Why is murder wrong if Many Worlds Interpretation of Quantum Mechanics is true?

[The Grand Nudger]

Basing your morality on anything resolving to how things are is fruitless.  Morality concerns what ought to be, not what is.  As Aegon pointed out - it's unclear how any theory in physics can attain or achieve moral warrant for some act x.

I strongly disagree. I think science has quite a few things to say about morality.

So do I.  It remains the case, however, that morality is concerned with what ought to be, not what is.  Science is great at telling us what is, and so it's useful when we consider how to get from there, to what ought to be. 

One obvious example: which animals are capable of suffering? It makes very little sense to fight for the rights of animals that the scientific consensus is that they are not capable of feeling any suffering, not even physical pain (such as fish). On the other hand, it makes a lot of sense to fight for the rights of animals that are capable of feeling pain (birds and mammals).

Fish do feel pain, which is why they're anesthetized and/or stunned (among other things) before the kill in operations with an ethical focus.  Moreover, the idea of cruelty free protein goes further than just what we know an animal might feel.  The creature is treated well even if there's no clear evidence of a benefit to or awareness of it being treated well.  Animals can suffer - but humans can be cruel to animals that don't suffer, or don't suffer from some specific thing.  

Fish, for example, don't suffer at all from TAN below .5mg/l.  The lower you want your TAN the larger and more labor intensive your biofilter needs to be.  Fish do suffer from TAN shock - which is to say that if you maintain a lower TAN and then your larger more labor intensive biofilter fails for any reason, you risk the entire growout tank and anything in-line to it.  In most systems, TAN is kept around 2mg/l.  However, in intensive recirculating aquaponics you want your TAN as high as the fish can physically tolerate and probably a little but into discomfort for the fish (to say nothing of their stocking density (which has a linear relationship to TAN in growout) so that nitrosomonas and nitrobacter can convert the fish waste into nitrite, then nitrite into nitrate, respectively - which is accessible fertility for plants and exponentially less detrimental to fish at each conversion.  The flip side to that coin is that the higher your TAN the higher your risk of contamination in the end product and the more "off" flavor the protein holds.  With full conversion, you needn't remove any of the nitrogen in the stocks life cycle.  They'll be harvested and the tank drained or let to sit in the tank culture version of laying fallow.  Or you may chemically scrub it with direct biofloc before the next cohort is scheduled for insertion (which has the added benefit of establishing an edible colony for the transplanted fry out of unconverted protein in waste and waste feed).  

So there's the shape of what you can do.  The science of RAS in a single paragraph.  Fish can survive in an environment filled with their own excrement to a surprising extent, and for the most part the fish you buy in stores here in the us is either that - or wild caught.  However, an ethically motivated consumer will not purchase your fish..or, if they do, they will not pay a premium.  Since the fish in the stores tends to come from places with very low labor rates - that means you'll fail to secure the ethical niche and be unable to compete in economies of scale with foreign labor.  

What you can do, the science of aquaculture, tells you alot...but it does not tell you what you should do, or what will satisfy that consumer.  In my experience, they want TAN as low as possible.  Crystal clear water.  It's good for the fish, practically, yes, but beyond a point it has nothing to do with the fish and everything to do with the level of apparent care and detail in their culture.  It demonstrates a willingness to forego profits and make significant investment in things that will not increase your productivity and may greatly decrease your total output.  This is actually the number one reason that small operators fail in the first three years, on average.  The physical infrastructure and technical expertise required to satisfy the moral demands of the niche consumer are eliminative factors, and the producer demographics are both unlikely to possess that expertise and unlikely to agree with the normative demands of their market.

"They're just fish, I can grow them this way for a profit" says the average producer as he views his roiling cauldron of fish and feces. "Yes, they are fish - and yes, you can grow them that way" replies the ethically minded consumer "but that doesn't mean that you should". The "you monster"...is implied. You abso-fucking-lutely do not want them catching you saying dumbo shit. Fish don't feel pain, they're made of rubber, ya see!