EFFECTS OF BUOYANCY ON THE DIVING BEHAVIOR OF NORTHERN ELEPHANT SEALS

Marine mammals experience radical seasonal changes in body composition , which would be expected to affect their buoyancy in the water, The a im of this study was to examine the relationship between such changes in buoyancy and diving behavior in northern elephant seals Mirounga an gustirostris. This was achieved by modifying the buoyancy of 13 juveni le elephant seals translocated from Ano Nuevo State Reserve, CA, USA, and released at various sites in Monterey Bay, CA, USA. The buoyancy o f each seal was calculated and was increased or decreased using syntac tic foam or lead weights, and their diving behavior was recorded as th ey returned to Ano Nuevo, The seals were divided into three groups: in creased buoyancy (B+), reduced buoyancy (B-) and control seals (Bc), M ean descent rates were 0.77+/-0.3 m s(-1) for the B+ seals, 0.82+/-0.2 m s(-1) for the control seals and 0.87+/-0.3 m s(-1) for the B- seals , and were significantly different. Mean ascent rates for the three tr eatments were 0.82+/-0.3 m s(-1) for the B+ seals, 0.86+/-0.3 m s(-1) for the control seals and 0.82+/-0.3 m s(-1) for the B- seals, All the B+ seals ascended faster than they descended, while four of the five B- seals descended faster than they ascended. There was a significant negative correlation between buoyancy and descent rate, with less buoy ant seals descending faster than more buoyant seals. There was, howeve r, no correlation between ascent rate and buoyancy. This suggests that seals may use negative buoyancy to drift passively during descent, bu t that all seals may swim continuously during ascent. There was a sign ificant correlation between buoyancy and the drift descent rate of C-t ype drift dives, including upwards drift in the most buoyant seal. Buo yancy was not correlated with diving depth, trip duration, dive durati on or surface-interval duration. This study demonstrates that buoyancy plays a significant role in shaping diving behavior in northern eleph ant seals and that elephant seals may adjust their behavior to suit th eir buoyancy, rather than adjusting their buoyancy to suit a dive, Thi s study also validated the truncated cones method of calculating body composition in this species by comparing it with body composition dete rmined using tritium dilution.