Tetsuo Shimada

Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

Size-dependent ontogenetic diet shifts to piscivory documented from stable isotope analyses in an introduced population of largemouth bass

Piscivorous largemouth bass (Micropterus salmoides) have been introduced in several regions outside of their native range in North America, resulting in significant disturbance to native fish communities. This species exhibits an ontogenetic diet shift from zooplanktivory to piscivory as juveniles. An early switch to piscivory allows 0+ bass to increase their growth rate prior to winter, resulting in reduced mortality. However, little is known about the dietary switch at the population level during the first year. We used carbon stable isotope analyses to examine the diets of age 0+ individuals in Lake Izunuma, Japan. The onset of the shift to piscivory occurred at a smaller size than in native or other non-native areas [>40xa0mm total length (TL)]. We found a positive correlation between TL and δ13C throughout summer and autumn. Small individuals had δ13C values that were similar to those of zooplankton, whereas large individuals had δ13C values that were similar to those of cyprinid prey species. This suggests that the smaller 0+ individuals remain zooplanktivorous until late autumn, whereas the larger individuals shift to piscivory as early as June, soon after the breeding season ends. Our results also suggest that a significant number of 0+ bass failed to switch to piscivory until the winter of their first year, despite the smaller size threshold for the onset of piscivory.

Comparison of the exploitation of methane-derived carbon by tubicolous and non-tubicolous chironomid larvae in a temperate eutrophic lake

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) in three sympatric species of larval chironomids were analyzed in a temperate eutrophic shallow lake in Japan. Markedly lower δ13C values were reported in Chironomus plumosus (−51.2xa0‰) and Tanypus sp. (−43.5xa0‰) than those in photoautotrophic carbon sources [particulate organic matter (POM) and sediment]. There were positive correlations between δ13C and δ15N in the two chironomid species. These results indicated that they assimilated carbon derived from biogenic methane by exploiting methane-oxidizing bacteria (MOB). In contrast, Propsilocerus akamusi exhibited similar δ13C values to those of POM or sediment. A δ13C-based isotope mixing model was used to estimate the dietary contributions of MOB to each chironomid species. The mean contributions ranged from 11 to 15xa0% in C. plumosus, 13 to 19xa0% in Tanypus sp., but only up to 5xa0% in P. akamusi. In an aquarium, we observed that individuals of C. plumosus and Tanypus sp., which exhibited low δ13C values, built U-shaped larval tubes in the sediment, and an oxidized layer developed around these tubes. Propsilocerus akamusi did not exhibit this behavior. These results suggest that tube building may provide larval chironomids with greater access to methane-derived carbon through increased opportunities to feed on MOB.

Ontogenetic changes in the trophic position of a freshwater Unionidae mussel

Freshwater mussels have often been used as indicators of a trophic baseline (i.e., primary consumers) in the food web. To assess the utility of a large filter-feeding mussel, Cristaria plicata, as an isotopic indicator, we compared the carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios of the mussel with those of its potential food sources, particulate organic matter (POM) and sediment organic matter (SOM). The delta C-13 values of large mussels (shell length > 140 mm, -31 parts per thousand to -29.2 parts per thousand) were similar to those of POM (-30.2 parts per thousand) rather than those of SOM (-27.6 parts per thousand), indicating that mussel carbon was derived mainly from POM (mainly phytoplankton). In contrast, the mussels exhibited 6.3 parts per thousand to 9.0 parts per thousand higher delta N-15 values than did POM. Assuming a previously reported trophic enrichment factor (+3.4 parts per thousand), the trophic level estimate of the mussel ranged from 2.9 to 3.6, indicating that they functioned as secondary rather than primary consumers. Our results also revealed positive correlations between shell lengths and delta N-15 values, suggesting that the mussels changed their trophic position from primary consumer (shell length 140 mm) with growth. A significant inter-annual difference was found in the y-intercepts of the regression lines between shell length and delta N-15, indicating that mussels can reflect inter-annual changes in the isotopic baseline (i.e., primary producers).

Application of a high-resolution acoustic video camera to fish classification: An experimental study

In this study, we try for fish classification using high resolution acoustic video camera at 3.0 MHz center frequency (ARIS). Newly observation method with wide in vertical and narrow in horizontal beam (WVNHB) was proposed for getting high quality acoustic image of swimming fish, and tested an image processing algorithm basing on Normalized Cross Correlation (NCC) for fish classification. This high-frequency acoustic video camera was applied to fish classification for the first time. Using the proposed observation method, we successfully obtained high quality acoustic images of fish and performed an image processing algorithm on the images.

Assessing the biological process of Hydrilla verticillata predation in a eutrophic pond using high-resolution acoustic imaging sonar

Most Hydrilla verticillata disappeared soon after planting in unsuccessful restoration efforts carried out at Lake Izunuma (northeastern Japan). Furthermore, the disappearance of aquatic plants has been speculated to be a result of crayfish predation. However, the precise cause remains unclear because of the low transparency of the lake water. In this study, to identify the source of Hydrilla verticillata predation and characterize the potential behaviors of predators, we used an acoustic monitoring system based on an image-processing program that we developed. A field experiment was performed in a small eutrophic pond adjacent to Lake Izunuma, and 24 Hydrilla verticillata were planted on the pond bottom. A DIDSON unit was mounted on an original frame with a 3° concentrator lens. Data were collected at 1.8xa0MHz and at a maximum range of 3.0xa0m. All Hydrilla verticillata disappeared within a day of planting. Using the image tracking approach, we estimated the walking speeds of two potential predators, namely crayfish (Procambarus clarkii) and pond snails (Cipangopaludina japonica), and distinguished them accordingly. Using this method, we identified crayfish as the main source of predation and characterized their foraging behavior in terms of the decreasing amount of Hydrilla verticillata. The foraging behavior of crayfish mostly ended after 17xa0h of planting, possibly because all plants had been eaten by that time.

Hypoxia within macrophyte vegetation limits the use of methane-derived carbon by larval chironomids in a shallow temperate eutrophic lake

Methane-derived carbon (MDC) can subsidize lake food webs. However, the trophic transfer of MDC to consumers within macrophyte vegetation is largely unknown. We investigated the seasonality of δ13C in larval chironomids within Nelumbo nucifera (Gaertn.) and Trapa natans var. Japonica (Nakai) vegetation in the shallow, eutrophic Lake Izunuma in Japan. Over the past several years, N. nucifera has rapidly expanded across more than 80% of the lake surface. Prior to the expansion of N. nucifera (2007–2008), a previous study reported extremely low larval δ13C levels with peak sediment methane concentrations in August or September. After the expansion of N. nucifera (2014–2015), we observed extreme hypoxia as low as or lower than 1xa0mgxa0l−1 among the macrophyte coverage during June and August. During August and September, no larvae could be found among N. nucifera, and larvae in T. natans showed relatively high δ13C levels (>u2009−u200940‰). In contrast, larvae were markedly 13C–depleted (down to −u200960‰) during October and November. The renewed supply of oxygen to the lake bottom may stimulate MOB activity, leading to an increase in larval assimilation of MDC. Our results suggest that macrophyte vegetation can affect the seasonality of MDC transfer to benthic consumers under hypoxic conditions in summer.

New method of fish classification by using high-resolution acoustic video camera-ARIS and local invariant feature descriptor

A new observation method of fish classification by using acoustic video camera (ARIS) at 3.0 MHz center frequency was developed. Using the method, high-quality acoustic image was obtained. After getting the acoustic image of fish by ARIS, we utilized a method called Scale-Invariant Feature Transform (SIFT) [1] into fish classification instead of the method that we used before, which is called Normalized Cross Correlation (NCC) [2], and then we compared and contrasted position matching rate of the two methods of NCC and SIFT, and improved the SIFT algorithm to increase the accuracy.

Zoobenthos are minor dietary components of small omnivorous fishes in a shallow eutrophic lake

We examined whether small omnivorous fishes (smaller than ~100mm long) integrate littoral, pelagic and benthic pathways in a shallow, eutrophic lake (Lake Izunuma, Japan). The surface of the lake was covered by a dense vegetation of floating-leaved macrophytes, and small species dominated the icthyofauna. We determined the δ13C and δ15N ratios of five dominant species of small omnivorous fishes. Using a stable isotope analysis in the R mixing model, we determined the possible contribution of three potential food sources (epiphytic algae, zooplankton and zoobenthos (larval chironomids)) to omnivorous fish tissue compositions. Four omnivorous fishes (Gnathopogon elongatus elongatus, Pseudorasbora parva, Biwia zezera and Tridentiger obscurus) subsisted largely on epiphytic algae and zooplankton, whereas zoobenthos contributed little to their diets. Acheilognathus rhombeus subsisted mostly on epiphytic algae. Thus, in this shallow, eutrophic lake, omnivorous fishes incorporated both littoral and pelagic production into the food web, but rarely benthic production. The dominant benthic chironomid larvae often burrow several centimetres into the sediment, and the low dietary contribution of zoobenthos to small fishes may be due to inefficiency at foraging on buried benthos associated with fish body size.

Observation of aquatic biota in eutrophied pond using stationary acoustic monitoring system

Active acoustic monitoring system using a dual frequency identification sonar (DIDSON) is applied for the follow-up observation of aquatic biota after Hydrilla verticillata planting. The field experiment was performed in a small pond adjacent to Lake Izunuma (northern latitude of 38.43 degrees and east longitude of 141.04 degrees; surface area, 1,584 m2), and 24 Hydrilla verticillata were prepared and planted at the pond bottom. The DIDSON unit was mounted on an original frame with a 3 degree concentrator lens. The data were collected at 1.8 MHz (high-frequency mode) and at a maximum range of 3.0 m from the imaging sonar. The frame rate was 2 fps and the recording time was about 20 hours. Successfully, we found out the cause of plants disappearance by the acoustic imaging data. All Hydrilla verticillata were disappeared in a day and strongly affected by the predation pressure of crayfish, and whole scene of feeding was recorded. Other aquatic creatures, such as snail, frog, and several species of fish, were also appeared in the acoustic images. Then, we have originally developed an image processing program and quantified the biological features of the aquatic creatures for the better understanding of aquatic biota in freshwater environment.