Evaluation of a sonic telemetry system in three habitats of an estuarine environment

Abstract

A directional sonic telemetry system in small embayment, seagrass, and channel habitats in Core Sound, North Carolina was evaluated. We compared point location estimates calculated after correcting for system bias in three different ways: using test station (i.e. known location transmitting–receiving stations) angle errors, and using a within test site and an among test site mean angle error estimate. Estimates are necessary because, when tracking an animal, system bias cannot be corrected for using test station angle errors. In addition, telemetered animals may move beyond test areas or into different habitats. We found no significant difference ( P>0.05) among point location estimates, suggesting that a within or an among mean angle error was an acceptable estimate. Choosing the appropriate angle error estimate must be done carefully because both presented limitations. A within test site mean angle error was the more conservative approach, avoiding biases caused by significant ( P<0.05) interhabitat angle error variability. An estimate containing interhabitat variability (i.e. among test site angle error) might be more robust for correcting system bias when the instrumented animal moves outside test areas or into a heterogeneous area. Seagrass habitat polygons in southern Core Sound range in size from 0.1 to 3189 ha. Attained levels of accuracy and precision from this study suggest that work could be conducted in areas where polygons are ≥6.9 ha, which represents >97% of the seagrass habitat in Core Sound. Although the majority (80%) of the polygons are small (<10.0 ha), they represent <5% of the total seagrass area. In addition, classifying use of habitat in areas where polygons are ≤6.9 ha is possible because small polygons have a contagious distribution; hence, their areas may be additive. Risks of misclassifying use of habitat can be reduced also by controlling the size of confidence areas ( A e ) by adjusting the distance between observers and the tracked animal. The confidence area as a function of distance can be predicted because location error varied linearly and significantly with geometric mean distance ( D g ). On the basis of this relationship, D g must be <326 m for A e 95 to be <10.0 ha.