Abstract

The goliath grouper, Epinephelus itajara, is the largest grouper in the Atlantic Ocean, exceeding 2 m in total length and 400 kg. Juveniles (< 1 m TL) inhabit fringing red mangrove (Rhizophora mangle) shorelines, whereas adults inhabit coral reefs, patch reefs, reef ledges, and artificial reefs. After reaching commercial extinction in the late 1980s, the species has been protected in U.S. state and federal waters since 1990. Underwater visual census to study distribution and abundance of juveniles are critical to evaluate population recovery and essential fish habitat but are often limited due to low visibility in mangrove shorelines. Here we present a new type of juvenile goliath grouper surveys using high-resolution dual-frequency sonar (DIDSON). DIDSON technology uses multiple narrow hydroacoustic beams (96 at 1.8 MHz and 48 at 1.0 MHz) to produce high-resolution, near-field image streams similar to that of a black-and-white video camera. It can record the shape and size of targets (i.e. fish), and function in low visibility conditions. A paired DIDSON – digital underwater camera system was used to survey the mangrove shorelines of Niles Channel, in the Lower Florida Keys. Calibration occurred by recording juvenile goliath grouper both with DIDSON and video in high visibility conditions. The specific profile obtained during calibration was used to record and detect juveniles with DIDSON in low visibility. However, DIDSON could not detect immobile juveniles resting on underwater structures. On-going and future research directions using this system are discussed.

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