ACCURACY TESTS FOR SONIC TELEMETRY STUDIES IN AN ESTUARINE ENVIRONMENT

We evaluated accuracy and precision of a directional sonic telemetry s ystem and 2 positioning systems to study sea turtle (Cheloniidae) use of estuarine habitat in Core Sound, North Carolina. Accuracy and preci sion of location estimates affect the power of statistical tests for u se of habitat studies and define the amount of movement that can be re liably measured. Angle errors associated with the sonic system average d -2.5 degrees +/- 5.67 (SD) for a 95% error are of +/-11.34 degrees(r ange - 17-12 degrees). We obtained 45 location estimates after correct ing 90 bearings for bias. Location errors (E), obtained from stationar y positions at 400-1,200 m, ranged from 14.6 to 281.0 m with a median of 75.6 m. The 90 and 95% confidence areas for these data were 11.1 an d 21.2 ha, respectively. Location error varied (P < 0.01) with geometr ic mean distance (D-g) between receivers and transmitters. Areal measu res of confidence obtained at the D-g 500-600 m were the smallest (2.1 -2.8 ha) among 3 distance intervals within 500-1,200 m. Attained level s of accuracy and precision were adequate to determine turtle movement and distribution in relation to selected fisheries activities, but of limited value for use of habitat studies. Inaccurate position estimat es of monitoring platforms (e.g., boats) also affect location estimate s. Precision of position estimates of a stationary boat anchored at a known location (i.e., channel marker) were poor, averaging 62 m, when obtained from Long Range Navigation System (LORAN). In contrast, posit ions obtained from Differential Global Positioning System (DGPS) varie d by 3 m. The DGPS did not affect (P = 0.94) location estimation. Aver age difference between estimates using known location coordinates and those obtained from DGPS was 0.56 with a 95% confidence interval of +/ -1.29 m. We recommend that DGPS be used when evaluating sea turtle use of habitat. The DGPS was more accurate than LORAN and was unaffected by geography.