Shin-ichiro Abe

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.

Habitat Use of the Grazing Goby (Sicyopterus japonicus) in Response to Spatial Heterogeneity in Riparian Shade

ABSTRACT The local density of the grazing goby (Sicyopterus japonicus) and benthic algal production were investigated in a 720 m reach of the Sate River in the northern part of Okinawa Island, Japan. The reach was spatially heterogenous in riparian shade and was divided into three segments (open-canopy, shaded and patchy segment) based on vegetation cover. In the reach, the goby was more abundant in the open-canopy segment and open-canopy patches than in the adjacent shaded segment and shaded patches. Between the open-canopy and shaded segments there was no significant difference in the algal biomass; however, the algal growth rate was greater in the open-canopy segment than in the shaded segment. These results indicate that the spatial heterogeneity in riparian shade controls the habitat use of the goby, with open-canopy areas with high algal renewal rates being preferred.

Territorial defense of an excess food supply by an algal grazing fish, ayu

The size of the territory defended by the grazing fish ayu, Plecoglossus altivelis, often increases with decreasing intruder pressure. This territorial extension occurs even after additional growth of the fish reaches a plateau due to a surplus supply of algal food. This raises questions about the ecological advantages of defending an apparently enlarged territory. Enclosures were used to carry out observations on fish maintained as: (i) individuals with algae attached to the whole bottom area (2.4 m2); (ii) individuals with algae covering 0.8 m2; and (iii) with three conspecifics (competitive use) with algae attached to 2.4 m2. In the competitive use enclosures, one (occasionally two) fish that exclusively foraged the area appeared in each of the groups. Algal resources were sufficiently renewable that no significant difference in growth rate was detected between the dominants of the competitive groups and the two types of solitary enclosures, indicating that the defended area supplied algal resources that exceeded the dominants’ ability to grow. All the territorial holders realized an increase in relative body weight, suggesting the occurrence of inhibition, which corresponds to Verner’s concept of superterritory. However, as inferred from the pheophytin/chlorophyll a ratios, algae within a territory showed less senescence, perhaps owing to cropping by the dominant, and this was considered to be an adaptive response by the fish to an unpredictable food supply. In conclusion, the ayu conditionally defends a territory for long-term benefits, and inhibition is a consequence of facultative territory size and not a cause of it.

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.