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We tested alternative methods to diver visual censuses of juvenile goliath grouper Epinephelus itajara in turbid mangrove habitat. We used a dual-frequency identification sonar (DID- SON) acoustic camera. DIDSON generates high resolution images of fish with a high-frequency multibeam sonar. The system can record the shape, size and movement of fish under low visibility conditions. A paired DIDSON-digital underwater video camera system was used to survey the man- grove shorelines of Niles Channel, in the Lower Florida Keys, USA. In high visibility conditions, the specific E. itajara DIDSON acoustic signal was obtained and used to evaluate DIDSON's performance for E. itajara surveys in turbid conditions. Under both clear and turbid conditions DIDSON enabled the quantification and measurement of juvenile E. itajara in mangrove undercuts and submerged dead trees that were not visible or detected using video or by a diver. DIDSON could not detect immobile juveniles, as it was unable to discern them from the substrate or the underwater structures where they rested. We demonstrate that mangrove microhabitat underwater structures do not inter- fere with acoustic fish signals. This study represents the first known successful application of a DID- SON acoustic camera in mangrove shorelines to achieve juvenile E. itajara detection and sizing.

Using dual-frequency sonar to detect juvenile goliath grouper Epinephelus itajara in mangrove habitat