Spatio-temporal variation in water quality and phytoplankton community structure in Changwang, Meishe, and Wuyuan Rivers in Hainan Island, China.

For the first time, this study explored spatio-temporal variation in water quality and phytoplankton community structure in Changwang, Meishe, and Wuyuan Rivers in tropical Hainan Island, China. Phytoplankton samples and water were collected between March and December 2019 and analyzed using standard methods. Two-way ANOVA revealed significant spatial and seasonal variation in physico-chemical parameters (p < 0.05). Wuyuan had high TP (0.06 ± 0.04 mg L-1), TN (1.14 ± 0.71 mg L-1), NH4+-N (0.07 ± 0.09 mg L-1), Secchi depth (2.28 ± 3.79 m), salinity (3.60±5.50 ppt), and EC (332.50 ± 219.10 µS cm-1). At the same time, Meishe had high TP (0.07 ± 0.03 mg L-1), TN (1.04 ± 0.74 mg L-1), NH4+-N (0.07 ± 0.10 mg L-1), EC (327.61 ± 63.22 µS cm-1), and turbidity (40.25 ± 21.16 NTU). In terms of seasons, spring recorded high average TP, TN, NH4+-N, COD, and DO, while summer had a high temperature, Chl-a, salinity, and EC. Generally, the physico-chemical parameters met the China water quality standard limits (GB 3838-2002). Overall, 197 phytoplankton species belonging to Cyanophyta, Chlorophyta, Cryptophyta, Bacillariophyta, Pyrrophyta, Euglenophyta, Xanthophyta, and Chrysophyta were identified, with Cyanophyta being dominant. Phytoplankton density showed spatial changes varying from 18 × 106 cell L-1 to 84 × 106 cell L-1. The phytoplankton diversity ranged from 1.86 to 2.41, indicating a mesotrophic state. One-way ANOSIM showed no significant spatial dissimilarity in phytoplankton composition (R = 0.042, p = 0.771) but indicated a significant seasonal difference (R = 0.265, p = 0.001). Therefore, SIMPER analysis revealed that Lyngbya attenuata, Merismopedia tenuissima, Cyclotella sp., Merismopedia glauca, Merismopedia elegans, and Phormidium tenue contributed to the seasonal differences. Furthermore, CCA demonstrated that TP, TN, NH4+-N, COD, Chl-a, and Secchi depth greatly influenced the phytoplankton community. This study shows the spatio-temporal variation in water quality and phytoplankton communities, useful for managing riverine quality.

Speciation-specific Cr bioaccumulation, morphologic and transcriptomic response in liver of Plectropomus leopardus exposed to dietary Cr(III) and Cr(VI).

Trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) are the two mainly stable oxidation states of Cr in aquatic environments, while the difference of their bioavailability and toxicity by dietary exposure has been rarely known in aquatic organisms. Using juvenile coral trout (Plectropomus leopardus), Cr(III) and Cr(VI) as model system, this study tested the hypothesis that the dietary Cr bioaccumulation and toxicity in fish were highly dependent on Cr speciation. The fish were chronically exposed to 200 mg kg-1 of dietary Cr(III) and Cr(VI) for 8 weeks, and then the Cr bioaccumulation, morphologic change, and RNA-Seq in fish liver were determined. The results showed that dietary Cr(III) and Cr(VI) exposure significantly induced fish weight gain, while 1.17 folds and 1.26 folds increased in relation to Control group, respectively. Cr contents in liver was increased significantly in dietary Cr(VI) but not in Cr(III) groups. Both Cr treatment induced lipid deposition in liver tissue structure, moreover, pancreatic part was increased in dietary Cr(III) but its reduced in Cr(VI) exposure. RNA-Seq in fish liver were significantly different as well. Specifically, there were 138 differentially expressed genes (DEGs) in dietary Cr(III) group, including 76 up-regulated and 62 down-regulated, and these DEGs were mainly involved in lipid metabolism, while there were 175 DEGs in dietary Cr(VI) group, including 85 up-regulated and 90 down-regulated, and these DEGs were mainly involved in immune system. The qRT-PCR confirmed the RNA-seq data were reliable. Overall, these results supported our hypothesis that the chronic dietary Cr(III) and Cr(VI) exposure resulted in apparently different Cr bioaccumulation and toxicity in fish. Our findings here help us to fill in a big gap in our knowledge of speciation-specific Cr bioavailability and toxicity in aquatic organisms, which has been largely unclear previously. CAPSULE: Dietary Cr(III) increased lipid metabolism and dietary Cr(VI) activated immune system in liver of coral trout at transcription levels.

Capturing response differences of species distribution to climate and human pressures by incorporating local adaptation: Implications for the conservation of a critically endangered species.

Considering local adaptation has been increasingly involved in forecasting species distributions under climate change and the management of species conservation. Herein, we take the critically endangered Chinese giant salamander (Andrias davidianus) that has both a low dispersal ability and distinct population divergence in different regions as an example. Basin-scale models that represent different populations in the Huanghe River Basin (HRB), the Yangtze River Basin (YRB), and the Pearl River Basin (PRB) were established using ensemble species distribution models. The species ranges under the future human population density (HPD) and climate change were predicted, and the range loss was evaluated for local basins in 2050 and 2070. Our results showed that the predominant factors affecting species distributions differed among basins, and the responses of the species occurrence to HPD and climate factors were distinctly different from northern to southern basins. Future HPD changes would be the most influential factor that engenders negative impacts on the species distribution in all three basins, especially in the HRB. Climate change will likely be less prominent in decreasing the species range, excluding in the YRB and PRB under the highest-emissions scenario in 2050. Overall, the high-emissions scenario would more significantly aggravate the negative impacts produced by HPD change in both 2050 and 2070, with maximum losses of species ranges in the HRB, YRB, and PRB of 83.4%, 60.0%, and 53.5%, respectively, under the scenarios of the combined impacts of HPD and climate changes. We proposed adapted conservation policies to effectively protect the habitat of this critically endangered animal in different basins based on the outcomes. Our research addresses the importance of incorporating local adaptation into species distribution modeling to inform conservation and management decisions.

Consumer trophic positions respond variably to seasonally fluctuating environments.

The effects of environmental seasonality on food web structure have been notoriously understudied in empirical ecology. Here, we focus on seasonal changes in one key attribute of a food web, consumer trophic position. We ask whether fishes inhabiting tropical river-floodplain ecosystems behave as seasonal omnivores, by shifting their trophic positions in relation to the annual flood pulse, or whether they feed at the same trophic position all year, as much empirical work implicitly assumes. Using dietary data from the Tonle Sap Lake, Cambodia, and a literature review, we find evidence that some fishes, especially small piscivores, increased consumption of invertebrates and/or plant material during the wet season, as predicted. However, nitrogen stable isotope (δ15 N) data for 26 Tonle Sap fishes, spanning a broader range of functional groups, uncovered high variation in seasonal trophic position responses among species (0 to ±0.52 trophic positions). Based on these findings, species respond to the flood pulse differently. Diverse behavioral responses to seasonality, underpinned by spatiotemporal variation at multiple scales, could be central for rerouting matter and energy flow in these dynamic ecosystems. Seasonally flexible foraging behaviors warrant further study given their potential influence on food web dynamics in a range of fluctuating environments.

Behavioral response of juvenile rainbow trout exposed to an herbicide mixture.

Fish are capable of sensing water-borne chemicals at sub-lethal concentrations. Inadequate behavioral responses to physiological and environmental stimuli owing to adverse effects of aquatic toxicants can have serious implications for survival. In this study we exposed juvenile rainbow trout (Oncorhynchus mykiss) during 5 days to a low-concentration mixture of three co-occurring herbicides: atrazine, linuron and metolachlor, at maximum concentrations of 4.5, 4.9 and 13.4 µg L(-1), respectively. Our hypothesis was that fish behavior - swimming activity and interactions between individuals - would be modified due to exposure to the mixture. We studied these behaviors by observing fish twice-daily throughout the exposure period at 30-s intervals for 5 min, registering the vertical distribution of fish in the water column and the number of agoniztic acts between all individuals. Fish exposed to the mixture of herbicides were hypoactive and spent more time in the lower parts of the aquaria in comparison to non-exposed controls, reflecting inhibited swimming activity. Average swimming height of exposed fish decreased significantly with the number of agoniztic acts, whilst in control groups there was no significant relationship between the two behaviors. Overall, behavior of fish exposed for a short time to the herbicide mixture was altered in comparison to control-fish behavior. The behavioral endpoints chosen here were easily observed, simple to quantify, and of ecological relevance.

Using phylogenetic information and chemical properties to predict species tolerances to pesticides.

Direct estimation of species' tolerance to pesticides and other toxic organic substances is a combinatorial problem, because of the large number of species-substance pairs. We propose a statistical modelling approach to predict tolerances associated with untested species-substance pairs, by using models fitted to tested pairs. This approach is based on the phylogeny of species and physico-chemical descriptors of pesticides, with both kinds of information combined in a bilinear model. This bilinear modelling approach predicts tolerance in untested species-compound pairs based on the facts that closely related species often respond similarly to toxic compounds and that chemically similar compounds often have similar toxic effects. The three tolerance models (median lethal concentration after 96 h) used up to 25 aquatic animal species and up to nine pesticides (organochlorines, organophosphates and carbamates). Phylogeny was estimated using DNA sequences, while the pesticides were described by their mode of toxic action and their octanol-water partition coefficients. The models explained 77-84% of the among-species variation in tolerance (log10 LC50). In cross-validation, 84-87% of the predicted tolerances for individual species were within a factor of 10 of the observed values. The approach can also be used to model other species response to multivariate stress factors.

The spatiotemporal variations of microbial community in relation to water quality in a tropical drinking water reservoir, Southmost China.

It is well-known that water quality has great significance on microbial community composition in aquatic environments. In this study, we detected water column indicates the microbial community composition of nine sampling sites over two seasons using Illumina TruSeq sequencing in Songtao Reservoir, Hainan Province, Southmost China. The study indicated that the dominant phylum was Actinobacteria, Proteobacteria, Bacteroidetes, and Cyanobacteria. The diversity parameters showed that the microbial community composition had significant spatiotemporal variations, including the significantly higher Shannon index and Simpson index upstream than those midstream and downstream. Besides, there were significantly higher Chao1 index, Shannon index, and Simpson index in winter than in summer. Principal coordinates analysis (PCoA) showed the microbial structural composition had significant seasonal differences. The results of microbial community composition further revealed that the eutrophication level upstream was higher than that of midstream and downstream. The redundancy analysis (RDA) diagram indicated that the abundance of microbiology species significantly correlated with temperature, total phosphorus, Se, and Ni. Furthermore, the mantel's test showed that the temperature and total phosphorus significantly affected the community composition of archaea and bacteria. Overall, our finding here partially validated our hypothesis that the spatiotemporal variations of microbial community composition are significantly related to nutrients, physicochemical factors and metals, which has been unknown previously in tropical drinking waterbodies. This study substantially contributed to understanding of the composition of microbial community in tropical drinking water reservoirs and the main environmental driving factors in tropical zones. It also provided a reference for the management of reservoir operation to ensure drinking water safe.

Morphological variation among wild populations of Chinese rare minnow (Gobiocypris rarus): deciphering the role of evolutionary processes.

Gobiocypris rarus Ye et Fu ( 1983 ) is an endemic cyprinid fish in China, and is considered to be an endangered species. From a conservation perspective, its population structure is interesting. In the present study, morphological variation, including morphometric and meristic traits, was assessed among wild samples collected in the upper Yangtze River basin. There were no significant meristic differences between sexes or among populations, except for scales in lateral line (LS). However, there were significant morphometric differences not only between sexes but also among populations. In discriminant function analysis, the first four discriminant functions explained 75.5% and 78% of the among-population morphometric variation for males and females, respectively. Almost all the truss network morphometric traits significantly contributed to population discrimination. By using all of the truss network morphometric traits, overall random assignments of male and female individuals into their original population were both 73.5%. In addition, the degree of differentiation in phenotypic traits (PST) significantly exceeds that in neutral molecular markers (FST). However, no significant correlation between PST and FST was found in males or females. Overall, these results suggest that two evolutionary processes, including phenotypic plasticity and natural selection, may contribute to the morphological patterns observed in G. rarus.

Long-term data show alarming decline of majority of fish species in a Lower Mekong basin fishery.

Overexploitation, habitat fragmentation, and flow alteration are major threats to freshwater biodiversity that can lead to fisheries collapse and species extinction. These threats are particularly alarming in poorly monitored ecosystems where resource use supports the livelihoods of numerous people. The Tonle Sap Lake in Cambodia is such an ecosystem, supporting one of the world's largest freshwater fisheries. Tonle Sap Lake fishes are the focus of indiscriminate harvest affecting species stocks, community composition and food-web structure. Changes in the magnitude and timing of the seasonal flood pulse have also been linked to declines in fish stocks. Yet, changes in fish abundance and species-specific temporal trends remain poorly documented. Analyzing 17 years' time series of fish catch data for 110 species, we show that fish populations have declined by 87.7 %, owing to a statistically significant decline for >74 % species, particularly the largest ones. Despite large variations in species-specific trends - going from locally extinct to >1000 % increase - declines were found across most migratory behaviors, trophic positions or IUCN threat categories, though uncertainty regarding the magnitude of effect precluded us drawing conclusions in some cases. These results, reminiscent of alarming declines in fish stocks in many marine fisheries, provide unequivocal evidence that Tonle Sap fish stocks are increasingly depleted. The consequences of this depletion on ecosystem function are unknown but will undoubtedly affect the livelihoods of millions of people, stressing the need to set-up management strategies aimed to protect both the fishery and its associated diversity. Flow alteration, habitat degradation / fragmentation - especially deforestation of seasonally inundated areas and overharvest - have been reported as major drivers in population dynamics and community structure, highlighting the need for management efforts aimed at preserving the natural flood pulse, protecting flooded forest habitats, and reducing overfishing.

Eutrophication induces functional homogenization and traits filtering in Chinese lacustrine fish communities.

Rapid anthropogenic nutrient enrichment has caused widespread ecological problems in aquatic ecosystems and the resulting eutrophication has dramatically changed fish communities throughout the world. However, few studies addressed how fish communities responded to eutrophication in terms of multidimensional functional and taxonomic structure, especially how eutrophication acted as an environment filter on functional traits. The aim of the present study was to investigate the effects of eutrophication on fish species composition, community metrics and species functional traits in 26 shallow lakes from the middle reaches of Yangtze River basin, China. This study validated that eutrophication is an important factor shaping the fish community structure. Regression analyses showed that eutrophication favored higher total biomass and lower functional diversity of fish communities but had little effect on species richness. Despite the fact that some pelagic zooplanktivorous species were more abundant in the most eutrophic lakes, multivariate analyses of the relationships between species traits and environmental variables revealed weak relationships between feeding traits and eutrophication. In contrast, species with a benthic life stage were negatively associated with eutrophication while those with a large body size and high absolute fecundity showed the opposite trend. Due to demersal habitat degradation, and to a lesser degree, to changes in trophic resources availability, eutrophication caused functional simplification of fish communities by increasing functional traits homogeneity among the most tolerant species. Some relationships between functional traits and eutrophication well established in the western palearctic have not been observed in this study, emphasizing the importance of biases resulting from specific evolutionary histories. This work will provide useful insights on on-going restoration and management of shallow lakes in the Yangtze River basin.

Fish biodiversity declines with dam development in the Lower Mekong Basin.

Hydropower dams are a source of renewable energy, but dam development and hydropower generation negatively affect freshwater ecosystems, biodiversity, and food security. We assess the effects of hydropower dam development on spatial-temporal changes in fish biodiversity from 2007 to 2014 in the Sekong, Sesan, and Srepok Basins-major tributaries to the Mekong River. By analyzing a 7-year fish monitoring dataset, and regressing fish abundance and biodiversity trends against cumulative number of upstream dams, we found that hydropower dams reduced fish biodiversity, including migratory, IUCN threatened and indicator species in the Sesan and Srepok Basins where most dams have been constructed. Meanwhile, fish biodiversity increased in the Sekong, the basin with the fewest dams. Fish fauna in the Sesan and Srepok Basins decreased from 60 and 29 species in 2007 to 42 and 25 species in 2014, respectively; while they increased from 33 in 2007 to 56 species in 2014 in the Sekong Basin. This is one of the first empirical studies to show reduced diversity following dam construction and fragmentation, and increased diversity in less regulated rivers in the Mekong River. Our results underscore the importance of the Sekong Basin to fish biodiversity and highlight the likely significance of all remaining free-flowing sections of the Lower Mekong Basin, including the Sekong, Cambodian Mekong, and Tonle Sap Rivers to migratory and threatened fish species. To preserve biodiversity, developing alternative renewable sources of energy or re-operating existing dams to increase power generation are recommended over constructing new hydropower dams.

Species-specific bioaccumulation and health risk assessment of heavy metal in seaweeds in tropic coasts of South China Sea.

Seaweeds are widely known superfood in coasts where most anthropogenic heavy metal discharges are inputted and stored. The present study analyzed 11 seaweed species and 13 heavy metals to test the hypothesis that the species-specific capacity of heavy metal bioaccumulation had great significance to health risk of human. The seaweeds were collected from tropic coasts of Hainan Island. We comparatively determined the bioaccumulation level of metals in different species. The results revealed that the red algae mainly concentrated V, Se, Mn, Ni, and Ag. The brown algae mainly concentrated Cr, Co, Cu, Cd, As and Fe, while the green algae mainly concentrated Zn and Pb. The cluster analysis, principal component analysis and metal pollution index indicated that Padina crassa, Sargassum thunbergii, Caulerpa racemosa and Asparagopsis taxiformis showed similar metal bioaccumulation behavior. The health risk assessment revealed that the overall hazard index (HI) of seaweeds consumption to adults was less than 1, while the HI of Sargassum oligocystum, Turbinaria ornate, Sargassum polycystum and Sargassum thunbergii consumption to children was greater than 1, suggesting a moderate or high risk to children. Moreover, the exposure amount and the carcinogenic risk parameter indicated that As and Cr were the limiting factor for seaweeds consumption. Overall, our findings here largely supported our hypothesis that the heavy metal bioaccumulation behavior and health risk was highly variable and complex among different species. We thus suggested that the species-specific health risk of heavy metals in seaweeds should be cautiously evaluated in natural environments.

Species range shifts in response to climate change and human pressure for the world's largest amphibian.

The Chinese giant salamander, Andrias davidianus, the world's largest amphibian, is critically endangered and has an extremely unique evolutionary history. Therefore, this species represents a global conservation priority and will be impacted by future climate and human pressures. Understanding the range and response to environmental change of this species is a priority for the identification of targeted conservation activities. We projected future range shifts of the Chinese giant salamander under the independent and combined impacts of climate change and human population density (HPD) variations by using ensemble species distribution models. We further evaluated the sustainability of existing nature reserves and identified priority areas for the mitigation or prevention of such pressures. Both climate change and increasing HPD tended to reduce the species range, with the latter leading to greater range losses and fragmentation of the range. Notably, 65.6%, 18.0% and 18.4% of the range loss were attributed solely to HPD change, solely to climate change and to their overlapping impacts, respectively. Overall, the average total and net losses of the species range were 52.5% and 23.4%, respectively, and HPD and climate changes were responsible for 71.4% and 28.6% of the net losses, respectively. We investigated the stability of the remaining species range and found that half of the nature reserves are likely vulnerable, with 57.1% and 66.7% of them likely to lose their conservation value in 2050 and 2070, respectively. To effectively protect this salamander, conservation policies should address both pressures simultaneously, especially considering the negative impact of human pressures in both contemporary periods and the near future. The species range shifts over space and time projected by this research could help guide long-term surveys and the sustainable conservation of wild habitats and populations of this ancient and endangered amphibian.

Spatial pattern and determinants of global invasion risk of an invasive species, sharpbelly Hemiculter leucisculus (Basilesky, 1855).

Invasive species have imposed huge negative impacts on worldwide aquatic ecosystems and are generally difficult or impossible to be eradicated once established. Consequently, it becomes particularly important to ascertain their invasion risk and its determinants since such information can help us formulate more effective preventive or management actions and direct these measures to those areas where they are truly needed so as to ease regulatory burdens. Here, we examined the global invasion risk and its determinants of sharpbelly (Hemiculter leucisculus), one freshwater fish which has a high invasive potential, by using species distribution models (SDMs) and a layer overlay method. Specifically, first an ensemble species distribution model and its basal models (developed from seven machine learning algorithms) were explored to forecast the global habitat-suitability and variables importance for this species, and then a global invasion risk map was created by combining habitat-suitability with a proxy for introduction likelihood (entailing propagule pressure and dispersal constraints) of exotic sharpbelly. The results revealed that (1) the ensemble model had the highest predictive power in forecasting sharpbelly's global habitat-suitability; (2) areas with high invasion risk by sharpbelly patchily spread over the world except Antarctica; and (3) the Human Influence Index (HII), rather than any of the bioclimatic variables, was the most important factor influencing sharpbelly' future invasion. Based on these results, the present study also attempted to propose a series of prevention and management strategies to eliminate or alleviate the adverse effects caused by this species' further expansion.

Evidence of parasite-mediated disruptive selection on genetic diversity in a wild fish population.

Identifying the processes maintaining genetic variability in wild populations is a major concern in conservation and evolutionary biology. Parasite-mediated selection may strongly affect genetic variability in wild populations. The inbreeding depression theory predicts that directional selection imposed by parasites should act against the most inbred hosts, thus favouring genetic diversity in wild populations. We have tested this prediction by evaluating the strength and shape of the relationship between the load of a harmful fin-feeder ectoparasite (Tracheliastes polycolpus) and the genome-wide genetic diversity (i.e. heterozygosity measured at a set of 15 microsatellites) of its fish host, the rostrum dace (Leuciscus leuciscus). Contrary to expectation, we found a nonlinear relationship between host genetic diversity and ectoparasite load, with hosts that were either homozygous or heterozygous harbouring significantly fewer parasites than hosts with an intermediate level of heterozygosity. This relationship suggests that parasites could increase the variance of global heterozygosity in this host population through disruptive selection on genetic diversity. Moreover, when genetic diversity was measured at each locus separately, we found two very strong positive associations between host genetic diversity and the ectoparasite load. This latter result has three main implications: (i) genome-wide effect cannot alone explain the nonlinear relationship between global heterozygosity and ectoparasite load, (ii) negative non-additive allelic interactions (i.e. underdominance) may be a mechanism for resisting ectoparasite infection, and (iii) ectoparasites may favour homozygosity at some loci in this host population.

What do artificial neural networks tell us about the genetic structure of populations? The example of European pig populations.

General and genetic statistical methods are commonly used to deal with microsatellite data (highly variable neutral genetic markers). In this paper, the self-organizing map (SOM) that belongs to the unsupervised artificial neural networks (ANNs) was applied to analyse the structure of 58 European and two Chinese pig populations (Sus scrofa) including commercial lines, local breeds and cosmopolitan breeds. Results were compared with other unsupervised classification or ordination methods such as factorial correspondence analysis, hierarchical clustering from an allele sharing distance and the Bayesian genetic model and with principal components analysis and neighbour joining from allelic frequencies and genetic distances between populations. Like other methods, SOMs were able to classify individuals according to their breed origin and to visualize similarities between breeds. They provided additional information on the within- and between-population diversity, allowed differences between similar populations to be highlighted and helped differentiate different groups of populations.

Why do parasitized hosts look different? Resolving the "chicken-egg" dilemma.

Phenotypic differences between infected and non-infected hosts are often assumed to be the consequence of parasite infection. However, pre-existing differences in hosts' phenotypes may promote differential susceptibility to infection. The phenotypic variability observed within the host population may therefore be a cause rather than a consequence of infection. In this study, we aimed at disentangling the causes and the consequences of parasite infection by calculating the value of a phenotypic trait (i.e., the growth rate) of the hosts both before and after infection occurred. That procedure was applied to two natural systems of host-parasite interactions. In the first system, the infection level of an ectoparasite (Tracheliastes polycolpus) decreases the growth rate of its fish host (the rostrum dace, Leuciscus leuciscus). Reciprocally, this same phenotypic trait before infection modulated the future level of host sensitivity to the direct pathogenic effect of the parasite, namely the level of fin degradation. In the second model, causes and consequences linked the growth rate of the fish host (the rainbow smelt, Osmerus mordax) and the level of endoparasite infection (Proteocephalus tetrastomus). Indeed, the host's growth rate before infection determined the number of parasites later in life, and the parasite biovolume then decreased the host's growth rate of heavily infected hosts. We demonstrated that reciprocal effects between host phenotypes and parasite infection can occur simultaneously in the wild, and that the observed variation in the host phenotype population was not necessarily a consequence of parasite infection. Disentangling the causality of host-parasite interactions should contribute substantially to evaluating the role of parasites in ecological and evolutionary processes.

Reproductive pattern and population dynamics of commercial red swamp crayfish (Procambarus clarkii) from China: implications for sustainable aquaculture management.

BACKGROUND: The red swamp crayfish, Procambarus clarkii (Girard, 1852), is one of the most promising freshwater species for aquaculture in China. Understanding its reproductive pattern and population dynamics is crucial for sustainable management, but there is currently a lack of fundamental knowledge of commercial P. clarkii populations. Therefore, the purpose of this study was to investigate the reproductive pattern and population dynamics of commercial P. clarkii throughout the yearly cycle. METHODS: A total of 2,051 crayfish (1,012 females and 1,039 males) were collected from March 2016 to February 2017 in the area of Selection and Reproduction Center of Crayfish. The reproductive pattern was evaluated by the gonadosomatic index (GSI), hepatosomatic index (HSI), ovarian development and fecundity. Growth, mortality rates and exploitation rate were estimated by electronic length frequency analysis by R package "TropFishR" based on data of cephalothorax length (CTL). RESULTS: Our results demonstrated that spawning activities of P. clarkii took place from September to November, with a mean fecundity of 429 ± 9 eggs per female. There were two recruitments yearly, a major one from October to November and a minor one from March to May. With respect to population growth, five growth cohorts were identified for both females and males. Crayfish grew faster but attained smaller asymptotic maximum CTL as indicated by higher growth coefficient (K), growth parameter index (Ø') and lower asymptotic CTL (Linf ). The estimates of total mortality rate (Z), natural mortality rate (M) and fishing mortality rate (F) were 1.93, 1.02, 0.91 year-1 for females and 2.32, 0.93, 1.39 year-1 for males, which showed that the mortality of male crayfish was mainly caused by fishing. The estimates of exploitation rate (E) indicated that male crayfish were overexploited, with the values of 0.47 and 0.60 year-1 for females and males, respectively. DISCUSSION: P. clarkii spawned from September to November while two recruitments were observed yearly. We inferred that some eggs, prevented from hatching by low water temperature in winter, were more likely to hatch in the next spring. Moreover, the fishing mortality rate was relatively high for males, which might be related to the males-directed selection during the reproductive period. The higher values of exploitation rate in our study confirmed that males P. clarkii were overexploited and were under high fishing pressure. We thus suggest reducing fishing intensity on immature crayfish and avoid sex selection during the reproductive period to improve the overall sustainability of commercial P. clarkii populations.

Age, growth, mortality and recruitment of thin sharpbelly Toxabramis swinhonis Günther, 1873 in three shallow lakes along the middle and lower reaches of the Yangtze River basin, China.

Despite being the most dominant and widespread small fish species in the lakes along the middle and lower reaches of the Yangtze River basin, Toxabramis swinhonis has been paid little attention by fisheries scientists and little is known about its population characteristics. For this reason, we estimated age, growth, mortality and recruitment of this species based on three shallow lakes, Biandantang Lake, Shengjin Lake and Kuilei Lake (BDT, SJH and KLH, respectively) in this region. A total of 13,585 (8,818 in BDT, 2,207 in SJH and 2,560 in KLH) individuals were collected during monthly sampling from July 2016 to September 2017. The results revealed that the age structures of T. swinhonis consisted of four age groups (0+-3+), with 0+-1+ year old fish comprising more than 98% of the samples. Allometric growth patterns were displayed by fish from all sampling sites and the von Bertalanffy growth functions estimated were L t = 173.25 (1 - e-1.20 (t + 1.09)): BDT; L t = 162.75 (1 - e-1.20 (t + 1.08)): SJH and L t = 215.25 (1 - e-1.20 (t + 1.12)): KLH, respectively. The rates of total mortality (Z), natural mortality (M) and fishing mortality (F) at BDT, SJH and KLH were computed as 5.82, 5.50 and 4.55 year-1; 1.89, 1.87 and 1.75 year-1; 3.93, 3.63 and 2.80 year-1, respectively. Meanwhile, growth performance indices (φ') were 0.68 (in BDT), 0.66 (in SJH) and 0.62 (in KLH), which indicated that T. swinhonis were overfished slightly in all study areas. Area-specific recruitment patterns were similar to each other, displaying evidence of batch spawning, with major peaks in April and August, accounting for 92.21% (BDT), 88.21% (SJH) and 88.73% (KLH) of total recruitment, respectively. These results showed that brief generation-time, fast growth rate, relatively high natural mortality rate and strong reproductive capacity (r-strategies) are reasons why this species became the most dominant species in many lakes of China.