Abstract
The vertical distribution, body orientation and mobility of pike free embryos were studied at 10 °C in 25-cm individual water columns. Five steps were identified: (1) a resting phase, up to day (D) 4 post-hatching; (2) an ascent phase (D1–D4); (3) a plateau (D5–D8), characterized by a wall-attachment behaviour in the subsurface zone; (4) a break-off phase (D9–D13); and (5) a final localization between mid-water and surface, corresponding to the air filling of the swim bladder and to the transition to active, horizontal swimming (D14–D17). Free embryos could be: side-lying (phase 1, late phase 4 and phase 5); lying upright (phases 4 and 5); vertically suspended with adhesion to the wall (phases 2–3 and early phase 4); and in oblique or horizontal position (phase 5). The opening of mouth and anus occurred during phases 2 and 3 and swim bladder filling started on days 13–14. Free embryo mobility was low and stable during the first 5 days and increased regularly thereafter to reach maximal levels on days 14–16. This research provides evidence for a more complex and more variable temporal pattern of free embryo vertical distribution, posture and mobility than previously thought.
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