Scientists have now demonstrated that fish can recognize and learn to respond appropriately to various colors, shapes, and patterns. Of course, any fishermen could have told them that.

From The New York Times:

A series of studies has recently revealed that reef fish are surprisingly adaptable. Freshly caught wild fish quickly learn new tasks and can learn to discriminate among colors, patterns and shapes, including those they have never encountered. These studies suggest that learning and interpreting new stimuli play important roles in the lives of reef fish.