Hydroacoustics for the discovery and quantification of Nassau grouper (Epinephelus striatus) spawning aggregations

Egerton, JP, AF Johnson, L Le Vay, CM McCoy, BX Semmens, SA Heppell, and JR Turner. 2017. Hydroacoustics for the discovery and quantification of Nassau grouper (Epinephelus striatus) spawning aggregations.

Coral Reefs. 36 (2): 589-600

The Grouper Moon Project is always looking for new and/or better ways of accurately estimating the number of spawning Nassau Grouper at the aggregation sites being monitored. In 2014, we tested the use of a split-beam echosounder as a tool for surveying the abundance and size of fish at the aggregation site; the results of the study are detailed in this peer-reviewed paper. We found that the echosounder performs fairly well at providing an index of abundance, although the absolute accuracy of the method was not sufficient to replace other survey methods (e.g. mark and recapture monitoring). After calibrating the method with diver-based fish length surveys, the tool was able to accurately capture estimates of aggregating fish sizes. Surveys on all 3 islands (Little Cayman, Cayman Brac, and Grand Cayman) showed that the average size of Nassau Grouper on Little Cayman was significantly larger than on both Brac and Grand. On the other hand, the sizes of Nassau Grouper on Brac and Grand were not significantly different. Based on this study, the echosounder is a potentially useful tool for surveying aggregations, but is likely best use to complement more intensive diver-based survey methods. 

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