Target strength of a common kelp species, Saccharina japonica, measured using a quantitative echosounder in an indoor seawater tank

Abstract

Knowledge about kelp forests spatial distribution and biomass is necessary for ecosystem conservation and sustainable use of fishery resources in coastal waters. The acoustic method has been suggested useful for estimating kelp forest thickness and spatial distribution in a large region; and is expected to be useful for biomass estimation; however, echo intensity per unit of kelp [i.e., target strength (TS)], which is a necessary parameter for estimating biomass, has not been revealed. The objective of this study was to obtain TS of the most common and commercially important kelp species, Saccharina japonica. We sampled 12 specimens of various weights in the coastal waters off Hakodate, Hokkaido, Japan, and measured the length, wet and dry weights, target strength of the blade body individually after they were folded and fixed to the same shape (0.3 × 0.3 m) using a quantitative echosounder at 120 kHz frequency in an indoor seawater tank. The TS of each specimen varied when they swayed slightly owing to the derived water currents, and a positive correlation was confirmed between the TS and length, wet weight (p < 0.05). Mean TS (TSmean) of samples were from –58.23 to –41.6 dB re 1 m2. Moreover, a positive linear correlation was found between TSmean and the logarithm weight (TSWmean ＝ 15.3 logWkg − 43.4, r2 = 0.79), which indicates the possibility for estimating biomass of kelp forests using TSmean by acoustic method, and the confirmation and accuracy increasing is expected.