Effective altruism is based on the core belief that all people count equally. We unequivocally condemn Nick Bostrom’s recklessly flawed and reprehensible words. We reject this unacceptable racist language, and the callous discussion of ideas that can and have harmed Black people. It is fundamentally inconsistent with our mission of building an inclusive and welcoming community.
— The Centre for Effective Altruism
That’s a good question, and is part of what Rethink Priorities are working on in their moral weight project! A hedonistic utilitarian would say that if fulfilment of the fish’s desire brings them greater pleasure (even after correcting for the intensity of pleasure perhaps generally being lower in fish) than the fulfilment of the human’s desire, then satisfying the fish’s desire should be prioritised. The key thing is that one unit of pleasure matters equally, regardless of the species of the being experiencing it.