Distinguishing Pelagic and Demersal Swimming of Deepwater Flatfish by Recording of Body Angles
Ole Thomas Albert, Yvan Lambert, Tone Vollen, Carla Freitas, and Lise Heggebakken
Abstract.—Based on previous investigations of flatfish swimming performance, three hypotheses regarding frequency distribution of body angles (pitch and roll) during periods of pelagic excursions and periods when the fish swim along the bottom were formulated and evaluated. A tagging experiment on Greenland halibut Reinhardtius hippoglossoides was made with archival tags (DST) recording body angles each 15 min together with ambient depth. A total of 19 tags were recovered, representing 80,000 h of recording. The paper shows that for periods when pelagic or demersal swimming behavior can be deduced from the depth trajectories alone, the angular spectra were consistent with the hypothesized patterns for pelagic and demersal phases, respectively. Based on this distinction, the percentage of pelagic occupancy was estimated for each month of the year. With almost no occurrences of vertical swimming positions, the common perception that Greenland halibut adopts such position during pelagic phases was rejected.