Scientists can't tell you how to define morality, but much of moral reasoning starts from a shared set of premises that establish an ethical framework, and from there it's about empirical questions.

In this case the moral axiom would be that it is bad to cause suffering and good to cause joy. The science here is telling us that dolphins are capable of a full range of experiences, just as we are. It's up to you to interpret that information in your moral framework, but I think it's reasonable to believe many people will share a basic set of axioms about what is moral that make the implications here clear.