than the collection of an occasional porthole -- and I agree that is true.
I find the fishing issue very difficult. It is a fact that much of the world is way overfished and it only gets worse each year. A lot of focus is on the larger nations -- China in particular -- having ravenous and expanding appetites for marine life, but fishing feeds a large part of the less-wealthy world as well, on a localized, survival and historical basis. Limiting fishing in any meaningful way will be a difficult battle, however I believe that battle will come sooner rather than later. As for why fishing is not worse than aquarium collecting, that should be obvious from my posts. I don't believe in collecting things simply for the sake of collecting. It deprives others for no valid reason. Once an aquarium owner takes a fish, I don't care if it lives or dies -- it's no longer part of the world it belongs in.
I look at things like this and then expand it somewhat. What if everyone that went diving took one fish? Where I like to dive there might be one frogfish at any given time. Last year we had one Great Barracuda for several months. Lionfish are somewhat rare. You look at yourself as one collector, but what if there were ten of you? How about 100? At my dive site that would decimate the population. Before anyone says "that could never happen", you may be interested in knowing that the number of tourist spearfishers in Hawaii is way up this year. Spearfishing is this year's InstaMadness and in Hawaii any idiot with a spear can take any fish that can get close to. At the moment they do not even need a license, although that is changing soon. As I said, laws do catch up with what should be common sense, sometimes they are just a little late.