Satoshi Kitano

Species diversity and abundance of freshwater fishes in irrigation ditches around rice fields

The relationships between environmental variables, distribution of freshwater fishes and their diet were investigated at 40 sites on irrigation ditches for rice fields in central Japan. Multivariate analyses showed that fish species richness and diversity in ditches was high when fishes were able to easily invade the rice fields. Natural stream beds not covered by concrete had greater total number and biomass of fishes in irrigation ditches. The connection between ditches and rice fields was of secondary importance. Canonical correlation analysis indicated that the two principal fish community variables, abundance and diversity, were respectively represented by the presence of the minnow, Gnathopogon elongatus elongatus, and the absence of the loach, Misgurnus anguillicaudatus. Most fishes in the irrigation ditches predominantly preyed upon aquatic insects such as ephemeropteran nymphs, chironomid larvae and trichopteran larvae, but aerial insects and benthic algae were also fed upon by a few species. Small fishes (<4 cm SL) preyed more on chironomid and less on trichopteran larvae than large (≥4 cm SL) fishes. The diet overlap between species was extremely high for small fishes in ditches weakly connected with rice fields. The recent changes in rice fields and irrigation ditches to increase production efficiency has led to a decrease in fish diversity. For the coexistence of rice production and fish diversity in rice areas, a new irrigation system, where fishes can easily invade rice fields and where plenty of invertebrates are available for fishes, should be developed.

Competitive interactions for foraging microhabitat among introduced brook charr, Salvelinus fontinalis, and native bull charr, S. confluentus, and westslope cutthroat trout, Oncorhynchus clarki lewisi, in a Montana stream

Competitive interactions for foraging microhabitat among introduced brook charr, Salvelinus fontinalis,and native bull charr, S. confluentus, and westslope cutthroat trout, Oncorhynchus clarki lewisi, were studied by species removal experiments in a tributary of the Flathead Lake and River system, northwestern Montana, focusing on brook charr influences on bull charr. When the three species were in sympatry, they interacted with each other, forming a size-structured, mixed-species dominance hierarchy in two stream pools. The influences of interference interactions were examined by measuring changes in five characteristics of foraging microhabitat and behavior, focal point height and velocity, cover use, and foraging rate and distance, after the successive removal of two species. Cutthroat trout removal resulted in increased foraging rates and distances, and decreased cover use for brook charr, but no changes for bull charr. After removal of brook charr from the two-species system, bull charr also increased foraging rates and distances and occupied more exposed positions. Moreover, total fish densities, which had initially decreased owing to the removal experiments, were partly compensated for by subsequent bull charr immigration, implying that competitive interactions with brook charr are an important factor in the mechanisms responsible for the regulation of bull charr densities, at least on a local scale.

Evolution, Ecology, and Conservation of Dolly Varden, White spotted Char, and Bull Trout

Abstract We review the ecology and conservation of three lesser-known chars (genus Salvelinus): Dolly Varden (S. malma), white-spotted char (S. leucomaenis), and bull trout (S. confluentus). Dolly Varden is distributed across the northern Pacific Rim and co-occurs with bull trout and white-spotted char at the southern extremes of its range. In contrast, bull trout and white-spotted char are naturally isolated, with the former restricted to North America and the latter distributed in northeastern Asia. Though the range of Dolly Varden overlaps with the two other chars, it is most closely related to Arctic char (S. alpinus), whereas bull trout and white-spotted char are sister taxa. Each species exhibits diverse life histories with respect to demographic characteristics, trophic ecology, and movement. This diversity appears to be tied to environmental variability (e.g., temperature, habitat connectivity), resource availability (e.g., food), and species interactions. Increasingly, these interactions involve ...

Phylogeography of White-Spotted Charr ( Salvelinus leucomaenis ) Inferred from Mitochondrial DNA Sequences

The white-spotted charr (Salvelinus leucomaenis) is a coldwater-adapted fish distributed in far-eastern Asia. To assess phylogeographic patterns of this species over most of its range in the Japanese archipelago and Sakhalin Island, Russia, we examined nucleotide sequences of the mitochondrial DNA (mtDNA) cytochrome b region (557 bp) in 141 individuals from 50 populations. A total of 33 (5.5%) nucleotide positions were polymorphic and defined 29 haplotypes. Phylogenetic analysis assigned the observed haplotypes to four main clades, which were characterized by the idiosyncrasies and discontinuity of geographic distributions. The nested clade analyses revealed that the geographical distribution patterns of some haplotypes and clades were explained by historical event such as past fragmentation. Although substantial genetic differentiation was found among the four main clades, their geographic distributions overlapped extensively in several regions. Since white-spotted charr can potentially use both freshwater and marine environments, coexistence among different lineages can be attributed to secondary contact through range expansion by migratory individuals during multiple glacial periods after interglacial isolation. Finally, our data demonstrate that the current subspecies designation does not reflect the phylogeography of this species based on mtDNA analysis. Hierarchical analysis (AMOVA) also showed that genetic variation was far more pronounced within subspecies than among subspecies (i.e., among discrete regions). These results suggest that each population, rather than each subspecies, must be treated as an evolutionarily significant unit.

Spawning behavior of bull trout in the upper flathead drainage, Montana, with special reference to hybridization with brook trout

Abstract We observed spawning behavior of adfluvial bull trout Salvelinus confluentus in a stream in the Flathead River basin where this species coexists with resident brook trout S. fontinalis. Bull trout were much larger in total length than brook trout. During spawning activities of bull trout, smaller paired males or females were displaced by larger individuals. Smaller satellite males, especially brook trout, remained near spawning bull trout pairs for long periods. During these periods, the smaller brook trout were attacked by the paired male as frequently as were larger bull trout satellite males. In one case, a small satellite male brook trout was observed to release sperm after sneaking into a redd during spawning by a pair of large bull trout. This activity may account for some instances of hybridization observed in these populations.

Evidence for interspecific hybridization between native white‐spotted charr Salvelinus leucomaenis and non‐native brown trout Salmo trutta on Hokkaido Island, Japan

Hybrids between native white-spotted charr Salvelinus leucomaenis and non-native brown trout Salmo trutta were identified in streams of Hokkaido, Japan, using both appearance and genetic characters. The DNA analyses indicated that the specimens were hybrids between female S. leucomaenis and male S. trutta. Occurrence of such hybrids implies increased mating opportunities between these species in wild streams.

Local specialists among endangered populations of medaka, Oryzias latipes, harboring in fragmented patches

Habitat fragmentation seriously damages local biodiversity of widespread organisms, or so-called common species, in agricultural habitats. We hypothesized that specialists adaptive to local particular conditions makes a population of generalists vulnerable to habitat fragmentation. To evaluate the extinction-proneness of common rural species, we determined the extent of phenotypic divergence using paddy fish, medaka, Oryzias latipes. Despite its wide geographical range, a rapid population decline threatens its persistence, and remnants persist in fragmented patches. We studied niche profiling of populations from different habitats for a factor that possibly lies behind the species being abundant within particular areas. Measurements of behavioral and morphological characteristics provided comparable variables between populations. Principal component analysis summarized these variables into compounded elements relevant to foraging and predator avoidance. Detection of association between behavioral and morphological traits showed a limited number of phenotypes specific to a local habitat, through which individuals adapted to specific narrow niches. Medaka maintains its status by accumulating a variety of local specialists. Because of the limited-dispersal ability, specialized individuals are vulnerable to isolation in less suitable patches that are caused by the destruction of the habitat-network. From a conservation point of view, the results suggest that preservation of habitats that also serve as corridors is recommended for enhancing the richness of common species that are composed of adaptively diversified phenotypes.

Temporal change in the distribution and composition of native, introduced, and hybrid charrs in northern Japan

Introductions of non-native species have caused various negative impacts on native species and their ecosystems. Hybridization is particularly prevalent among closely related species, and can result in displacement, hybrid swarms, or the disruption of a locally adapted gene complex. Although hybridization between native and non-native species is widespread, long-term monitoring is generally lacking. In this study, we compared the distribution and composition of native white-spotted charr (Salvelinus leucomaenis), introduced brook trout (Salvelinus fontinalis), and their hybrids in the upper Sorachi River, Hokkaido, Japan in 2003 and 2013, especially focusing on (1) if genetic introgression or hybrid swarm has occurred and (2) if white-spotted charr have declined, since a previous study indicated a potentially harmful asymmetric hybridization with the mothers of hybrids being all white-spotted charr. We found no evidence of decline in native white-spotted charr; rather, the distribution and abundance of introduced brook trout had decreased. Of 142 charr (i.e., genus Salvelinus) collected, 18 individuals (13%) were hybrids but no unidirectional hybridization was observed. However, most of the hybrids were post-F1 individuals with biased mating with white-spotted charr. The effects of long-term introgression on native white-spotted charr should be further examined.

Stable carbon isotope variability associated with taxonomic composition of lotic benthic algae

We examined the relationship between the δ13C and taxonomic composition of benthic algae collected from a riffle (fast current habitat) of a non‐shaded mountain stream, which is a tributary of the Kiso River, Japan. The benthic algal δ13C ranged from −20.6 to −14.2‰ and tended to be 13C‐depleted with increasing relative abundance of upright filamentous cyanobacteria and 13C‐enriched with increasing relative abundance of prostrate filamentous cyanobacteria. Using isotopic mass balance equations, the relative abundance of the dominant taxa, upright and prostrate filamentous cyanobacteria, small diatoms and others, explained 74% of δ13C variability. This study shows a case where the difference in taxonomic composition is a possible source of the isotopic variability of benthic algae, which is a mixture of taxa with distinct isotopic signatures.