Phocid seals go through dramatic seasonal changes in body mass and composition as a result of the spatial and temporal separation of foraging, reproduction and moulting. These changes in body fat content and body mass result in seasonal changes in buoyancy, which in turn may influence diving behaviour. We examined the longitudinal changes in buoyancy and diving behaviour of 14 adult grey seals (Halichoerus grypus) during two periods that represent maximal contrast in body mass and composition. During both the post-moulting (PM) and pre-breeding (PB) periods, grey seals were negatively buoyant. However, buoyancy increased by 47.9 % between the PM and PB periods. Descent rate was significantly faster during the PM period (1.0+/−0.07 m s(−1)) than during the PB period (0.7+/−0.06 m s(−1)), suggesting that seals were aided by negative buoyancy during the downward portion of dives. Ascent rate was also significantly faster during the PM period (0.8+/−0.06 m s(−1)) than during the PB period (0.6+/−0.05 m s(−1)), contradicting the prediction that more buoyant animals should ascend faster. The effects of drag could not account for this discrepancy. Dive depth and surface interval between dives did not differ significantly between the two periods. Similarly, the distribution of dive shapes used by individuals did not differ between the two periods. However, dive duration was significantly longer during the PB period than during the PM period (5.5+/−0.25 min compared with 4.4+/−0.24 min, respectively) as was time spent at the bottom of the dive (3.1+/−0.22 min compared with 2.5+/−0.15 min, respectively).

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