Pier 39

This is a twilight view of San Francisco's Golden Gate Bridge from Pier 39 on Fisherman's Wharf.

The crowd is watching the haul-out of harbor seals.

Harbor seal physiology requires them to routinely haul-out on land. When in cold ocean waters, their core body temperature is maintained by an insulating layer of blubber and increased metabolic heat production. Restricted blood flow to the skin and extremities lowers heat loss by reducing the flow of blood outside the insulating blubber layer.

Despite these adaptations to conserve heat, the seals' metabolic heat production increases the longer they remain in the water. To avoid depleting their blubber layer, this increased heat production must be fueled by eating more fish, which in turn requires spending more time in the water foraging. This cycle can be potentially harmful.

Regular hauling out on land reduces the thermal stress on the seals, permitting increased blood flow to the skin and extremities, which allows any cuts or wounds to heal normally, and reducing the need for metabolic heat production. These benefits underlie the seals' physiological need to haul-out on land. When captive seals are deprived of haul-out time for an extended period, they compensate for the loss by hauling almost continuously for days after the option is restored.

(from Harbor Seals in the Bay by Diane Kopec)

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