This clip from the BBC series Wild China looks at this old practice, and describes the cormorants as slaves. Is this a fair assessment? Are all forms of domestication best understand as enslavement of animals, or partnerships? When and what the birds can eat is limited, but eat they do. In fact, as opposed to a caged bird, the cormorants get to enter their natural habitat. They are trained, and not fully wild, intelligent and able to bend the rules.
The relationship is exploitative, but not just one way. The birds are employed to hunt for food, but are not themselves food. This kind of relationship may not extend much further into the 21st century, but it's a fascinating example of how humans can relate to non-humans in a non-consumptive fashion (except of course for the fish!)