Hiroki Yasuma

Measurements of density contrast and sound-speed contrast for target strength estimation of Neocalanus copepods (Neocalanus cristatus and Neocalanus plumchrus) in the North Pacific Ocean

The mass density and sound-speed contrasts against surrounding seawater (g and h, respectively) of Neocalanus copepods (N. cristatus and N. plumchrus) were measured in 2006 and 2007 to compute the theoretical target strength (TS). The values of g ranged from 0.997 to 1.009 in N. cristatus and from 0.995 to 1.009 in N. plumchrus. There were no correlations between prosome length (PL) and g. The values of h ranged from 1.006 to 1.021 in N. cristatus and from 1.013 to 1.025 in N. plumchrus and varied with changes in temperature. TS was estimated with the theoretical sound scattering model using the values of g and h based on the temperature, salinity, and depth of the location where the specimens were collected. Regressions of the tilt-averaged TS versus PL were obtained at 38, 120, and 200xa0kHz. The averaged TS of N. cristatus and N. plumchrus at 120xa0kHz, which is widely used as a high frequency, ranged from −110.0 to −103.1xa0dB and from −121.4 to −109.7xa0dB, respectively. There was a positive correlation between frequency and averaged TS: the higher the frequency, the higher the value of averaged TS. The TS at 120 and 38xa0kHz varied from 14.8 to 16.4xa0dB in N. cristatus and from 17.9 to 18.7xa0dB in N. plumchrus, respectively; that at 200 and 120xa0kHz varied from 2.9 to 5.5xa0dB in N. cristatus and from 5.3 to 6.5xa0dB in N. plumchrus, respectively.

Density and sound-speed contrasts, and target strength of Japanese sandeel Ammodytes personatus

Sound-speed and density contrasts (h and g, respectively), important acoustic material properties, of Japanese sandeel Ammodytes personatus were measured to estimate theoretical target strength (TS). The measured sound-speed contrast of adult fish varied between 1.016 and 1.023 (mean 1.020), and showed temperature dependence. The measured density contrast differed significantly between juvenile and adult. The density contrast of juvenile varied between 1.017 and 1.024 (1.021), and that of adult varied between 1.026 and 1.038 (1.032). Using these results, TS at 38 and 120xa0kHz in the fishing season were estimated by an empirical sound scattering model. TS of an individual fish varied significantly with change of tilt angle. TS of near-dorsal aspect (TSmax) and tilt-averaged TS (TSave) differed by up to 7xa0dB. At both frequencies, two different TSave−length relationships (TSavexa0=xa0a logxa0Lxa0+xa0b) were obtained for adult and juvenile. The coefficients of logxa0L of adult were close to 20, suggesting that backscattering strength was proportional to square of body length. These values were larger in juvenile (34.0 at 120xa0kHz, 56.5 at 38xa0kHz), suggesting that backscattering strength varied drastically with the cube or fifth power of body length.

Swimming angle and target strength of larval Japanese anchovy (Engraulis japonicus)

The swimming angle of larval Japanese anchovy (Engraulis japonicus) was measured in a tank, and target strength (TS) was calculated using a theoretical scattering model. The mean swimming angle was 12.8° (SD ±22.1). Increased speeds of flow led to increased mean swimming angles. The mean swimming angle at flow of 5xa0cmxa0s−1 was higher than at other speeds. TS values were estimated using a distorted-wave Born approximation model for two cases. Average values were 1–3xa0cmxa0s−1 (11.5°xa0±xa022.1) and 5xa0cmxa0s−1 (16.6°xa0±xa021.7) for cases 1 and 2, respectively. For case 1, TS ranged from −92.0 to −74.7xa0dB with a mean of −79.4xa0dB at 120xa0kHz. For case 2, TS ranged from −92.2 to −75.2xa0dB with a mean of −79.9xa0dB. The mean TS in case 2 was lower than that in case 1, with the maximum difference being 1.0xa0dB at 120xa0kHz (standard length 22.0xa0mm). However, there were no significant differences between the regression lines of cases 1 and 2. Thus, changes in flow speed altered the swimming angle of larval Japanese anchovy, but had little influence on TS.

Quantitative mapping of kelp forests (Laminaria spp.) before and after harvest in coastal waters of the Shiretoko Peninsula, Hokkaido, Japan

In the Shiretoko Peninsula, a World Natural Heritage site, the sustainable management of kelp forests has drawn public attention because of the economic and ecological importance of kelp. We spatially estimated the distributions of kelp forests in the Shiretoko Peninsula before and after harvest. Field surveys were conducted in coastal waters (23.74xa0km2) at the ends of July and August 2008, immediately before and after harvest. Data on the presence or absence and thickness of the kelp forests were collected via acoustic observation. The data were interpolated using geostatistical methods. Before harvest, the kelp forests were continuously distributed over 5.64xa0km2 (thickness 33–132xa0cm), especially near the north part of the study area. After harvest, they were sparsely distributed over 2.73xa0km2 (thickness 35–105xa0cm). In the southern part of the study area, the influence of harvests was observed as declines in forest area. In addition, relatively thickly forested areas formed the majority of the part most likely to be harvested. Selective harvesting for area and size was confirmed though quantitative mapping of kelp forests. The quantitative mapping of both the distribution and harvest of kelp forests was successful.