Tetsuya Tsuruta

Effect of fish in rice-fish culture on the rice yield

Rice-fish culture, which means the simultaneous culture of rice and fish, is one of the best options to increase food production from limited land and is practiced in many countries in the world. Although many researchers and farmers believe that the rice yield is increased by fish farming in paddy fields, this hypothesis has never been fully tested. Here, we report ecological processes leading to higher rice yields in the rice-fish culture using crucian carp (Carassius complex), which have adapted to the paddy field ecosystem in Japan. We compare the rice-fish and rice-only plots in the experimental paddy field for biota, water quality, and rice yield. Coverage of duckweed and densities of zooplankton and benthic invertebrates in the rice-fish plots were lower than those in the rice-only plots, indicating that fish utilized them as food. NO3–N concentration in the rice-fish plots was higher than that in the rice-only plots, indicating that the increase in NO3–N concentration results from excretion of unutilized food nutrients by the fish. Consequently, rice yield in the rice-fish plots was 20% higher than that in the rice-only plots. The fertilizing effect of the fish excrement probably increased rice yield.

Changes in prolactin mRNA levels during downstream migration of the amphidromous teleost, ayu Plecoglossus altivelis

Changes in mRNA levels of prolactin (PRL) during seaward migration and after experimental transfer from fresh water (FW) to seawater (SW) were examined in larvae of the amphidromous fish, ayu Plecoglossus altivelis. In the field study, ayu larvae caught in the surf zone showed lower levels of PRL mRNA than those in the river, while growth hormone (GH) levels showed no significant change. Decrease in PRL gene transcription was also observed 24h after direct transfer from FW to SW, whereas there was no significant influence of water temperature. On the other hand, there was no significant change in GH mRNA levels in relation to SW transfer or environmental temperature. In a raceway with a vertical salinity gradient, PRL mRNA levels of ayu larvae showed a significant reduction during spontaneous migration from FW to SW, which mimicked the results from the field observation and the transfer experiment, and then a gradual increase during the course of development. Whole body water and sodium contents of larvae in a salinity gradient were stable during migration to SW. Results in this study indicated the importance of regulation of PRL gene expression in the downstream migration and acclimation to SW during the early development of ayu.

Diet and food selectivity of Japanese fluvial sculpin Cottus pollux (large-egg type) in the upper regions of the Chikuma River

To clarify food resource use and niche breadths of the Japanese fluvial sculpin Cottuspollux (large-egg type), listed by over half the prefectures as an endangered species, we examined the diet and food selectivity of 168 specimens at 7 locations in 3 tributaries that drain into the upper regions of the Chikuma River. A stomach pump was used to assess the stomach contents. In total, 2,833 aquatic organisms from more than 22 families were found in the stomachs. An index of relative importance (IRI) indicated that chironomid, baetid, and ephemerellid larvae were the major prey for the sculpin. Despite considerable variation in the composition of aquatic organisms among locations, sculpin exhibited strong food selectivity for baetid, glossosomatid, chironomid, and tipulid larvae at multiple locations. Each individual sculpin preyed upon 17.2 prey items on average, and the number of prey items found in the stomach positively correlated with the number of prey families, regardless of sculpin body size. Of 22 prey families found form all stomachs we examined, each individual sculpin preyed upon only 3.54 families on average. These findings suggest that sculpin may have relatively narrow niche breadths regardless of a considerable number of prey individuals in their stomachs. GLM results indicated that a model that incorporated water depth and sculpin body size was the most parsimonious model to explain the prey diversity index in the stomachs of the sculpins. The diversity index of aquatic organisms positively correlated with the mean water depth at each location. These data suggest that rivers with deeper water depths might allow colonization of a variety of aquatic organisms, which might be expected to enhance availability of prey potentially available for the sculpin.

Winter Feeding of the Common Cormorant (Phalacrocorax carbo hanedae) in a Temperate River System in Japan

ABSTRACT In Japan, the numbers of common cormorants (Phalacrocorax carbo hanedae) feeding in inland waters have shown a rapid increase since the 1980s, which has resulted in concerns of predation impacts on inland fisheries. We had an opportunity to examine stomach contents of cormorants in an inland area during winter, 2007–2008. Of the 23 cormorants examined, 19 had fish in their stomachs. The mean wet weight of identifiable prey fish in stomachs (129.3 g) amounted to 25.7% of the daily food requirement of the cormorants. Of the four fish species found in the stomachs of the cormorants, pale chub (Zacco platypus) and Japanese dace (Tribolodon hakonensis), school-forming species, numerically dominated. Sixteen of 19 cormorants had either or both of these two species in their stomachs; only four cormorants preyed upon the barbel steed (Hemibarbus labeo), which implies a benthic foraging mode. A calculated index of relative importance indicated that pale chub and Japanese dace were important prey for the cormorants. Coefficient of variation of prey fish size was large for crucian carp (19.0), pale chub (18.8), and Japanese dace (15.9). We conclude that fish forming dense size-structured schools are vulnerable to predation by the cormorants, and cormorants may have started to use inland pools of the Chikuma River during winter as feeding environments.