When it comes to many ocean fisheries, we are literally scraping the bottom of the barrel, unfortunately.

Take for example slimeheads (marketed as "orange roughy" in the USA). These fish live deep in the ocean, can take decades to reach reproductive maturity and a marketable size, and can live up to 149 years. Harvested by bottom trawling, their commercial exploitation causes tremendous damage to the seafloor. Just some food for thought the next time you sit down in a restaurant and dine on a fish that is likely older than your parents.

They aren't even good fish to eat.

High in mercury, low in omega-3 acids. And being from the twilight zone, something tells me it will taste like all the other scavenger-type fish and sea creatures.

Only reason to eat one over a farmed salmon would be for the novelty factor.

Unfortunately as many waters have been overfished and thus are no longer productive, trawlers have to go deeper and deeper to keep our supermarkets well stocked with endangered seafoods like Halibut.

Your general point is correct, but Halibut is not endangered. Seafood Watch lists some types of Halibut as a "Good Alternative".

http://www.seafoodwatch.org/seafood-recommendations/groups/h...

There's perhaps disagreement about this, but Atlantic Halibut is classified as endangered (IUCN Red list) and is also on Greenpeace's red list:

https://en.wikipedia.org/wiki/Atlantic_halibut

In any case, nobody would disagree that Halibut stocks have been depleted (or even eradicated) in many areas where it was once plentiful.