Optimal stroke frequency during diving activity in seabirds

Diving animals have to counter both drag and buoyancy when moving through the water column. The magnitude of these forces depends upon an animal's swim speed and current depth. Feet, fins or flippers create a motivating force, and different stroke strategies give a variable level of output power, providing both fast swim speeds and reduced travel times to any given target depth. However, while feet, fins or flippers may confer a real advantage, they also create considerable drag and engender high metabolic (oxygen consumption) costs. Consequently, there should be an optimal relationship between output power and stroke frequency during diving. Here we develop diving models to predict the optimal pattern of stroke frequency during both the descent and ascent phase of divers in water, while maintaining a minimum cost of transport (COT, J kg(-1) m(-1)). We also test predictions derived from these models using diving data for foot-propelled South Georgian shags Phalacrocorax georgianus diving to deeper than 80 m. Our predictions include: (1) output power (stroke frequency) should decline during the descent and ascent phases of a dive, but constant output power should be evident when the diving depth is shallow, (2) initial thrust for dives to deeper target depths should be smaller during the descent phase and greater during the ascent phase, and, (3) swim speed should be constant during the descent phase and should decrease first or remain constant, then increase during the ascent phase because of a declining output power strategy. Our empirical data on shags support only some of these predictions. Predictions (1) and (3) were supported by our observations, but Prediction (2) was not, possibly due to unknown but specific factors concerning diving seabirds. These findings suggest that diving seabirds adjust and control their stroke frequency pattern in order to minimize COT during diving, generating the observed changes in swimming speed.

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