The Halieutica opens by representing the sea as a vast, impenetrable, and unpredictable environment, its myriad inhabitants both fascinating and baffling. As often in ancient thought, the sea is imagined to bring both terror and adventure, mortal danger and new frontiers. For Oppian this is also a realm that marks an epistemological boundary, and the start of the poem is structured by a tension between what can be known and what cannot. The sea is in part available to be catalogued and described, yet its limits lie tantalisingly out of reach, and its depths have yet to be conquered by man. Book 1 – which catalogues sea-creatures according to habitat (1.93–445), before discussing their mating practices (1.446–797) – is not structured according to the moral qualities that dominate the following three books; it is concerned less with the failings of fish than with delineating the environment in which these creatures live and breed.1 In placing weight on the unfathomable enormity of the sea, I argue, the poet writes his work into a didactic epic tradition that draws attention to the process of gathering and disseminating knowledge, foregrounding not only the possibilities but also the epistemological challenges generated by the drive to catalogue such an immense and daunting realm.