Keishi Matsuda

Exposure of a herbivorous fish to 134Cs and 137Cs from the riverbed following the Fukushima disaster

Ayu Plecoglossus altivelis, a herbivorous fish, is an important fishery resource and key component of the foodweb in many Japanese streams. Radionuclide contamination of this species is likely transferred to higher trophic levels, include humans, in the food chain. After the Fukushima accident in March 2011, ayu were exposed to highly contaminated silt while feeding on algae attached to the riverbed stones. To understand the route by which herbivorous fish are exposed to radionuclides, the activity concentrations of sum of (134)Cs and (137)Cs (radiocesium) were analyzed in riverbed samples (algae and silt) and in the internal organs and the muscle of ayu in five river systems in the Fukushima Prefecture between summer 2011 and autumn 2013. Although there was a positive correlation between the radiocesium activity concentrations in the muscle and the internal organs of ayu, the median activity concentration in the muscle was much lower than those in the internal organs. The activity concentrations of radiocesium in the riverbed samples and the internal organs and the muscle of ayu were correlated with contamination levels in soil samples taken from the watershed upstream of the sample sites. The results of the generalized linear mixed models suggest that the activity concentrations in both the internal organs and the muscle of ayu declined over time. Additionally, the activity concentrations in the internal organs were correlated with those in the riverbed samples that were collected around the same time as the ayu. The activity concentrations in the muscle were correlated with ayu body size. Our results suggest that ayu ingest (134)Cs and (137)Cs while grazing silt and algae from the riverbed, and a part of the (134)Cs and (137)Cs is assimilated into the muscle of the fish.

Comparison of visual acuity and visual axis of three flatfish species with different ecotypes

The visual acuity, visual axis and visual accommodation of pointhead flounder, slime flounder, and red halibut were determined to obtain basic knowledge for developing appropriate fishing gear and fishing methods for sustainable fisheries. Each of these species has a different ecotype in terms of habitat, depth and prey species. Thus, it was hypothesized that they may differ in terms of visual acuity, visual axis and visual accommodation. Few studies have compared these characters in flatfishes from different ecotypes. We used histological methods to determine visual acuity (i.e. cone cell density) and visual axis (i.e. cone cell distribution) in each of these species. The maximum visual acuity was 0.127 in pointhead flounder (total length, TL 344 mm), 0.092 in slime flounder (TL 372 mm) and 0.109 in red halibut (TL 336 mm). Based on the cone cell distribution in the retina, the visual axis was upward and forward in pointhead flounder, forward and downward in slime flounder, and downward in red halibut. Finally, the mean angle of lens movement was −2° in pointhead flounder, −13° in slime flounder and −32° in red halibut. This measurement of lens movement indicated that the average near-point distance was 0.87× TL in pointhead flounder, 0.65 × TL in slime flounder and 1.02 × TL in red halibut. At similar TL (336–355 mm), the visual acuity of these species differs depending on the direction in which they are looking.

Visual acuity and spectral sensitivity of the elkhorn sculpin Alcichthys alcicornis

In this study, the visual acuity, visual accommodation, and spectral sensitivity of the elkhorn sculpin were determined. The elkhorn sculpin Alcichthys alcicornis is a typical groundfish that has conspicuously large eyes. They are caught by bottom gillnets, trawl nets, bottom line angling, and other fishing methods. Although fish behavior arising from visual stimuli is important in these fishing methods, there is little information available regarding the visual sensitivity of the elkhorn sculpin. Three specimens, with total lengths of 300–310 mm, were studied. Visual acuity was calculated, from the maximum cone density in the retina, to be between 0.10 and 0.14. The minimum separable angles were calculated to be 7.14–10.37 min. Measurement of lens movement in five specimens indicated that the near-point distance was between 0.87 and 1.53 times the total length. Lens direction was estimated to be 11–22° in the downward visual axis. Spectral sensitivity was also determined electrophysiologically by recording S-potentials. Three types of response were obtained from 21 specimens, including L responses that showed hyperpolarization at all wavelengths of stimulating light, and two C responses that showed wavelength-dependent hyperpolarization and depolarization. Based on these results, the elkhorn sculpin has color vision and its sensitivity is most acute in light with a wavelength of 554 nm.

Comparison of the Radioactive Cesium Contamination Level of Fish and their Habitat Among Three Lakes in Fukushima Prefecture, Japan, After the Fukushima Fallout

Levels of radiocesium (134Cs + 137Cs) contamination in lake water, bottom sediment, plankton, and fish were investigated in three geographically separated lakes in Fukushima Prefecture (Lake Hayama, Lake Akimoto, and Lake Tagokura) between June 2012 and November 2013. Levels of contamination differed among the three lakes, with the highest levels in each measured component found in Lake Hayama, followed by Lake Akimoto, and the least contamination in Lake Tagokura. Among the lakes, the magnitude of contamination decreased with distance from the Fukushima Dai-ichi Nuclear Power Plant. Mean radiocesium concentrations were higher in piscivorous fish than in other fish, possibly reflecting differences in trophic levels. Radiocesium concentrations of the lake water, bottom sediment, plankton, and fish were significantly correlated with surface soil radiocesium content near lake sites.

Radiocesium Concentrations and Body Size of Freshwater Fish in Lake Hayama 1 Year After the Fukushima Dai-Ichi Nuclear Power Plant Accident

We measured radiocesium (134Cs + 137Cs) concentrations in five freshwater fish species in Lake Hayama, Fukushima Prefecture, 1 year after the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident in March 2011. The five species included bluegill (Lepomis macrochirus), Carassius spp. (Carassius auratus langsdorfii and Carassius cuvieri), Japanese dace (Tribolodon hakonensis), largemouth bass (Micropterus salmoides), and smallmouth bass (Micropterus dolomieu). We observed a “positive size effect” for radiocesium concentrations in fish muscle, but the coefficient of determination was low for bluegill, Carassius spp., and Japanese dace. In contrast, the coefficient of determination was high for the exponential relationship between body size and radiocesium concentrations in largemouth and smallmouth bass. The geometric mean radiocesium concentration in each body size class was generally higher for carnivorous fish than for omnivorous and herbivorous fish.

Spatiotemporal Monitoring of 134Cs and 137Cs in Ayu, Plecoglossus altivelis, a Microalgae-Grazing Fish, and in Their Freshwater Habitats in Fukushima

Ayu, Plecoglossus altivelis, is a herbivorous fish that is an important fishery resource and a key component of the food web in many Japanese streams. After the Fukushima Daiichi Nuclear Power Plant (FNPP) accident in March 2011, ayu were exposed to highly contaminated silt while feeding on benthic microalgae attached to riverbed stones. To understand the effects of radioactive contamination on ayu, radiocesium (134Cs + 137Cs) concentrations were analyzed in riverbed samples (microalgae and silt) and in the internal organs and muscle of ayu in five river systems in the Fukushima Prefecture between summer 2011 and autumn 2013. The concentrations of radiocesium in both the internal organs and the muscles of ayu declined over time. The radiocesium concentrations in the muscle were correlated with, but much lower than, those in the internal organs. The concentrations in the internal organs were correlated with those in the riverbed samples. The concentrations in the muscle were further correlated with ayu body size. Our results suggest that ayu ingest radiocesium while consuming silt and microalgae from the riverbed, and that a small proportion (about 15 %) is assimilated into the muscle of the fish.