CAPILLARY-TUBE DEPTH GAUGES FOR DIVING ANIMALS: AN ASSESSMENT OF THEIR ACCURACY AND APPLICABILITY

Gauges to measure the maximum depths attained by diving animals were con- structed from plastic tubing lined with soluble indicator powder, at a cost of $0.10 each. In our tests, the differences between real and estimated depths averaged <3% with single immersions of gauges to any depth up to 140 m. With multiple immersions errors were usually <10% and always <25%. The accuracy of the gauges was not affected by depth or duration of dive, rate of descent, or underwater movements simulating a bird's swimming. Errors resulted from severe jarring of the devices underwater, plunge-diving, accumulation of moisture within the tubes, and use of excessive hydrophilic indicator. Potential errors associated with high air-sea temperature gradients were not realized in our tests. With careful construction and deployment the gauges should provide accurate depth estimates without adversely affecting free-living animals. Methods for attaching the gauges to birds are reviewed.