There are a lot of issues around ocean conservation worth caring about, studying, and accepting a cost to solve. However, it is worth pointing out that fisheries management works well and has been working where it is deployed to the extent that many many populations that were overfished and in serious trouble as recently as the 1990s are now healthy and can be ethically fished. Many issues around overfishing wild populations are occurring in regions of the world where fisheries management is difficult or impossible due to a combination of logistical (they're fishing in the middle of the ocean, not returning to shore for many months at a time) to political (they're from a country with a government that cannot or will not intervene in fishing practices) to technical (we need data on extraction and population dynamics to be able to manage fish populations) reasons.[1]

I take serious issue with the idea that the ethics of eating animals is related to how similar they are to humans ("it's ok to kill them for food because I perceive them to have low intelligence"), but I don't really think it is worth arguing over. Sustainable harvest is much more important than your personal feelings around the ethics of individual animal or plant extraction. If you kill one less wild fish, what will you eat instead and what are the impacts of that?

Eating things that are very old is not unethical persay, IMO, except to the extent that with fish populations older animals contribute disproportionately to reproduction (a 2x bigger fish produces >2x more eggs; I find it hard to convince people that the logical conclusion is that it is more ethical to take younger animals). It just comes down to whether they're being taken at a rate that is sustainable or not. I used to do rockfish research in California and individuals of species that are routinely taken as commercial and sport fish are well over 50 yrs old (China rockfish, vermillion rockfish, yellow-eye rockifsh)[2]; most consumers who have fish and chips or a "cod" sandwich or whatever in California are eating one of these species, but at the point of sale you can almost never find out which species. Is that an issue? I think so because I believe traceability is a key component of a good, modern, sustainable food system that I'd like to see. But it isn't a problem necessarily if whatever species it is was caught within the confines of a well managed fishery.

I've had many dinner parties with too many academic ecologists (I am/was one of them) and I get that they can be depressing because many are so focused on things going poorly. I think it is worth being reminded that when we take cooperative approaches grounded in decent science, we can all win, Earth first and foremost.

[1] One example paper: https://www.pnas.org/doi/10.1073/pnas.1909726116
[2] https://www.bookdepository.com/Rockfishes-Northeast-Pacific-... I don't have a copy of this book or institutional access any more, but this is the one place I have ever seen a whole bunch of age-growth curves of Sebastes printed in a single location; it is a great reference if you are curious about these species or about these issues in the context of California ecosystems.