Spatial distributions of floating seaweeds in the East China Sea from late winter to early spring

Abstract

Floating seaweeds play an important role as a habitat for many animals accompanying or attaching to them in offshore waters. It was in 2000 that the first report described abundant distributions of floating seaweeds in offshore waters in the East China Sea in spring. Young individuals of the yellowtail Seriola quinqueradiata are captured for aquaculture purposes from floating seaweeds in the East China Sea. Therefore, a sound understanding of the distributions of floating seaweeds in the East China Sea is needed. Detailed information is especially important during the late winter to early spring, which corresponds to the juvenile period of the yellowtail. Thus, field surveys using R/V Tansei-Maru were conducted in the Japanese Exclusive Economic Zone in the East China Sea from late winter to early spring in 2010 and 2011. We obtained positions of the vessel by GPS and transversal distances from the vessel to a raft by visual observation. Distance sampling method (Thomas et al. 2010) was applied to estimation of floating seaweed densities (rafts km^-2). Seaweed rafts were also randomly sampled using nets during the research cruises. In the East China Sea, seaweed rafts were distributed mainly on the continental shelf west of the Kuroshio, especially in waters between 26° N and 30° N. Collected rafts consisted of only one species, Sargassum horneri (Turner) C. Agardh. Taking into account surface currents and geographical distribution of S. horneri, it is estimated that these floating seaweeds originated from natural beds along the coast between mid and south China. Considering the approximate travel times, it is suggested that floating patches are colonized by yellowtails early on during their trips, i.e., close to the Chinese coast.

Keywords: Biomass; East China Sea; Floating seaweed; Kuroshio; Sargassum horneri.

Fig. 1

Map showing sampling stations in the East China Sea and Pacific Ocean south of Shikoku Island. Triangles and circles indicate stations in 2010 and 2011, respectively. Filled and open marks are stations where floating seaweed rafts consists of only S. horneri and many seaweed species, respectively

Fig. 2

Visual survey transects for floating seaweed rafts (solid lines) in the East China Sea during R/V Tansei Maru cruises of KT10-1 (a) and KT11-1 (b), and floating seaweed rafts (green circle) in intervals of 20 km distance of transect. Size of circle depends on number of rafts. Black dash line represents 200 m isobaths

Fig. 3

Regression curve (R ² = 0.796) showing the relation between diameter (Di) and wet weight (Wt) of floating seaweed rafts collected during the R/V Tansei Maru cruises of KT10-1 (closed squares) and KT11-1 (open triangles)

Fig. 4

Estimated biomass of floating seaweeds (kg ww km⁻²) along the observation transects (solid lines) during R/V Tansei Maru. Cruises of KT10-1 in February 2010 (a), March 2010 (b), and KT11-1 in February 2011 (c). Light blue stripe represents the path of Kuroshio Current. (source: Hydrographic and Oceanographic Department of Japan Coast Guard, 2010 and 2011)