Seasonal variations in food resource partitioning among four sympatric gudgeon species in the upper Yangtze River

Is resource partitioning between two sympatric species of Gracilinanus (Didelphimorphia: Didelphidae) related to trophic and spatial niches?

Small mammal species may occur in sympatry, and it is relevant to investigate the mechanisms that lead to coexistence of the closely related species. Despite this, studies evaluating the coexistence of closely related Neotropical marsupials are insipient. The aim of this study was to analyse the mechanisms of resource partitioning between the sympatric species of mouse opossums Gracilinanus agilis and G. microtarsus (Didelphidae), evaluating their trophic and spatial niche. We hypothesized that G. agilis and G. microtarsus differ in at least one niche dimension (space use or food preferences) as a mechanism of coexistence. In the study we analysed trophic niche by evaluating the frequency of occurrence of food items present in Gracilinanus faeces. Also, we analysed spatial niche by comparing the vegetal stratum where the individuals were captured and its relationship with the environmental complexity. Both species presented a high trophic niche overlap, with differences in the frequencies of occurrence of the consumed items. Although the most frequent items in the faeces were Hymenoptera, Isoptera and Coleoptera, their proportion differed significantly between the two species. The variety of food items also differed, with flowers being more frequent in G. agilis diet, whereas fruits were more recorded for G. microtarsus. The spatial niche was very similar, with both species being more captured in the forest understory. However, habitat complexity influenced the abundance of both species, especially litter depth and its biomass of leaves. The results indicated that the coexistence of the two species is associated with the trophic and spatial niche, and that this niche segregation could be allowing closely related species of Gracilinanus to coexist.

Spatial and seasonal patterns of fish assemblages in mountain streams of the Ren River, southwest China

The spatial-temporal patterns of fish assemblages in lotic systems can provide useful information in developing effective conservation measures. This study aimed to explore the spatial and seasonal changes in fish assemblages and their association with environmental factors in mountain streams of the Ren River, southwest China. Field investigations were conducted at 18 sites during the rainy and dry seasons in 2017. A total of 1,330 individuals, belonging to three orders, eight families, 19 genera, and 21 species, were collected. Analysis of similarities (ANOSIM) showed that the structure of fish assemblages varied significantly at the spatial scale, but not at the seasonal scale. In low-order sites, fish assemblages were mainly dominated by cold-water and rheophilic species (e.g., Rhynchocypris oxycephalus, Scaphesthes macrolepis, Metahomaloptera omeiensis, and Gnathopogon herzensteini), while those in high-order sites were predominated by warm-water and eurytopic or stagnophilic species (e.g., Squalidus argentatus, Hemiculter leucisculus, and Zacco platypus). Canonical correspondence analysis (CCA) showed that the fish assemblages were structured by a combination of large-scale landscape factors (e.g., altitude and C-link) and small-scale habitat features (e.g., channel width, water temperature, and depth). Among these factors, landscape had the greatest influence on fish assemblages, while local habitat variables were less important or were only significant in certain seasons.

Spatiotemporal Distribution and Species Composition of Fish Assemblages in the Transitional Zone of the Three Gorges Reservoir, China

The river–reservoir transitional zones play an important role in maintaining riverine fish biodiversity and abundance in highly regulated rivers. The Upper Yangtze River, which contains 286 fish species, is experiencing intense hydropower development. However, few studies have been conducted on the spatial distribution of fish in the river–reservoir transitional zones in the Upper Yangtze River, and it is therefore difficult to predict how this development will impact the region. In this study, a 200-kHz split-beam acoustic system in combination with biological samplings was used to investigate the composition and spatiotemporal distribution of the fish in the transition zone of the Three Gorges Reservoir (TGR) in 2017. The biological samplings comprised 7434 individuals from 61 species, 38 genera, 15 families, and 5 orders. The dominant fish species were Rhinogobio cylindricus, Coreius guichenoti, Hemiculter bleekeri, and Pelteobagrus vachelli. Acoustic detection identified the mean target strength (TS) to be −55.6 in May, −57.8 in August, and −55.8 dB in December. Accordingly, the average fish density was 4.96, 6.96, and 4.42 ind/1000 m3. Significant differences were found in the TS and fish density between flood season (August) and non-flood season (May and December). Spatially, fish preferred the meandering sections at 10–20 m deep. A size-dependent vertical distribution pattern was observed, with larger fish in the deeper layers and smaller fish in shallower waters. Lastly, our results suggest that meandering deep channels should be assigned high priority with respect to fish conservation in the Upper Yangtze River.