Abstract
Salmo trutta, S. gairdneri, Cristivomer namaycush, and Coregonus clupeaformis failed to fill their swim bladders while being reared without access to an air surface for 84, 50, 22, and 56 weeks after hatching, respectively. When given access to an air surface they filled them. It is concluded that, like other physostomes, they must swallow air for the initial filling, but even if filling is delayed their pneumatic ducts remain open. In control trout, filling occurred at the beginning of feeding, and in whitefish controls, 2 to 3 months after hatching (at 20–22 mm length). Lake trout fry with unfilled swim bladders swam up vertical distances ranging up to 900 ft without marked fatigue, indicating that fish hatched in deep water can swim to the surface with relative ease to fill their swim bladders.

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