The sad truth is we are fishing fish out of existence. Roughly 80% of stocks are being fished at or above their limits. The effect has been particularly bad for big predatory fish (tuna, swordfish, cod, flounder, etc.), whose populations have been decimated since 1950 — cut literally to a tenth of their size. That’s why you see less of them at restaurants and markets these days and more salmon and tilapia, which are farmed.
I doubt there is a mainstream value system in the world, religious or secular, that would condone the knowing elimination of species that neither harm nor compete with us.
But ethics aside, there is the impact on ocean ecosystems to consider. To kill off multiple species in the blink of an evolutionary eye is to court major disruption if not downright disaster.
Fortunately, it is not too late to save fish species — and we even know how. By creating and enforcing science-based catch limits on fish stocks, we can rebuild them. In fact, we’re doing that successfully in the U.S. now where some of the most effective legislation for protecting fish is in place.
The trouble lies mainly in other nations’ waters and on the high seas where weaker laws, if any, are in effect.
However, since Americans are the third largest consumer of fish, we can have an impact in the marketplace. Key is buying wild fish (because farmed fish are usually raised in an environmentally detrimental way that puts extra pressure on wild fish stocks), buying domestic fish (because American protections and environmental standards are better) and buying smaller fish (because they are less likely to be threatened).
Read Eat More Fish? on nrdc.org for more specific guidelines for eating fish sustainably and a fuller discussion of the problem.