An Inexpensive Depth Gauge for Penguins

We describe an inexpensive autoradiographic depth gauge for use on penguins. The device costs -$10, has a mass of about 25 g, and records the total time that the penguins spend at each depth underwater. J. WILDL. MANAGE. 48(4):1077-1084 Penguins spend more time in the water than do any other birds. Dive times may be as long as 18 minutes and depths of 265 m have been recorded (Kooyman et al. 1971). Estimates of their swimming speeds range from 7 to 58 km/hour (Kooyman et al. 1971). Surface-swimming penguins are difficult to see due to their low profiles and birds at sea approached by a boat or a diver will generally dive or swim away rapidly. It is, therefore, not surprising that data on penguin behavior in their marine environment are meager. Siegfried et al. (1975) gave an account of jackass penguins (Spheniscus demersus) social behavior at sea, though all the observations were made when the birds were on the surface of the water. Deductions can be made about foraging areas and depths by analyzing stomach contents (Emison 1968, Croxall and Furse 1980, Croxall and Prince 1980). Because it is almost impossible to directly observe penguins foraging, information can only be obtained indirectly from a recording device strapped to the free-swimming bird. Kooyman et al. (1971) used a capillary depth gauge to determine maximum diving depths of the emperor penguin (Aptenodytes forsteri), and they also used a depth/time recorder weighing 700 g. Recently, smaller electronic devices (weighing 95 g) have been constructed to record the frequency of dives of king penguins (A. patagonicus) within set depth ranges (Kooyman et al. 1980, 1982). Unfortunately, these devices are expensive. Losses are liable to be high in this type of field study so the devices should be as inexpensive as possible. We describe the design and field tests of an inexpensive depth gauge for penguins that records continuously the total time spent at each depth integrated over many dives. We thank J. Hanekom, Nuclear Dev. Corp., for his careful preparation of the radioactive beads, and K. Blake, Med. Phys. Sect., Groote Schuur Hospital, and P. Bestbier, South Afr. Bur. of Stand., for the use of their densitometers. R. P. Wilson acknowledges financial support from the J. W. Jagger Scholarship Fund, Univ. of Cape Town, and the South Afr. Nat. Found. The Sea Fish. Res. Inst. is thanked for permission to undertake research on jackass penguins. DESIGN AND PRINCIPLE OF OPERATION We modified a commercially available (Profundimetro 110, cost about $6), wristattached, capillary-type depth gauge commonly used by SCUBA divers (Fig. 1). The gauge is 70 x 45 x 10 mm, with a weight in air of about 25 g. When the capillary tube, closed at one end and with a constriction at the other, is immersed in water, air is trapped in the tube and a meniscus forms near the open end. As the capillary tube is lowered to greater depths, J. Wildl. Manage. 48(4):1984 1077 This content downloaded from 157.55.39.128 on Mon, 18 Jul 2016 04:21:08 UTC All use subject to http://about.jstor.org/terms 1078 DEPTH GAUGE FOR PENGUINS * Wilson and Bain 70 mm ADepth (m.) 30 AA