Yingqiu Zhang

The strength of the interspecific interactions between non‐native and native species influences the subsequent invasion impacts, with stronger interactions and thus more severe impacts predicted when the species are phylogenetically close and co‐exist at lower latitudes. Although work demonstrates non‐native fish are more likely to invade areas which have phylogenetically closely related species present, the impacts of these invaders on the native species remain poorly understood. Accordingly, we conducted a comprehensive analysis of global freshwater fish occurrence data to test the phylogenetic niche conservatism hypothesis in co‐occurring native and non‐native freshwater fishes, assess whether non‐native fishes drive extinctions of phylogenetically closely related native species, and evaluate how this varies with latitude. Phylogenetic niche conservatism was evident in co‐occurring non‐native and native fishes, with their morphometric trait dissimilarity significantly correlated with their phylogenetic distance. Extinct freshwater fishes were found to be more closely related to co‐occurred non‐native species than were species of other conservation statuses. However, the relationship between the extinction probability of native freshwater fishes and their phylogenetic relatedness to non‐native species was context‐dependent, varying with latitude and across biogeographical realms. At higher latitudes, native fishes closely related to non‐native species had a higher probability of extinction, whereas at lower latitudes, closely related native species were less likely to become extinct. Thus, the extinction risks posed by non‐native fish vary spatially and depend on their phylogenetic relatedness to native species, both of which are recommended for consideration in invasion risk management programmes.

Understanding the mechanisms that promote coexistence between functionally analogous non-native and native species that share similar prey is important for predicting the ecological consequences of their competitive interactions. Theory predicts that species coexistence is facilitated by stabilizing mechanisms that enhance inter-specific trophic niche partitioning, and/ or the equalizing mechanisms that reduce differences in the average fitness. Inter-specific niche partitioning, influenced by the ecological opportunity in prey resources, also affects intra-specific niche partitioning and individual niche expansion. These theories were tested using the non-native mrigal carp Cirrhinus mrigala and its congener native mud carp C. molitorella that coexist across the Pearl River basin, Southern China, with their trophic interactions analysed using DNA metabarcoding on fish gut contents sampled across the basin in both the dry and wet seasons of 2019 and 2021. The results revealed that the inter-specific trophic niche overlap was lower than their intra-specific trophic niche overlap, suggesting their coexistence was supported by stabilizing mechanisms. The ecological opportunity in prey resources was driving inter-specific niche partitioning and this partitioning then positively influenced intra-specific niche partitioning and individual niche expansion, especially in mrigal carp that had larger body sizes and occupied higher trophic positions at constricted individual niches. Fish body size also affected inter-specific niche partitioning and the relationship between inter- and intra-specific niche partitioning, with positive effects on both in mrigal carp but negative effects on both in mud carp. The results thus provide a mechanistic explanation on how closely related non-native and native species coexist and highlight the importance of dietary plasticity in facilitating the invasion of non-native species.

Understanding the strength, direction and consequences of the trophic impacts of invasive fish on functionally analogous native fishes is important in invasion impact assessment and for informing management programmes. As functionally analogous native and invasive fishes can share prey resources, there is high potential for competitive interactions, which could affect trophic niche sizes and positions. The highly invasive mrigal carp Cirrhinus mrigala and native mud carp Cirrhinus molitorella now co‐exist in many freshwaters in southern China, raising concern on the consequences for mud carp. The trophic interactions of the two species and their consequences (for growth rates and trophic niche sizes as the isotopic niche, calculated using δ¹³C and δ¹⁵N) were tested in a pond enclosure experiment based on additive and substitutive treatments using fish in allopatric and sympatric contexts, with results compared with invaded wild communities. In the experiment, specific growth rates decreased significantly as fish abundance in treatments increased. These density‐dependent effects were observed in both species and were independent of whether the treatment was allopatric or sympatric. In allopatric treatments, isotopic niches of mud carp increased in size with fish number. Conversely, in sympatric treatments, mud carp isotopic niche size decreased with increased mrigal carp number. For mrigal carp, isotopic niche sizes decreased as fish number increased in both allopatric and sympatric treatments. In sympatric experiment treatments, the isotopic niches of mud carp and mrigal carp were always divergent. This was, however, in contrast to four invaded wild communities, where the isotopic niches of the two fishes always had some overlap. Experimental and field data suggest that although native mud carp could be impacted by mrigal carp, these impacts might only manifest when the species are at very high abundances and inter‐specific competitive interactions could be intense. These results also suggest the two species have mechanisms that reduce their inter‐specific interactions to minimise their competitive interactions, thus any ecological impacts from mrigal carp might manifest more from indirect than direct processes.

Predicting the ecological impacts of invasive species on analogous native species has great significance for native population conservation. We tested how the strength of interspecific competition from a coexisting invasive species varies with abundance and how this compares with intraspecific competition. The model species was the native mud carp, and the coexisting invasive species was mrigal carp. The replicated mesocosm used each species in an allopatric control (n = 10), in intraspecific competition treatments (n = 20, 30), in a sympatric control (n = 5 for both species) and in interspecific competition treatments (n = 10, 15, 20 for both species). The treatment effects were the specific growth rate (SGR), position and size of trophic (isotopic) niches. Compared to mrigal carp, mud carp had a significantly lower survival rate (median: 0.8) and SGR. The mud carp SGR significantly decreased when the con- and inter-specific densities increased, showing a density dependence, but there was no significant difference between intra- and inter-specific competition. Compared to intraspecific competition in which mud carp expanded their niche, the presence of mrigal carp drove the mud carp trophic niche toward broader sizes at lower trophic positions, but intensified interspecific competition drove the mud carp trophic niche toward constriction. Both mud carp and mrigal carp expanded their niches in invaded communities, and their niches overlapped with each other. Their niche expansions implied plasticity in resource utilization. These results also suggested a complexity in how ilyophagous invasive species integrate into food webs and alter their structure.

The Pearl River is a global hotspot of fish biodiversity, yet has the most threatened endemic fish species in China. Since the establishment of the Changzhou Dam in the lower reach, changes in hydrological rhythm have negatively impacted fish downstream of the dam, but their spatiotemporal distribution in response to flood alteration has received little attention. In this study, hydroacoustic surveys were undertaken monthly in 2016 to monitor the distribution and behavior of fish. Fish densities were higher during the water discharge rising stage than during the falling stage, indicating that the fish aggregate during flooding (coefficient of variation [CV] > 100%) and depart after flooding (CV < 100%), especially aggregations of large fish. The target strength (TS) was allocated to two groups as per their frequency distributions, defined as small fish (−55 dB < TS < −40 dB) and large fish (TS > −40 dB). The sizes of both groups were significantly larger during the rising stage when compared to those during the falling stage (p < 0.01). Comparatively more fish were present with a greater average TS, and a substantially greater proportion of large fish was detected during rising stages. Hydrological variation importantly influences fish aggregations, including the numbers and sizes present, with the differences being particularly pronounced between the rising and falling stages. Combined with relevant studies, it is suggested that water releases from the Changzhou Dam should be regulated to satisfy fish spawning and migration demands during the main breeding season.

This study aimed to determine the age, growth, mortality, and population structure of the economically important cyprinid silver carpHypophthalmichthys molitrix(Valenciennes, 1844) in the middle and lower reaches of the Pearl River. A total of 297 silver carp were sampled quarterly from the catches of gillnet fishermen, at six sites, between June 2019 and September 2020. Standard length of the specimens ranged from 130 to 585 mm, and body weight ranged from 45.5 to 3930 g. The length–weight relationship parameterbvalues reached 3.015. Age was determined through examination and measurements of fish scales, and the age composition of the sampled silver carp varied from 0+ to 4+. Fitting the new data to the von Bertalanffy growth model, we obtained an asymptotic size (L_∞) of 1107 mm,kof 0.135, andt₀of −0.666 for silver carp in this stretch of the river. The calculated growth performance indexϕand estimated longevityt_maxwere 5.22 and 21.56, respectively. The rates of total mortality, natural mortality, and fishing mortality were calculated as 0.4997, 0.1621, and 0.3377, respectively, while the exploitation ratio was evaluated as 0.6757. The overall results confirm overexploitation of this resource in the middle and lower reaches of the Pearl River. It was concluded that this species should be protected from capture until at least 790 mm in standard length, representing an optimal minimum size for capture to benefit conservation of the species and to sustainably develop this valuable fishery.

Understanding fish migration patterns and habitat use is essential for fisheries management. We conducted an acoustic fine-scale tracking experiment from March to June 2017 to determine the common carp movement pattern and habitat use in the downstream area of the Pearl River. The eight tagged common carp were detected for 39.38 ± 29.57 d, on average, with a total average detection period of 42.12 ± 28.02 d. A general linear mixed model suggested they were detected more during the night than during the day. Common carp preferred habitats near the sandbar and riparian shallow habitats, as revealed by their movement tracks and habitat selection indexes. The general additive mixed model (GAMM) analysis of their shortest distances to riverbanks and depth indicated that they migrated to shallower riparian habitat more frequently during the night than during the day. The continuous wavelet spectrum further confirmed the significant diel pattern of their vertical movement. Their habitat use was also influenced by the water temperature, water level, and river discharge, as suggested by the GAMM. Our results provide novel information on common carp spatiotemporal movement and habitat use patterns, and have important implications for its native population restoration and the invasive population control/exclusion.