Some other scientists might say that most of the fish bones seen are from young fish, and thus might have been caught more easily off the coast as opposed to in open waters. While this may be the case, it's still not easy to catch tuna--it requires nets set in deep water.
Does the underlined sentence mean that the fish might have been caught off the coast rather than in open waters, because it was easier to do so off the coast?
Am I right in my understanding? So there is difference in the distance from the land between "off the coast" and "in open waters"?
Thanks for your patience and help!
Yes to both questions.
Read about the continental shelf.