Aquaculture is the fastest growing animal-production sector in the world. This leads to the question how we should guarantee fish welfare. Implementing welfare standards presupposes that we know how to weigh, define, and measure welfare. While at first glance these seem empirical questions, they cannot be answered without ethical reflection. Normative assumptions are made when weighing, defining, and measuring welfare. Moreover, the focus on welfare presupposes that welfare is a morally important concept. This in turn presupposes that we can define (...) the capacities of fish, which is an empirical undertaking that informs and is informed by ethical theories about the moral status of animals. In this article we want to illustrate the need for a constant interaction between empirical scientific research and ethics, in which both fields of research make their own contribution. This is not a novel claim. However, the case of fish sheds new light on this claim, because regarding fish there is still much empirical uncertainty and there is a plurality of moral views on all levels. Therefore, we do not only want to show the necessity of this interaction, but also the added value of a cooperation between ethicists and empirical scientists, such as biologists, physiologists, and ethologists. We demonstrate this by considering the different steps in the process of reflection about and implementation of fish welfare. (shrink)