SUMMARY Buoyancy is one of the primary external forces acting on air-breathing divers and it can affect their swimming energetics. Because the body composition of marine mammals (i.e. the relative amounts of lower-density lipid and higher-density lean tissue) varies individually and seasonally,their buoyancy also fluctuates widely, and individuals would be expected to adjust their stroke patterns during dives accordingly. To test this prediction, we attached acceleration data loggers to four free-ranging Baikal seals Phoca sibirica in Lake Baikal and monitored flipper stroking activity as well as swimming speed, depth and inclination of the body axis(pitch). In addition to the logger, one seal (Individual 4) was equipped with a lead weight that was jettisoned after a predetermined time period so that we had a set of observations on the same individual with different body densities. These four data sets revealed the general diving patterns of Baikal seals and also provided direct insights into the influence of buoyancy on these patterns. Seals repeatedly performed dives of a mean duration of 7.0 min(max. 15.4 min), interrupted by a mean surface duration of 1.2 min. Dive depths were 66 m on average, but varied substantially, with a maximum depth of 324 m. The seals showed different stroke patterns among individuals; some seals stroked at lower rates during descent than ascent, while the others had higher stroke rates during descent than ascent. When the lead weight was detached from Individual 4, the seal increased its stroke rate in descent by shifting swimming mode from prolonged glides to more stroke-and-glide swimming, and decreased its stroke rate in ascent by shifting from continuous stroking to stroke-and-glide swimming. We conclude that seals adopt different stroke patterns according to their individual buoyancies. We also demonstrate that the terminal speed reached by Individual 4 during prolonged glide in descent depended on its total buoyancy and pitch, with higher speeds reached in the weighted condition and at steeper pitch. A simple physical model allowed us to estimate the body density of the seal from the speed and pitch(1027-1046 kg m-3, roughly corresponding to 32-41% lipid content,for the weighted condition; 1014-1022 kg m-3, 43-47% lipid content,for the unweighted condition).
to total body metabolic rate during the breath-hold. Extreme hypoxemic tolerance in these seals was demonstrated by arterial P O 2 values during late apnea that were less than human thresholds for shallow-water blackout. Despite such low P O 2s, there was no evidence of significant anaerobic metabolism, as changes in blood pH were minimal and attributable to increased P CO 2. These findings and the previously reported lack of lactate accumulation during these breath-holds are consistent with the maintenance of aerobic metabolism even at low oxygen tensions during rest-associated apneas. Such hypoxemic tolerance is necessary in order to allow dissociation of O 2 from hemoglobin and provide effective utilization of the blood O 2 store.
Some pinniped species appear to forage during both daylight and darkness. To determine any differences in the foraging tactics between day and night, we attached data loggers with a newly developed time-scheduled release system to 2 free-ranging female Baikal seals Phoca sibirica. The system released the loggers from the seals 24 h after deployment, and allowed us to retrieve the loggers via VHF radio signals. We obtained the first time-series diving data on Baikal seals. The seals dived almost continuously, to an average of 68.9 m, with dives deeper than this (>150 m) being concentrated around dusk and dawn. They showed distinctly different diving patterns between day and night in terms of swimming speed profile and the sequential pattern in maximum dive depth . In the daytime, dives were characterized by higher swimming speeds (mean 1.2 m s -1 ) and upward-directed acceleration events. At night, dives were shallower around midnight and characterized by lower speeds (mean 0.9 m s -1 ) and undirectional deceleration events. We suggest that these differences reflect predation on pelagic fishes by seals using visual cues during the day and predation on swarming crustaceans by seals using tactile cues at night.
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