Halibut spawn between November and March. Though it is believed they spawn annually, there is evidence to suggest they may "skip spawn", or reproduce every other year. During the winter months, halibut will migrate from their relatively shallow feeding grounds to deeper waters along the edge of the continental shelf (180-450 meters).
Pop-up Archival Transmitting tag data have recorded periods where halibut swim up off the bottom and drift back down to the sea floor. They'll repeat this several times. While we're not certain what this behaviour is, it seems to conform with "spawning rises" witnessed in other flatfish, where females move up into the water column to release eggs while accompanying males fertilize them. This mechanism would allow for better egg dispersal. Numbers of eggs vary with the size of the female: a 50-pound (23 kg) fish will produce around 500,000 eggs while a 250 lb (113 kg) female may produce over 4 million.
Larva (Stage 1)
Approx. 9 mm in lengthAfter about 15 days, the drifting eggs hatch. Halibut start off in an upright position, with eyes on either side of its head. The larva are neutrally buoyant and are transported by the ocean currents. Note the large yolk sac which will sustain the fish till the early post-larva stage.
Post-larva (Stage 3)
Approx. 16 mm in lengthAt this point, the yolk sac has been fully absorbed and the fish will begin feeding on planktonic organisms.
Post-larva (Stage 7)
Approx. 21 mm in lengthThe young fish is still riding the currents. During its development, a post-larva can travel hundreds of miles in the Alaska Stream, which runs counter-clockwise in the Gulf of Alaska. Currents can run in excess of a mile (1.6 km) per hour in some coastal regions, but, typically, speeds are 3-5 miles (5-8 km) per day.
Post-larva (Stage 9)
Approx. 25 mm in lengthThe metamorphosis begins: the left eye moves over the snout to the right side of the head and pigmentation on the left side fades.