The important contribution of terrestrial invertebrates to the energy budget of drift‐foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m−2 day−1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m−2 day−1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.

Intraspecific population diversity (specifically, spatial asynchrony of population dynamics) is an essential component of metapopulation stability and persistence in nature. In 2D systems, theory predicts that metapopulation stability should increase with ecosystem size (or habitat network size): Larger ecosystems will harbor more diverse subpopulations with more stable aggregate dynamics. However, current theories developed in simplified landscapes may be inadequate to predict emergent properties of branching ecosystems, an overlooked but widespread habitat geometry. Here, we combine theory and analyses of a unique long-term dataset to show that a scale-invariant characteristic of fractal river networks, branching complexity (measured as branching probability), stabilizes watershed metapopulations. In riverine systems, each branch (i.e., tributary) exhibits distinctive ecological dynamics, and confluences serve as "merging" points of those branches. Hence, increased levels of branching complexity should confer a greater likelihood of integrating asynchronous dynamics over the landscape. We theoretically revealed that the stabilizing effect of branching complexity is a consequence of purely probabilistic processes in natural conditions, where within-branch synchrony exceeds among-branch synchrony. Contrary to current theories developed in 2D systems, metapopulation size (a variable closely related to ecosystem size) had vague effects on metapopulation stability. These theoretical predictions were supported by 18-y observations of fish populations across 31 watersheds: Our cross-watershed comparisons revealed consistent stabilizing effects of branching complexity on metapopulations of very different riverine fishes. A strong association between branching complexity and metapopulation stability is likely to be a pervasive feature of branching networks that strongly affects species persistence during rapid environmental changes.

Motivation: We compiled a global database of long-term riverine fish surveys from 46 regional and national monitoring programmes and from individual academic research efforts, with which numerous basic and applied questions in ecology and global change research can be explored. Such spatially and temporally extensive datasets have been lacking for freshwater systems in comparison to terrestrial ones. Main types of variables contained: The database includes 11,386 time-series of riverine fish community catch data, including 646,270 species-specific abundance records, together with metadata related to the geographical location and sampling methodology of each time-series. Spatial location and grain: The database contains 11,072 unique sampling locations (stream reach), spanning 19 countries, five biogeographical realms and 402 hydrographical basins worldwide. Time period and grain: The database encompasses the period 1951-2019. Each timeseries is composed of a minimum of two yearly surveys (mean = 8 years) and represents a minimum time span of 10 years (mean = 19 years). Major taxa and level of measurement: The database includes 944 species of rayfinned fishes (Class Actinopterygii). Software format: csv. Main conclusion: Our collective effort provides the most comprehensive long-term community database of riverine fishes to date. This unique database should interest ecologists who seek to understand the impacts of human activities on riverine fish biodiversity and to model and predict how fish communities will respond to future environmental change. Together, we hope it will promote advances in macroecological research in the freshwater realm.

We examined the effectiveness of energetic potential (net energy intake [NEI]) estimated from bioenergetics models as an index of habitat quality for stream salmonids in seven streams within four watersheds in Hokkaido, northern Japan. In addition, we confirmed the utility of the NEI as an index of habitat quality by comparing it with several other habitat variables, including pool volume, pool area ratio, and prey density, that are often used as indices of habitat quality for stream salmonids. The mean NEI at each study reach was closely related to salmonid abundance, although the physical environment and drifting prey density differed considerably among study sites. In contrast, the relationships between habitat variables and fish abundance were weaker (drift density) or nonsignificant (pool volume and area). These results suggest that the NEI is more widely applicable as an index of habitat quality for drift‐feeding fish, although its validity should be tested in additional systems.

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Host dispersal is now recognized as a key predictor of the landscape-level persistence and expansion of parasites. However, current theories treat post-infection dispersal propensities as a fixed trait, and the plastic nature of host's responses to parasite infection has long been underappreciated. Here, we present a mark-recapture experiment in a single host-parasite system (larval parasites of the freshwater mussel and its salmonid fish host) and provide, to our knowledge, the first empirical evidence that parasite infection induces size-dependent host dispersal in the field. In response to parasite infection, large fish become more dispersive, whereas small fish tend to stay at the home patch. The observed plasticity in dispersal is interpretable from the viewpoint of host fitness: expected benefits (release from further infection) may exceed dispersal-associated costs for individuals with high dispersal ability (i.e. large fish) but are marginal for individuals with limited dispersal ability (i.e. small fish). Indeed, our growth analysis revealed that only small fish hosts incurred dispersal costs (reduced growth). Strikingly, our simulation study revealed that this plastic dispersal response of infected hosts substantially enhanced parasite persistence and occupancy in a spatially structured system. These results suggest that dispersal plasticity in host species is critical for understanding how parasites emerge, spatially spread, and persist in nature. Our findings provide a novel starting point for building a reliable, predictive model for parasite/disease management.

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Field studies to examine the influence of woody debris on rainbow trout (Oncorhynchus mykiss) abundance through habitat modification were conducted in two small streams, the Horonai and Uenae streams, running through secondary deciduous forest in south‐western Hokkaido, northern Japan. Reach‐based woody debris volume (total woody debris volume per 100 m2 of study reach) was significantly correlated with the total basal area of riparian stands along the margins of the Horonai stream, but no significant relationship was evident between them for the Uenae stream. This inconsistency between the streams was considered to be a result of the difference in stream size (width, depth and discharge). Woody debris was the principal agent for pool formation, although it had a far smaller volume than that found in streams draining old‐growth coniferous forest in North America, where most of the previous studies have been carried out. Untransported debris pieces of larger volume more effectively contributed to pool formation than smaller transported pieces. The volume of individual debris scour pools was positively correlated with the volume of woody debris associated with each. Similarly, reach‐based pool volume increased with total woody debris volume, but the relationship was less clear in the Uenae stream, having more abundant transported woody debris than the Horonai stream. The biomass of rainbow trout in individual pools, which were regarded as the most preferred habitat type for stream salmonids, was correlated with pool volume. A positive relationship also existed between reach‐based standing crop and pool volume. These results revealed that secondary deciduous forest, like old‐growth coniferous forest, plays an important role in enhancing the carrying capacity for rainbow trout by supplying woody debris which promoted preferred habitat formation.