Estimation of positioning error from an array of automated omnidirectional receivers in an artificial reef area

Artificial reefs (AR) have been shown to harbour increased numbers of fish compared to adjacent areas. To assess whether this is due to attraction through artificial structures or providing permanent habitat for the occupants, the behaviour of fish within ARs needs to be examined in more detail. Automated omnidirectional receivers and miniaturized pingers were applied to determine the hydrophones' detection range and study the variability of detections recorded at different distances, and to estimate pinger positioning error throughout the AR study area. Throughout the study site, positioning design and respective detection ranges of five receivers created 17 regions of overlap between two to five receivers. The detection range of the hydrophones was found to be 500 m. Weighted means of modified detection numbers were used to calculate the pinger's position in the study area. Pinger positioning error (Pe) was defined as the distance between the calculated geographical coordinates and the real ones. The artificial reef characterization results showed that the position error is related to the number of receivers overlapping. Minimal positional error was found in regions with four and five overlapping receivers (Pe is 54±24 m). The "barrier effect" of an artificial structure on signals detection depends on the distance between obstacle and transmitter. The omnidirectional hydrophones were unable to record any signal when the pinger was set inside the artificial structures. But, when the pinger was at least 1 m outside the pyramid, the positioning accuracy was found to be 132±54 m.