Comparison of fish assemblages aggregated at artificial reefs by using scuba diving and acoustic surveys

To determine the aggregation effect of artificial reefs (ARs), we conducted surveys in 16 artificial reefs in Taiwan by using the scientific echo sounder, Simrad EY-60 (200 KHz), along with sighting by scuba divers. Scuba diving was conducted within the reef structures (r = 20 m), whereas acoustic surveys were conducted over a wider range (r = 100 m), with five repeating transects over the ARs. We compared the results obtained from the two methods among 16 artificial reefs in the coastal waters of Taiwan to determine the consistency of the quantitative information between the two methods and to investigate the fish assemblage aggregation status of the ARs. The surveys were conducted in the summers of 2018 and 2019. In most cases, quantitative descriptors obtained by the acoustic surveys (e.g., TS, Sv, and NASC) and scuba diving (e.g., fish lengths and total number of fish) had consistency in variation. The mean fish length estimated by TS and scuba diving showed a synchronous change. The ARs with larger fish sighted by eye had larger fish detected by acoustics, and the ARs with higher diversity of aggregated fish had higher varia-tion of detected TS values. The consistency between the number of fish estimated by scuba diving and the NASC es-timated by acoustics are not as good as that of TS. A possible reason is that the acoustic surveying ranges are much wider than those of scuba diving and partly because the fish echoes were too close to the dead zone of the acoustic beam.