Program

Seagrass bed has been considered as an ecosystem which supports high species diversity and high biological production. In order to evaluate impact of the Tsunami in March 2011 on habitat condition, fish community structure and recruitment in seagrass Zostera marina beds of the Pacific coast of northern Japan, vegetation and fish assemblage were compared between the period before (summer in 2009 and 2010) and after (summer from 2011 to 2016) the Tsunami at three sites with different levels of disturbance. At the site with catastrophic impact, seagrass mostly disappeared after 2011. Mean number of fish species, fish abundance and biomass were significantly lower in 2011 and 2012 than in the years before the tsunami. Dominant fish species changed after the tsunami: seagrass-associated species were replaced with mud/sund-associated species. On the other hand, there was no significant year effect on the vegetation coverage, mean number of fish species, fish abundance and biomass at the two other sites with smaller impact by the tsunami. Changes in sea bottom and vegetation conditions (from seagrass to mud/sand bottom) are suggested to have induced the replacement of dominant fish species.

Effects of the 2011 Tohoku earthquake and tsunami on the surf zone fish assemblage during summer to autumn in Otsuchi Bay, Pacific coast of northeastern Japan, were evaluated based on the species composition and abundances of 46 fish species collected by bottom towing surveys from 2009 to 2015. The fish community was characterized by the juveniles of epibenthic species preferring bare sandy habitat, viz. Upeneus japonicus, Repomucenus curvicornis, R. lunatus, Paralichthys olivaceus, Tarphops oligolepis and Pleuronectes yokohamae, and the species associated with seagrass or macroalgal habitat, viz. Pholis crassispina and P. nebulosa and R. ercodes, according to SIMPER analysis. While no significant differences were found in the physical water condition, total abundance, or species diversity and richness indices, cluster and canonical correspondence analyses showed that the post-earthquake species composition was clearly distinct from the pre-earthquake communities by the increase of demersal fish species associated with sandy substrate, such as Favonigobius gymnauchen and flatfishes, in contrast to the reduction of seagrass or macroalgal dependent fish species. The results suggest that the disturbance of tsunami has affected the expansion of the nursery and/or feeding potential for epibenthic species, including many commercial pleuronectid flatfishes, caused by the replacement of substratum by sandy sediment.

The coastal areas of Miyako Bay, Pacific coast of northeastern Japan, were remarkably impacted by the tsunami following the Great East Japan Earthquake on 11 March 2011. In this study, we examined effects of the disturbance caused by the tsunami on habitat characteristics of seagrass Zostera marina beds (surface area, shoot density, and leaf length) and the structure of the associated fish communities (abundance, biomass and species richness) by comparing those before to after tsunami impact from June 2011 to December 2013. After the tsunami, the most seagrass vegetation disappeared at an innermost site of the bay. However, seagrass vegetation (surface area, shoot density, and leaf length) drastically recovered in the three years after the tsunami. The abundance and biomass of seagrass-associated fish (e.g. Gymnogobius heptacanthus, Pholis crassispina) were reduced with the decrease of the seagrass vegetation after tsunami impact, but pelagic species (e.g. Engraulis japonica) were temporarily increased in spite of the drastic changes in their habitat.　The abundance, biomass, and species number of seagrass-associated fish increased from 2011 to 2013 on the whole. The relationship between the seagrass shoot density and occurrence of seagrass-associated fish during the summer season (July-September) revealed that fish abundance and biomass tended to decrease when the seagrass shoot density exceeded 50 shoots / m². Therefore, to conserve seagrass beds appropriately as a nursery of fish, manipulations such as artificially thinning out seagrass could be effective.

Massive tsunamis induce catastrophic disturbance in marine ecosystems, yet they can provide unique opportunities to observe the process of ecological succession. Here we report the recovery of fauna after the 2011 tsunami in northeast Japan based on underwater visual censuses performed every two months over six years. The censuses were conducted at four locations in and around the Moune-Bay, Kesennuma, and species, size and abundance of fishes and conspicuous invertebrates were recorded. Both total fish abundance and species richness increased from the first to the second year after the tsunami followed by stabilization in the following years. Short-lived fish, such as the banded goby Pterogobius elapoides, were abundant in the first two years, whereas long-lived species, such as the black rockfish Sebastes cheni, increased in the third year or later. Tropical fish species were recorded only in the second and third years after the tsunami. The body size of long-lived fish increased during the survey period resulting in a gradual increase of total fish biomass. The recovery of fish assemblages was slow at one site located in the inner bay, where the impact of the tsunami was the strongest. Apart from fish, blooms of the moon jellyfish Aurelia sp. occurred only in the first two years after the tsunami, whereas the abundances of sea cucumber Apostichopus japonicus and abalone Haliotis discus hannai increased after the second and fourth years, respectively. We conclude that it takes approximately three years for coastal fish assemblages to recover from a heavy disturbance such as tsunami and that the recovery is dependent on species-specific life span and habitat. These findings provide some insights for the management of fisheries resources, particularly for designing marine protected areas and also for planning rotational harvest.

Environmental disturbances caused by tsunami can affect the ecology of aquatic organisms with different life histories. In this study, we focused on an amphidromous fish, ayu (Plecoglossus altivelis altivelis), and aimed to elucidate the ecological impacts of the huge tsunami in 2011 on the ayu population at Sanriku. Fish ascending to the river were captured and their hatching date, age and body size at ascending, and growth rate of juveniles during the sea period were determined by analyzing otolith increment and Sr/Ca ratio. Additionally, the total number of drifting larvae was used to estimate the abundance of adult ayu. The values obtained in 2011–2016 were compared with those observed before the tsunami. In 2011, all ascending fish were late-hatched individuals (hatched after October), whereas in 2010, early-hatched fish (hatched in September) had comprised the majority of ascending fish. This hatching-date composition differed from the one predicted based on temporal changes in the number of drifting larvae in 2010. Therefore, the tsunami likely induced high mortality of early-hatched juveniles distributed around the estuary just before entering the river. In the following years, the hatching-date composition was gradually restored to the original (before the tsunami). However, the growth rate during the sea period and the body size at ascending consistently declined. The number of drifting larvae was almost stable during 2011–2016 suggesting that no remarkable decrease had occurred in the abundance of the local population. However, CPUE (catch per unit effort) of ascending fish varied considerably; therefore, survival at sea likely contributed to determining the abundance of ascending fish. These results suggest that the long-term ecological changes in ayu had been influenced by low water temperatures in the Sanriku area, and the post-tsunami temporal changes in the coastal environment.

The impact of the great tsunami, which was generated by the Great East Japan Earthquake on 11 March in 2011, and subsequent temporal and local fishing ban due to contamination of radiocesium on the quality of a nursery ground and population abundance, respectively, of Japanese flounder Paralichthys olivaceus was evaluated. Growth rates of juveniles, as an index of the quality of the nursery ground, were compared between before and after the tsunami in a nursery ground <15 m in depth in Sendai Bay. Recent growth rates (RGR; mm day^-1) between 1 and 6 days before collection in 2011 – 2013, which were estimated from the width of otolith increments, ranged from 0.84 to 2.37 mm day^-1 for juveniles of 24.8 – 146 mm in total length. These figures are comparable to the maximum RGR for juvenile Japanese flounder in the literature. A linear model for observed growth rate (mm day^-1), which was calculated from the temporal changes in the juvenile size frequency distribution, suggested the growth rate did not change between before and after the tsunami. Additional information on prey abundance and sediment size distribution also support this conclusion. Fishing mortality coefficient (F) on the population after the ban during 2011-2014 was estimated as 28 % of F before the ban during 2006-2010 (F2006-2010). On the condition that the population continued to be targeted by the fisheries at the same level of F2006-2010, the abundance of the flounder in 2014 was predicted to be 0.49 times of the actual estimated abundance. Therefore, we conclude that the quality of the nursery ground for juvenile Japanese flounder has not been damaged by the great tsunami and one of the causes of increase in the flounder abundance is the fishing ban.

The waters off Fukushima form one of the most fertile fishing grounds for demersal fish species. However, since the Great East Japan Earthquake and tsunami, and subsequent Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident in March 2011, the commercial catch of demersal fish species off Fukushima had been zero and regular fishing operations have been restricted. In order to be able to re-start regular fishing operations, trial fishing operations [i.e., commercial fishing operations but effort and target fish (only the fish species <100Bq per kg/wet) are restricted] by coastal gill net and offshore bottom trawl fishing have been started off Fukushima from June 2012 and January 2015, respectively. Information on the abundance status of demersal fish stocks after the FNPP accident is necessary to manage them effectively in case regular fishing operations are re-started. We estimated the abundance of seven demersal fish stocks off Fukushima [Japanese flounder (Paralichthys olivaceus), Pacific cod (Gadus microcephalus), Yellow goosefish (Lophius litulon), Slime flounder (Microstomus achne), Stone flounder (Platichthys bicoloratus), Brown sole (Pseudopleuronectes herzensteini) and Marbled sole (Pseudopleuronectes yokohamae)] using catch per unit effort from coastal gill net and offshore bottom trawl fishing from January 2000 to December 2015, including the data from the trial fishing operations. All the estimated abundance of demersal fish stocks apparently increased since 2012, although the predicted abundance decreased or slightly increased in a simulated case the regular fishing operation had been continued. This indicates that the waters off Fukushima have effectively been serving as a marine protected area for the demersal fish stocks since the FNPP accident.

The Pacific cod (Gadus macrocephalus) population in northeastern Honshu, Japan has largely increased for a few years after the Great East Japan Earthquake. We estimated the reason for the increase through the comparisons of commercial fishing activity, age structure, recruitment level and decreasing process after recruitment among before and after the earthquake, and examination of nursery area of young fish. Pacific cod abundance in 2013 and 2014 was estimated to be more than 4-fold of the maximum amount before 2011. Fish was highly exploited from 1 year old before 2011 while fishing pressure has markedly decreased after 2011 in adjacent waters of Fukushima. The population in 2013 and 2014 was primarily composed of 2-4 year old fish (2010-2011 year classes) although the population in pre-2011 was primarily of 1 year old fish. The 2010-2011 year classes had almost the same population sizes until 1.3 years old to the pre-2009 year classes, but became much higher more than 2.8 years old. Pacific cod from 1.3 to 2.8 years old were distributed intensively off Fukushima waters. These results suggest that Pacific cod increased not by support of high recruitment but by lower mortality after recruitment due to reduced fishing mortality. The offing area of Fukushima is the primary nursery of this population, which is effectively serving as a marine protected area after the tsunami events. This increase occurred in the “moderate” recruitments such as 2010 and 2011 year classes. Therefore, adequate catch planning for the young fish will make it possible to sustain population level high without occurrence in strong year classes.

The Great East Japan Earthquake and the gigantic tsunami waves that struck northeastern Japan on 11 March 2011 caused a severe accident at Fukushima Dai-ichi Nuclear Power Plant (FDNPP), owned by the Tokyo Electric Power Company. As a consequence, marine and freshwater fishes in Fukushima Prefecture were broadly contaminated by radionuclides, especially radiocesium (¹³⁴Cs and ¹³⁷Cs), released from the devastated FDNPP. Results of monitoring by Fukushima Prefecture revealed that, in marine areas, demersal fishes that dwell in shallower areas south of the FDNPP tended to show higher radiocesium concentrations and tended to display a slower declining trend than pelagic fishes. However, in 2016, the percentage of demersal fish samples below the detection limit (about 8.3 and 7.4 Bq/kg for ¹³⁴Cs and ¹³⁷Cs, respectively) increased to over 90%, which clearly indicates that radiocesium contamination levels in marine fishes decreased drastically during the six years following the FDNPP accident. Based on those monitoring results, Fukushima’s coastal fisheries have targeted more areas and species on a trial basis. In freshwater fish species, although radiocesium concentrations decreased considerably overall, the percentage of samples below the detection limit remained about 40% in 2016, indicating longer-term contamination of freshwater fishes compared with marine fishes. Particularly, because some samples of salmonid species sometimes exceeded the Japanese regulatory limit of 100 Bq/kg, fishing activities even in the Abukuma River system, located in the central part of Fukushima Prefecture, have been restrained. Furthermore, radiocesium concentrations in fish inhabiting the exclusion zone, which are not covered by the monitoring survey, were still extremely high because radiocesium accumulation through contaminated prey items is continuing in freshwater ecosystems near the FDNPP. Long-term and careful monitoring should be continued to restore marine and freshwater fisheries in Fukushima.

Strontium-90 was released from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) to the atmosphere, which led to fallout on the sea. The waste water was directly released intermittently into the sea during the end of March and early April 2011. Additionally, the leakage of the waste water, including high concentration of strontium-90, was occurred on December 2011. The strontium-90 in seawater was spread across the ocean and diluted by the surrounding sea water to the background level, therefore, concentrations of strontium-90 derived from FDNPP accident in marine fishes would be expected to be lower than those of cesium-137. However, the public have been concerned about the strontium-90 contamination of marine fishes due to its long physical half-life and propensity to accumulate in bone tissue. In this study, we investigated the concentrations of strontium-90 in marine fishes of Japan. Strontium-90 concentrations in all samples collected outside of Fukushima prefecture were under the background level (< 0.046 Bq/kg-wet). The important thing is that the concentrations of strontium-90 derived from the FDNPP accident in marine fishes were notably lower than those of cesium-137. Our results with together analysis of other data sets indicated that the strontium-90 contamination of marine fishes was low level and limited to the area near the FDNPP. Thus, in marine environment, the FDNPP-derived strontium-90 did not a threat to sea food safety.

The accident at Fukushima Daiichi nuclear power plant dispersed radioactive cesium into the freshwater grounds used by fisheries of the herbivorous ayu. As a result, ayu might be exposed to highly contaminated silt while feeding on algae attached to stones on the riverbed. To understand the route by which herbivorous fish are exposed to radionuclides, the concentration of cesium-137 (¹³⁷Cs) was measured in the riverbed (algae and silt) as well as in the internal organs (i.e., stomach contents, stomach, intestine contents, intestine, liver, spleen, and gonad) and the muscle of ayu. Sampling was conducted in five river systems in the Fukushima Prefecture between summer 2011 and autumn 2016. The concentrations of ¹³⁷Cs in the riverbed, internal organs of ayu, and muscle of ayu were correlated with contamination levels in upstream watershed soil samples. The concentrations of ¹³⁷Cs in both the internal organs and the muscle of ayu declined over time across all five rivers. The median concentration of ¹³⁷Cs in the muscle was much lower than in the internal organs. Additionally, the concentration of ¹³⁷Cs in the internal organs was correlated with concentration in contemporary riverbed samples. Surprisingly, the concentration of ¹³⁷Cs in algae (after isolation from the riverbed samples) was only one-third the concentration in the full riverbed samples. The concentration of ¹³⁷Cs in the muscle of ayu was correlated to the concentration in the algae. Our results suggest that ayu ingest radioactive cesium while grazing algae and silt from the riverbed and that some of the ¹³⁷Cs in algae is assimilated into the muscle of the fish.

A Japanese governmental agency (Fisheries Agency) and local governments initiated monitoring programs soon after the Fukushima fallout to monitor radioactive contamination of freshwater and marine fishes and invertebrates in affected areas. The results revealed that radiocesium concentrations in most species have decreased over time to their currently low levels. However, some freshwater fishes in high deposition areas still exhibit radiocesium concentrations more than 100 Bq/kg-wet, which is the Japanese standard limit for radiocesium in foods. Here, I present field and experimental data to demonstrate temporal trends of radiocesium concentration in freshwater fishes. Large variation in the ecological half-life of 137Cs was found among sympatric fishes in Lake Chuzenji, central Honshu Island. Ecological half-lives of 137Cs in the omnivorous masu salmon, brown trout, and lake trout were much longer than those in the planktonic kokanee and smelt. The ecological half-lives of 137Cs in kokanee and smelt were comparable with the half-life of 137Cs in the water from Lake Chuzenji. Size-dependent individual variation in radiocesium concentrations (size effect) was also found in salmonid fishes collected in 2006. To understand the process of radiocesium uptake in salmonids, two captive-rearing experiments were conducted around Lake Chuzenji. These experiments used water and feed with controlled radiocesium concentrations. The results showed that direct radiocesium transfer from the water in Lake Chuzenji [42 - 93 mBq/L (134Cs+137Cs) during Nov. 2012 - Apr. 2013] to muscle tissue was negligible [< 1.28 Bq/kg-wet (134Cs+137Cs) at the termination of the experiment] at least during the ～90-day experimental period. Conversely, a rapid increase in muscle radiocesium levels was observed when fish were fed radiocesium-contaminated pellets. These results revealed that radiocesium contamination in salmonids is mainly via the food chain and that direct intake from water via the skin, gut, and/or gills has no major direct impact on muscle tissue concentrations.

Fukushima Daiichi Nuclear Power Station accident caused by tsunami disaster occurred at March 11, 2011 released a great deal of radioactive materials. Although six years have passed, much amount of radioactive materials, mainly ¹³⁷Cs, have been detected from the river muddy sediment and muscle of fishes in rivers of Fukushima. Rivers and ponds are the place where the radioactive materials flow from the surrounding forest and taken into an organism through food chain. High level of ¹³⁷Cs has been detected from the muscle of Oncorhynchus masou still now. In this study, the genetic effect of radioactive materials on DNA sequence of mitochondria DNA D-loop and Cyt b regions were examined. It is expected that the sequence of mtDNA in offspring are basically equal to maternal sequence.
Females of O. masou were collected from Mano River in Fukushima Prefecture and Fukushima Prefectural Inland Water Experimental Station (FP), at 2014 and 2015. Low level of ¹³⁷Cs was detected from the individuals from Mano River and it did not detect from the individuals from FP. From these females, next generations were produced by gynogenesis, since to remove the genetic effect of male. About 700bp of D-loop and Cyt b regions were sequenced, and compared with maternal sequence.
Single nucleotide substitutions were observed at seven positions of Cyt b and two in D-loop region in the offspring produced from the females of Mano River. On the other hand, no variation was observed in the offspring produced from the females of FP. In these substitutions, fiver types did not involve amino acid substitution, however two types involved amino acid substitution.
These results suggest that the radiation exposure affect to the DNA sequence as the substitution of base.