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Abstract

Acoustic methods can be used to assess seaweed meadows. The accuracy of the methods is a key factor in the estimation of seaweed distribution and conditions. We obtained and verified thickness and spatial distribution values measured using an echosounder. We determined the thickness of seaweed growing in the coastal waters off Higashidoori-mura, Aomori, Japan on June 5-6, 2013. Acoustic data were collected using an on-board quantitative echosounder at 120 kHz. The thickness was also directly measured at 14 points. The root mean square error (RMSE) of the thickness determined by the acoustic and direct methods was calculated. A survey to determine the spatial distribution was performed in Miyako-shi, Iwate on July 18, 2014. The estimated spatial distribution was determined from the thickness data collected by the acoustic method of varied transect lines by changing transect orientation and intervals. The kelp forest distribution was also observed directly at 106 points. Then, the concordance rates of the visual observations and estimated spatial distribution were obtained. The RMSE of the acoustic and directly measured thickness of the kelp forests (Saccharina japonica), was 0.06 m, similar to the vertical resolution of the echosounder. The concordance rate between the acoustic and directly measured values was 92% when the maximum transect interval was 21 m. Smaller transect intervals yielded higher accuracy. High accuracy for the thickness and acoustically derived spatial distribution were obtained by the acoustic method when suitable thresholds were used, which has important applications for the evaluation of seaweed stands.

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