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. NO 3 -N concentration in the rice-fish plots was higher than that in the rice-only plots, indicating that the increase in NO 3 -N concentration results from excretion of unutilized food nutrients by the fish. Consequently, rice yield in the ricefish plots was 20% higher than that in the rice-only plots. The fertilizing effect of the fish excrement probably increased rice yield.

The nest site characteristics and distribution patterns of nests of three sympatric ninespine sticklebacks, the brackish-and fresh-water types and Pungitius tymensis, were investigated in the Shiomi River, eastern Hokkaido, Japan. In this river, most of the males of the brackish-water type were found to build their nests in the lower reach with higher salinity (mean 13.5%) and water temperature (mean 13.3°C), whereas the males of the freshwater type and P. tymensis built their nests in the upper reach with lower salinity (mean 0.6 and 0.8%, respectively) and water temperature (both mean 11.3°C). The ranges of their nesting areas, however, overlapped within the river, because the nests of the brackish-water type were widely distributed in the river. A comparison of the nest site characteristics in the area where nest overlapped indicated that there were no significant differences for all of five characteristics (the distance from the nest to the shore, the distance from the nest to the bottom, the distance to the nearest nest, the nest density and the amount of cover around the nest). Using canonical discriminant analysis, the nests of the three sticklebacks could not be distinguished using the five nest characteristics. These results suggest that habitat isolation through physical factors functions as an important mechanism of reproductive isolation between the brackish-water type and the other two sticklebacks.

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The population structure and its effect on genetic diversity were investigated in two ecologically distinct forms (fresh-and brackish-water types) of ninespine stickleback (Pungitius pungitius L.), which coexist on the coast of eastern Hokkaido, Japan. Obvious population subdivision (ϕ ST = 0.102 to 0.668) between freshwater systems in the freshwater type compared with that in the brackish-water type (ϕ ST = <0 to 0.078) was revealed on the basis of mitochondrial DNA (mtDNA) control region sequence variations. Results of the nested clade analysis suggested that the geographic distribution patterns of mtDNA lineages were mainly due to the contemporary population structure, rather than population histories. Although local populations were sampled from a similar geographic range for the two types, gene and nucleotide diversities of the freshwater type were significantly greater (0.94 and 0.016) than those of the brackish-water type (0.55 and 0.003). The increment of genetic diversity should be attributable to the obvious population structure and existence of divergent mtDNA lineages in the former, which clearly increased the effective population size of a subdivided population. Although these two forms maintain distinct gene pools, small sequence differences among mtDNA haplotypes obtained from each type suggested that mtDNA introgression has occurred between them.Résumé : La structure de population et son effet sur la diversité génétique ont pu être étudiés chez deux formes écolo-giquement distinctes (d'eau douce et d'eau saumâtre) de l'épinoche à neuf épines (Pungitius pungitius L.) qui cohabitent sur la côte orientale d'Hokkaido au Japon. Contrairement à la forme d'eau saumâtre (ϕ ST = <0-0,078), il y a chez la forme d'eau douce une nette subdivision de la population (ϕ ST = 0,102-0,668) selon les systèmes hydrographiques, mise en évidence par les variations dans les séquences de la région de contrôle de l'ADN mitochondrial (ADNmt). Les résultats d'une analyse emboîtée des clades laisse croire que les patterns de répartition géographique des lignées d'ADNmt sont reliés à la structure contemporaine de la population plutôt qu'au passé de ces populations. Bien que les populations locales des deux formes aient été échantillonnées sur une étendue géographique semblable, la diversité des gènes et celle des nucléotides chez la population d'eau douce sont significativement plus élevées (respectivement, 0,94 et 0,016) que chez la population d'eau saumâtre (0,55 et 0,003). L'accroissement de la diversité génétique doit être attribuable à la structure manifeste de population et l'existence de lignées divergentes d'ADNmt chez la population d'eau douce, ce qui a clairement augmenté la taille effective de cette population subdivisée. Bien que ces deux formes conservent des pools génétiques différents, les faibles différences génétiques observées entre les haplotypes obtenus des deux formes font croire à l'existence d'une introgression de l'ADNmt entre les deux.[Traduit par la Rédaction] Takahashi et al. 432

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The fine scale geographic population structure of two types of nine-spined stickleback Pungitius pungitius (the widely distributed freshwater type and a local endemic, the Omono type) within the Omono River system, Japan, was investigated. A principal components analysis of allele frequencies and neighbour-joining tree for pair-wise F ST values, based on 10 allozyme loci, revealed that the Omono type was comprised of four regional groups with relatively high genetic divergence. This grouping was also supported by hierarchical analysis of molecular variance (AMOVA) with a higher variance component among the regional groups, and by an exact test with significant genotypic differentiation for all sample pairs among the regional groups. Moreover, in the clustering of individuals using the Bayesian method, most of individuals in each regional group were assigned the corresponding cluster. On the other hand, there were less pronounced regional groups of the freshwater type, although AMOVA, exact test for genotypic differentiation and Bayesian analysis indicated genetic divergence between two sampling sites in lower reach of the Omono River and other sites. The results suggest that the Omono type represented an earlier colonization, with subsequent invasion of the freshwater type.

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