CAPILLARY-TUBE DEPTH GAUGES FOR DIVING ANIMALS: AN ...

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of moisture within the tubes, and use of excessive hydrophilic indicator. Potential .... gauge, an estimated depth of 100 m could result from real depths between.
j. Field Ornithol., 59(4):345-354

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

ANIMALS:

ALAN E. BURGER l AND RORY P. WILSON 2 BarnfieldMarine Station Barnfield,British Columbia,VOR IBO, Canada

Abstract.--Gaugesto measurethe maximum depthsattainedby diving animals were constructedfrom plastictubing lined with solubleindicator powder, at a costof $0.10 each. In our tests,the differencesbetween real and estimateddepths averaged 0.05). Depths estimatedfrom actual readingswere much closerto the real depth than thosepredictedwith temperatureinducederrors. Other factors,suchas expansionand contractionof the plastic,might havenegatedthechanges in the air spaceinducedby temperaturechanges. Internal condensation causedby rapid coolingof hot air, with a high relativehumidity due to proximityof the sea,is anotherproblemlikely

Vol.59,No.4

Maximum Depth Gauges for Diving Birds

[351

TABLE1. The effectof divedurationat 50 m depthwith maximum depthgaugesof length 100 mm Time

and 145 mm.

at

100 mm gauges

depth (min) 0.5 1.0 2.0 4.0 8.0

145 mm gauges

Depth predicted(m) Error • (%) 51.0 51.4 51.4 50.6 50.2

ANOVA

+ + + + +

3.1 4.0 4.7 3.2 3.8

Depth predicted(m)

2.0 2.8 2.8 1.2 0.4

F = 0.188, P > 0.05

51.2 51.6 52.5 53.2 53.5

+ + + + +

1.9 2.0 2.4 3.1 3.4

Error (%) 2.4 3.2 5.0 6.4 7.0

F = 1.426, P > 0.05

• Deviation betweenthe real depth and the depth predictedfrom the indicatorreading using eq. 3.

to occurin hot areas.For example,air at 100% relative humidity contains up to 168 g/m 3 of water at 60 C, but only 9 g/m 3 at 10 C (Jorgensen 1979). Thus, a 100 mm longgauge,with an internal diameterof 1.6 mm (volumeca. 200 mm3), at 60 C and 100% R.H. contains3.4 x 10-5 g of water vapor.When immersedin water at 10 C, 95% of the water vapor will condense(i.e., 3.2 x 10-5 g). If all the condensed water remainsin the tube, approximately1 mm3 of water (causinga 0.5 mm error in Ld) will

condense in the lumen

after 30 dives. Under

extreme

conditions the

dropletsmay coalesceand run down the tube dissolvingthe indicator; this was experiencedwhen deployinggaugesmountedon dark-plumaged cormorants in southernAfrica (Wilson,pers.obs.).Condensation problems canbe reducedby minimizingthe periodof deploymentand by attaching gaugeson the undersidesof the birds to reduceinsolation. DISCUSSION

Our testsrevealedseveralreal or potential sourcesof error associated with maximum depthgauges.Accumulationof water dropletswithin the lumen, throughcondensation, prolongeddeployment,or useof excessive hydrophilicindicator,can lead to significanterrors. These, and errors resultingfrom imprecisionin readingLd, will have progressively larger effectsas depth increases.The probability of sucherrors can be reduced throughminimizingthe amountof hydrophilicindicatorused;minimizing deployment times;reducingcondensation by mountingdevices wherethey will be shadedby the animal's body;usinglongertubesfor deepdiving animals;andmaximizingreadingprecision.Inspection of recovered gauges will usuallyrevealevidenceof moistureaccumulation and affectedgauges can be discarded.

The mixing of the air and water columnswithin the gauge,resulting from severejarring, causedsignificanterrorsand might precludethe use of the techniqueon largesealsor whales.The useof narrow tubesreduced sucherrors.Underwater locomotionof birdsshouldnot affectthe accuracy

352]

A.E. Burger andR.P. Wilson

J.Field Ornithol. Autumn 1988

TABLE2. The effectof the rate of descenton depthsestimatedby maximum depth gauges of length 100 and 145 mm (n = 10 for eachtreatment).

Gaugelength (mm)

Depth (m)

Rate of descent Mean (_SD) predicted Paired t-test (m. s-l) depth (m) (t)

100

50

1.3

52.9 + 5.5

100

50

0.4

50.4

100

70

1.1

69.6 + 4.3

100

70

0.4

69.2

145

50

1.1

49.7 + 1.2

145

50

0.4

49.4

145

70

1.2

73.3 + 4.2

145

70

0.4

73.9

+ 3.4

+ 6.4

+ 0.8

+ 3.1

1.500, NS

0.318, NS

0.658, NS

0.286, NS

NS = No significantdifference(P > 0.05).

of maximum depthgauges,but gaugesusedon plunge-divingbirdsshould be testedthoroughlyfor potentialerrors. Repeatedsubmersionsto the samedepth led to overestimates of maximum depths,and this might affectresultsobtainedfrom animalsdiving regularly to fixed depths,suchas obligatebottomfeeders.The size of error dependson the frequencyof diving and the depth attained. Maximum depth estimateswill be more accuratewith epipelagicand midwater foragers,which tend to make infrequent forays into deeperdepths. Errors in depth estimatesaveraged