Why only wild? Frankly, farmed fish sounds like a good a ecological option. However, there are many environmental problems with fish raised in confinement systems similar to many of the problems associated with factory land farms: concentration of waste, pesticides, antibiotics, parasites and disease. All these issues are compounded by concentrating population of the selected species, elimination of predatory animals (eliminating 'Survival of the Fittest') allowing sickly fish to be included in the 'farmers' 'harvest'. The selected food is made by 'farmers' on their best guess what is healthy and needed for the fish. Well, in our assessment their guesses are wrong. Simply look at Farmed Salmon; first the 'farmer' adds artificial food coloring to the feed given to their 'crop' of fish, and then this artificially colored fish often looks pasty and sickly. We select against farmed until we can be convinced that this product is better for our customers and planet.
Sustainability? Well, this has to do with the survival of the planet, our planet. Some fishing methods are extremely damaging to our waterways and fish populations, like Persian Fishnet, dredge fishing, and bottom trawling, just to name a few. These destructive and indiscriminate fishing methods destroy ecosystems so future generations will not be able to reproduce in the targeted area until the coral, sea vegetables and ecosystem can rebuild. Perhaps the easiest way to visualize the gravity of this problem is to imagine the following: all fish sold in every market, every restaurant, every dock, every day, using methods like Perian nets catch the targeted fish AND other fish. This other fish is called bycatch - wasted catch of fish other than the target species, and is often thrown overboard as 'chum' - waste/trash. Here is the alarming part: bycatch accounts for 1/3 of all fish caught. We are killing our oceans at an alarming rate.