Takaharu Natsumeda

Immigration of the common cormorant Phalacrocorax carbo hanedae into inland areas of the northern part of Nagano Prefecture, eastern Japan, inferred from stable isotopes of carbon, nitrogen and sulphur

Immigration of common cormorants Phalacrocorax carbo hanedae into inland areas might result in substantial loss of inland fish stocks due to predation. We estimated the immigration and habitat of piscivorous common cormorants into inland areas using 3 stable isotope values (δ13C, δ15N and δ34S) collected from 24 cormorant specimens and 9 grey heron (Ardea cinerea) specimens for comparison; Samples were collected (shot) during the winter and spring of 2007–2008 in the northern part of Nagano Prefecture, an inland area of eastern Japan. Variation in carbon and nitrogen isotope values between cormorant individuals was low in the liver and in the pectoralis major muscle, with a rapid rate of turnover. Relatively higher variation in the primary and secondary feathers, the values of which reflect a moulting period, suggest inter-individual variation in habitat use and prey items during and after summer moulting. Predicted mean isotope values of cormorant prey, calculated from fractionation values of the primary feathers, did not support littoral feeding of the cormorants after moulting. Based on empirical measurement of δ34S thresholds, withxa0>10xa0‰ being indicative of tissue grown in a marine environment, the percentage of individuals feeding in seawater- dominated areas after moulting was only 16.7xa0% in cormorants and 0xa0% in grey herons, suggesting the majority of cormorants examined inhabited inland areas and preyed upon inland fish stock after summer moulting.

Size-mediated dominance and aggressive behavior of male Japanese fluvial sculpin Cottus pollux (Pisces: Cottidae) reduce nest-site abundance and mating success of conspecific rivals

Despite growing evidence for plasticity in the mating patterns of nest-holding animals in relation to the changes in nest abundance, the effects of aggressive interaction by dominant males on nest availability for conspecific rivals remains unclear. To quantify the effects of male–male competition on nest-site choice and mating success of the male Japanese fluvial sculpin Cottus pollux, we conducted experiments on 5 males from different 5 size classes under both sufficient and shortage nest-abundance conditions. Nest-choice experiments showed that both male size class and nest-abundance condition had significant effects on the nesting rates of males. Following the nest-choice experiments, 10 gravid females were added in the experimental tanks. Mating experiments revealed that male size, nesting rate before addition of females, and the number of courtship attempts on females were valid variables of male mating success, regardless of nest-abundance conditions. After achieving initial mating success, the largest nesting male exhibited more frequent aggressive interaction with other conspecific males than he did before obtaining eggs in his nest. Our results suggest that size-mediated dominance and aggressive behavior of males may disrupt nest acquisition of other conspecific males, and may consequently result in extreme variation in mating success among males even under sufficient nest-abundance conditions.

Environmental and biotic characteristics to discriminate farm ponds with and without exotic largemouth bass and bluegill in western Japan

We compared the environmental and biotic characteristics of farm ponds with and without the invasive fish, largemouth bass (Micropterus salmoides), and bluegill (Lepomis macrochirus), with varying degrees of aquatic vegetation cover in western Japan. Redundancy analysis (RDA) revealed that aquatic vegetation cover and pond area were significant environmental variables in explaining the variance in aquatic organisms. Aquatic vegetation cover predominantly affected Odonata and Hemiptera larvae, and the native cyprinid, Hemigrammocypris rasborella, while the pond area mainly affected the two exotic fishes (largemouth bass and bluegill), Viviparidae, Oligocheata, Ephemeroptera, and chironomid larvae. In the RDA biplot for aquatic organisms, the RDA1 axis appeared to separate the exotic fish group (bluegill, largemouth bass, Gammaridae, Oligochaeta, Viviparidae, Ephemeroptera, Trichoptera, and chironomid larvae) from the native fish group (H. rasborella, Oryzias latipes, Rhinogobius sp., Odonata, shrimps, and Hemiptera larvae). The best path model results indicated that the presence of piscivorous largemouth bass had a significantly negative effect on native fish numbers; largemouth bass also had a positive indirect effect on benthic organism numbers. Our data suggest that the depletion of native fishes via top-down effects by exotic largemouth bass may indirectly increase the number of benthic organisms as a result of trophic-cascading effects.

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.

Distinct foraging modes between two stream-dwelling benthic fishes, torrent catfish Liobagrus reini and Japanese fluvial sculpin Cottus pollux (large-egg type)

We observed feeding behavior, movement, focal point utilization, and diet of two stream-dwelling benthic fishes, torrent catfish Liobagrus reini and Japanese fluvial sculpin Cottus pollux (large-egg type), under experimental conditions. Torrent catfish are benthos feeders, cruise widely, and prey exclusively on chironomid and trichopteran larvae. Japanese fluvial sculpin are drift and benthos feeders, ambush prey, and select for ephemeropteran nymphs in addition to chironomid and trichopteran larvae. The different foraging modes of these two species might enable their sympatric distribution when their diets overlap.