Propagation of optical pulses in a resonantly absorbing medium is studied. Propagation time of nanosecond pulses was measured for the Rb D 1 transition. At the center of two absorption lines, delay of the pulse peak which is about ten times as large as the pulse width was observed, where zero delay is defined for the propagation with the light velocity in vacuum. On the other hand, at the peak of an absorption line, negative delay was observed for large absorption, where the advance time is as large as 25% of the pulse width. Simulation including the effect of absorption and phase shift reproduced well the experimental results.

Environmental DNA (eDNA) analysis has seen rapid development in the last decade, as a novel biodiversity monitoring method. Previous studies have evaluated optimal strategies, at several experimental steps of eDNA metabarcoding, for the simultaneous detection of fish species. However, optimal sampling strategies, especially the season and the location of water sampling, have not been evaluated thoroughly. To identify optimal sampling seasons and locations, we performed sampling monthly or at two‐monthly intervals throughout the year in three dam reservoirs. Water samples were collected from 15 and nine locations in the Miharu and Okawa dam reservoirs in Fukushima Prefecture, respectively, and five locations in the Sugo dam reservoir in Hyogo Prefecture, Japan. One liter of water was filtered with glass‐fiber filters, and eDNA was extracted. By performing MiFish metabarcoding, we successfully detected a total of 21, 24, and 22 fish species in Miharu, Okawa, and Sugo reservoirs, respectively. From these results, the eDNA metabarcoding method had a similar level of performance compared to conventional long‐term data. Furthermore, it was found to be effective in evaluating entire fish communities. The number of species detected by eDNA survey peaked in May in Miharu and Okawa reservoirs, and in March and June in Sugo reservoir, which corresponds with the breeding seasons of many of fish species inhabiting the reservoirs. In addition, the number of detected species was significantly higher in shore, compared to offshore samples in the Miharu reservoir, and a similar tendency was found in the other two reservoirs. Based on these results, we can conclude that the efficiency of species detection by eDNA metabarcoding could be maximized by collecting water from shore locations during the breeding seasons of the inhabiting fish. These results will contribute in the determination of sampling seasons and locations for fish fauna survey via eDNA metabarcoding, in the future.

Dolly Varden (Salvelinus malma) and Whitespotted Char (Salvelinus leucomaenis) are representative native fish of the family Salmonidae that inhabit the upper reaches of rivers on Hokkaido Island, Japan. They are threatened by the invasive Rainbow Trout (Oncorhynchus mykiss). In this study, environmental DNA (eDNA) real‐time polymerase chain reaction (PCR) assays to detect these three salmonids were developed and used to clarify the distribution pattern of these fish. A specificity test for each assay was conducted using DNA extracted from both target and closely related fish, and the specificity of each assay was confirmed. Then, we carried out eDNA surveys in two mountainous rivers around Mt. Daisetsu in winter, when snow depth was maximized. In the winter surveys, eDNA of all three species were successfully detected from river water samples, including under‐ice water samples. The results of eDNA detection corresponded with the results of an earlier distribution survey performed with Japanese‐style fly‐fishing and lure‐fishing. These results suggested that the eDNA assays developed in this study are applicable for inter‐seasonal surveys for these species.

Underwater sediments are a natural archive of biological information. Reconstruction of past fauna has been conducted for various taxonomic groups using morphological remains and DNA derived from these remains. However, information on past occurrences of fish species, the top predator of lake ecosystems, could have been reproduced only in exceptional environments, and past quantitative information on fish, particularly in lake ecosystems, has been a knowledge gap in reconstructing past fauna. Tracking the quantitative fluctuations of fish is essential for reconstructing multiple trophic levels of organisms in lake ecosystems. To acquire past quantitative fish information from lake sediments, we collected approximately 30 cm-length of underwater sediments in Lake Biwa. We extracted sedimentary environmental DNA (eDNA) and measured temporal fluctuations in the eDNA concentration of the native and fishery target species Plecoglossus altivelis and Gymnogobius isaza. For P. altivelis, we examined the possibility of tracking quantitative fluctuations by comparing sedimentary eDNA with recorded catch per unit effort (CPUE). The chronology of the sediments allowed us to obtain information on sediments collected in Lake Biwa over the past 100 years. The deepest depths at which sedimentary eDNA was detected were 30 and 13 cm for P. altivelis and G. isaza from the surface, corresponding to approximately 100 and 30 years ago, respectively. In the comparison of sedimentary eDNA concentrations and biomass, we found a significant correlation between the CPUE of P. altivelis and its sedimentary eDNA concentration adjusted to compensate for DNA degradation. Sedimentary eDNA fluctuations were observed in P. altivelis, possibly reflecting the abundance fluctuation due to variations in the main food resources of zooplankton. Our findings provide essential pieces for the reconstruction of past fauna of lake ecosystems. The addition of quantitative information on fish species will reach a new phase, for instance, by investigating population shifts or biological interactions in the reconstruction of past fauna in lake ecosystems.

In freshwater ecosystems, invasive salmonid fishes can have a significant impact on native fish species. Detecting the invasion and its negative effects is critical for the conservation of native fish communities. We examined the species composition and seasonal changes in the freshwater fish community, including salmonids, on the Kamikawa Plain, Hokkaido Island, Japan, using environmental DNA (eDNA) metabarcoding. We detected 23 fish species in 176 samples collected from 16 sites over 12 months (October 2018 – August 2019). Between 11 and 20 species were detected at each site, including five native salmonids (Oncorhynchus masou, Oncorhynchus keta, Parahucho perryi, Salvelinus leucomaenis leucomaenis and Salvelinus malma krascheninnikova). The invasive alien rainbow trout Oncorhynchus mykiss was detected at all 16 sites and it was the most commonly detected salmonid. Although we found no obvious competitive exclusion of native salmonids by rainbow trout in the study area, the invasive species occurred more often and at more sites than any of the natives. We also determined the occurrence and seasonal changes in the fish community, classified as native salmonids, invasive rainbow trout, Cypriniformes and other benthic fishes. There were fewer species overall in winter, but the sites with higher species richness in winter were on the lower reaches of the river. In addition, we detected domestic invaders, such as the topmouth gudgeon, Pseudorasbora parva, although they were less prevalent than rainbow trout. These results show the effectiveness of eDNA metabarcoding, which can be used for surveying species richness at an ecosystem scale. In particular, the detection of the early stages of establishment and spread of invasive species can be achieved by eDNA monitoring.