中国水产科学  2023, Vol. 30 Issue (02): 236-246  DOI: 10.12264/JFSC2022-0361
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引用本文 

倪文静, 邓华堂, 何春, 蒲艳, 田辉伍, 刘绍平, 陈大庆, 段辛斌. 长江中上游3种鲌的营养生态位比较[J]. 中国水产科学, 2023, 30(2): 236-246. DOI: 10.12264/JFSC2022-0361.
NI Wenjing, DENG Huatang, HE Chun, PU Yan, TIAN Huiwu, LIU Shaoping, CHEN Daqing, DUAN Xinbin. Trophic niche comparison of three species of Culter from the middle and upper reaches of the Yangtze River[J]. Journal of Fishery Sciences of China, 2023, 30(2): 236-246. DOI: 10.12264/JFSC2022-0361.

基金项目

国家自然科学基金项目(51909271);国家重点研发计划项目(2018YFD0900903);重庆市水产科技创新联盟项目(2021183);中国水产科学研究院创新团队项目(2020TD09);中国水产科学研究院基本科研业务费项目资助(2021CG02).

作者简介

倪文静(1998‒),女,硕士研究生,研究方向为渔业资源. E-mail:niwenjing0611@163.com

通信作者

通信作者:段辛斌,研究员,研究方向为渔业资源. E-mail:duan@yfi.ac.cn

文章历史

收稿日期:2022-10-14
修改日期:2022-11-18
长江中上游3种鲌的营养生态位比较
倪文静1,2,邓华堂2,何春3,蒲艳2,4,田辉伍2,刘绍平2,陈大庆2,段辛斌2,     
1. 上海海洋大学水产与生命学院,上海 201306
2. 中国水产科学研究院长江水产研究所,湖北 武汉 430223
3. 西华师范大学生命科学学院,四川 南充 637002
4. 西南大学生命科学学院,淡水鱼类资源与生殖发育教育部重点实验室,重庆 400715
摘要:为了解长江中上游鲌属鱼类种群动态和种间营养关系,本研究利用碳、氮稳定同位素分析技术,对来自长江中上游3个区域的达氏鲌(Culter dabryi)、蒙古鲌(Culter mongolicus)和翘嘴鲌(Culter alburnus)样品的碳、氮稳定同位素特征和营养生态位进行比较分析。结果显示,3种鲌的δ13C值在各区域均有显著性差异(P<0.05),而δ15N值仅在上游江段差异显著(P<0.05),表明三者间食物来源和营养级位置存在空间上的差异。多营养生态位指标结果显示,在长江中上游,蒙古鲌的食物来源更为广泛,占据的营养生态位空间总量较大,且在大多数情况下,蒙古鲌的生态位宽度大于达氏鲌和翘嘴鲌,这可能与蒙古鲌独特的摄食器官有关。3种鲌在长江中上游各江段的食性竞争存在差异,其中,蒙古鲌与翘嘴鲌、达氏鲌的重叠度相对较高,而翘嘴鲌与达氏鲌之间重叠度较低甚至无重叠,表明在长江中上游水域,蒙古鲌由于对不同饵料资源的利用能力强而易与其他两者出现食性竞争,而翘嘴鲌与达氏鲌因对各自优势饵料的摄食偏好性强,在食物资源维度上出现了一定的生态位分化。
关键词营养生态位        长江中上游    稳定同位素分析    凶猛性鱼类    
Trophic niche comparison of three species of Culter from the middle and upper reaches of the Yangtze River
NI Wenjing1,2,DENG Huatang2,HE Chun3,PU Yan2,4,TIAN Huiwu2,LIU Shaoping2,CHEN Daqing2,DUAN Xinbin,2    
1. College of Fisheries and Life Science, Shanghai Ocean University, Shanghai 201306, China
2. Yangtze River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Wuhan 430223, China
3. College of Life Science, China West Normal University, Nanchong 637002, China
4. Key Laboratory of Freshwater Fish Reproduction and Development, Ministry of Education; School of Life Sciences, Southwest University, Chongqing 400715, China
Abstract: Culter species, as carnivorous fishes in lakes and reservoirs in the middle and upper reaches of the Yangtze River, usually play the top predator role in the freshwater ecosystem. Culter dabryi, Culter mongolicus, and Culter alburnus are the most common and abundant Culter species in these regions. Because of sympatry and predatory habits, these Culter species seemingly have interspecific resources competition, which could be fierce especially when food resources are limited. In addition, the changes in their population could have an important impact on other fish species or communities through top-down effects. To understand the interspecific trophic relationships of Culter in the middle and upper reaches of the Yangtze River, carbon and nitrogen stable isotope analysis technology and isotope-derived metrics were used to examine stable isotope characteristics and trophic interrelations of these three Culter species samples from the upper reaches, reservoir reaches, and the middle reaches. A one-way analysis of variance (ANOVA) was used to test whether there were significant differences in δ13C and δ15N values among the three Culter species in each river section. Six quantitative metrics in the bi-plot space, including δ13C range (CR), δ15N range (NR), mean distance to centroid (CD), mean nearest neighbor distance (MNND), standard deviation of the nearest neighbor distance (SDNND), and total area (TA) derived from stable isotope data, were used to describe trophic niche space, trophic diversity, and redundancy. Instead of total area (TA), which is highly sensitive to sample size, standard ellipse area (SEA) was also used to quantify the trophic niche space of each species. Besides, to provide more robust metrics and allow for statistical comparisons among sites and species, Bayesian methodology was implemented to generate a posterior distribution of estimates of these metrics. The results showed that δ13C values of the three Culter species differed significantly (P<0.05) in each region, while the significant disparity of δ15N values only appeared in the upper reaches of the river (P<0.05), indicating that there were spatial differences in the food sources and trophic levels among the three species. The results of multi-trophic ecological niche indicators showed that C. mongolicus had the highest SEAB (Bayesian standard ellipse area) values in all river sections, and the probability of its SEAB being greater than the SEAB values of the other two Culter species ranged from 74.7% to 96.4%. Additionally, the Bayesian results of the CR, CD, and MNND indicators were also highest in the middle and upper reaches of the Yangtze River, indicating that C. mongolicus has a wider food source and occupies a larger trophic niche space. In most cases, the niche width of C. mongolicus is larger than those of C. dabryi and C. alburnus, which may be related to the fact that C. mongolicus has a wide mouth end position and a sparse gill rake that facilitates food passage. The results of stable isotope standard ellipse area overlap showed that there were spatial differences in the niche space overlap among three Culter species. Among them, the highest overlap of stable isotope ellipsoidal area (0.522) was found between C. mongolicus and C. alburnus in the midstream, which accounted for 62.4% and 76.3% of the total SEA of each species, respectively. The SEA overlap between C. dabryi and C. alburnus was lower, only about 0.1 in the upper or middle reaches and no overlap was observed in the reservoir area. Generally, the overlaps between C. mongolicus and C. alburnus or C. dabryi were higher than other pairwise comparisons. These results indicated that C. mongolicus are more liable to have feeding competition with the other two Culter species in the middle and upper reaches of the Yangtze River. Nevertheless, there appeared a certain niche differentiation of food resources between C. alburnus and C. dabryi due to their preference for feeding on their dominant food.
Key wordstrophic niche     Culter     middle and upper reaches of the Yangtze River    stable isotope analysis    carnivorous fish    

营养生态位一般是指生物能对其饵料资源实际和潜在占据与利用的部分,研究营养生态位有助于理解生态系统中物种间的营养相互作用及资源利用状况[1-2]。胃肠含物分析和稳定同位素技术是鱼类营养生态位研究中用于食性分析的两个主要手段。传统的胃肠含物分析法通常只能获得鱼类短时间内的食物信息,易忽视已被鱼类吸收的食物的情况;且对肠含物分析鉴定工作较为繁琐,难以正确辨认鱼类已经消化的食物。而稳定同位素技术能反映生物一定时间(数月到数年)内摄取并吸收的食物同位素组成,能有效弥补胃肠含物分析的不足[3-5]。此外,使用稳定同位素技术可以通过运用相关模型将同位素比值转换成不同同位素来源的摄食比例[6],这使得稳定同位素技术在营养生态位研究中得到了广泛应用[7-8]

翘嘴鲌(Culter alburnus)、蒙古鲌(Culter mongolicus)和达氏鲌(Culter dabryi)均隶属于鲤科(Cyprinidae)、鲌亚科(Cultrinae)、鲌属(Culter Basilewsky),是我国最常见的鲌属鱼类,广泛分布于除青藏高原和新疆外的各大江河、湖泊和水库[9]。在很多湖泊和大型水库中,鲌类的渔获量仅次于鲢(Hypophthalmichthys molitrix)、鳙(Aristichthys nobilis),是我国重要的经济鱼类[10],同时也是部分国家种质资源保护区的重点保护对象[11-12]。然而在长江全面禁渔之前,由于过度捕捞、栖息地环境变化等因素的影响,天然鲌类种群呈现低龄化和小型化趋势[13]。3种鲌同为淡水中上层捕食者,通常混栖,且食性近似,故三者间可能存在营养资源竞争的现象[3]。尤其在饵料生物资源有限时,3种鲌之间的竞争会更为激烈,进而发生种群变动,导致某种鲌类资源量下降[11]。另一方面,高营养级生物主导作用的下行效应对生态系统的动态平衡具有重要的调节作用[14],鲌类作为长江中上游湖泊和水库中较为常见的肉食性鱼类[15],在淡水生态系统中扮演着顶级消费者的角色,其种群变动对于鱼类及其他生物群落都具有重要影响[16]。在十年禁渔等长江大保护政策实施使得渔业资源逐步恢复的情况下,鲌类等顶级捕食者对禁捕水域鱼类群落结构组成及调控作用将越发凸显,因此对不同顶级捕食者间营养关系的研究显得十分必要,能够为水生生态系统结构与功能及演替趋势预测奠定理论基础。

目前,对鲌类的研究主要集中在生长特性[17-18]、营养学[19-20]、遗传学[21-22]等方面,有关摄食生态学的研究较少且研究区域大多局限在湖泊[11,14],对于长江中上游鲌属鱼类的营养生态位研究尤其是种间比较研究鲜见报道。因此本研究运用稳定同位素技术研究3种鲌之间的营养生态位关系,有助于了解长江中上游鲌属鱼类种群动态和种间营养关系、饵料资源利用状况,同时也为研究其他生态位相近的凶猛性鱼类的摄食生态提供参考,亦能为长江水生态系统鱼类群落结构及营养关系变动预测奠定基础。

1 材料与方法 1.1 研究区域

本研究所涉及的鱼类样品采集于2021年5—7月、11—12月,采样点分布于长江上游保护区江津流水江段(S1),库区丰都等静水区(S2)及长江中游干流监利江段(S3)。采样点地理位置详见图1

图1  采样点示意图 Fig. 1  The schematic diagram of sampling sites
1.2 样品采集与处理

本研究鱼类样品主要使用3层复合刺网以及地笼进行捕捞。共采集鲌类样品239尾,对于捕捞上来的鱼类进行种类鉴定和生物学测量,记录体长(mm)和体重(g)。取其背部白色肌肉2~3 g,用去离子水冲洗后,在60 ℃下连续烘干至恒重,最后用研钵研磨至均匀粉末,并干燥保存,用于碳、氮稳定性同位素比值测定,采样信息详见表1。考虑到个体差异影响,预先采用Pearson相关性分析检验所有鲌类个体的δ13C和δ15N值与其体长和体重的相关关系。最终选取其中140尾鲌类作为研究对象,目标样本信息详见表1

表1  长江中上游3种鲌类样品采集信息 Tab. 1  Sample collection information of three culters in the middle and upper reaches of the Yangtze River
1.3 稳定同位素分析 1.3.1 稳定同位素测定

对每一个研磨保存的样品使用百万分之一精密天平称取0.35 mg粉末装入锡杯(5 mm×8 mm)中进行包埋,利用元素分析-稳定同位素质谱仪测定样品的碳、氮稳定同位素比值(使用的分析仪器为美国Thermo公司制造的Finnigan DELTA V Advantage同位素比率质谱仪和Flash 2000 EA-HT元素分析仪)。所有样品的稳定性同位素比值测定在江汉大学生命科学学院进行。碳、氮稳定性同位素分析分别采用国际标准物质PDB (Pee Dee Belnite)和标准大气氮对比。分析结果表示为δ13C和δ15N:

δX (‰)=(Rsample/Rstandard–1)×1000

式中,X13C或15N; Rsample为样品所测得的同位素比值,即13C/12C或15N/14N; Rstandard为标准物质(PDB与大气中N2)的同位素比值。测定精度δ13C<0.1‰, δ15N<0.15‰。

1.3.2 营养生态位的确定

参考Layman等[23]的方法构建由δ13C-δ15N值双坐标图组成的二维空间生态位,计算6个量化种群营养生态位的指标。使用δ15N值变幅(δ15N range, NR)代表不同鱼类的营养级跨度;δ13C值变幅(δ13C range, CR)表示不同鱼类的摄食范围;平均离心距离(mean distance to centroid, CD)用于度量营养多样性的平均程度;凸多边形总面积(total area, TA)代表物种占据的营养生态位空间的总量;以上4种参数用来反映种群营养结构多样性。使用平均最邻近距离(mean nearest neighbor distance, MNND)和最邻近距离标准差(standard deviation of nearest neighbor distance, SDNND)两个参数研究鲌鱼种群的营养冗余度,前者用于表征营养密度,后者用于衡量种群营养均匀度。以校正标准椭圆面积(corrected standard ellipse area, SEAC)及贝叶斯模型估算的标准椭圆面积(Bayesian estimates standard ellipse area, SEAB)作为营养生态位宽度的度量指标,由于TA值易受样品大小和δ13C、δ15N极端值影响,而SEAC是对核心同位素生态位面积的校正,SEAB是通过贝叶斯方法进行估计,考虑到采样误差和返回概率分布,其在样本大小较小时也校正了对SEA的低估偏差,因此本研究以SEAB及SEAC指标为主来衡量3种鲌的营养生态位宽度。定义个体间的SEAC的重叠度(overlap area, OA)作为衡量个体间营养生态位重叠的度量指标[24-25]。以上各值均在R语言集成开发环境R Studio中使用SIBER、SIAR等数据包计算。

1.4 数据分析与处理

使用Kolmogorov-Smirnov检验对样品碳、氮稳定同位素数据进行正态分布检验,若检验数据服从正态分布和方差齐性,则使用单因素方差分析法(one-way ANOVA)分别检验各江段3种鲌之间的δ13C、δ15N值是否存在显著性差异,设定差异显著性水平P为0.05,当P<0.05时为差异显著,P>0.05时差异不显著。以上数据处理与分析均在Microsoft Excel (2019)和SPSS (IBM SPSS Statistics 26)中进行。用于衡量生态位的各指标值的计算以及营养框架图的绘制均在R Studio (4.1.2)中使用SIBER、SIAR等数据包完成。

2 结果与分析 2.1 长江中上游3种鲌的稳定同位素特征及种间差异

Pearson相关性分析结果显示,本研究对象的体长、体重与其δ13C和5N值均不存在显著的相关关系(P>0.05)。基于以上结果,本研究的体长和体重差异对结果无影响。

表2 比较了各区域3种鲌的稳定同位素值,结果显示三者的δ13C值在各江段均有显著性差异存在(P<0.05),而δ15N值仅在上游江段有显著差异(P<0.05)。在上游江段,蒙古鲌的δ13C值显著高于达氏鲌和翘嘴鲌的δ13C值(P<0.05);达氏鲌的δ15N值显著高于蒙古鲌和翘嘴鲌的δ15N值(P<0.05)。在库区,蒙古鲌的δ13C值亦显著高于翘嘴鲌(P<0.05),但和达氏鲌的δ13C值无显著性差异(P>0.05);三者δ15N均值无显著差异(P> 0.05)。在中游江段,达氏鲌的δ13C值显著高于蒙古鲌和翘嘴鲌(P<0.05);然而三者δ15N均值无显著差异(P>0.05)。

表2  长江中上游3种鲌类基础信息及稳定同位素差异比较 Tab. 2  Comparison of basic information and stable isotopes of three culters in the middle and upper reaches of the Yangtze River $\bar{x}\pm \text{SD}$
2.2 长江中上游3种鲌的营养生态位差异 2.2.1 生态位空间

3种鲌的贝叶斯标准椭圆面积(SEAB)显示,在各江段蒙古鲌的SEAB值均最高(图2)。使用贝叶斯迭代方法比较3种鲌的贝叶斯标准椭圆大小,结果显示,蒙古鲌的SEAB值大于达氏鲌的概率在上游、库区及中游分别是83.5%、91.6%、93.4%,蒙古鲌的SEAB值大于翘嘴鲌的概率在上游、库区及中游分别为90.6%、96.4%、74.7%,而达氏鲌与翘嘴鲌的SEAB值在库区相差不大(50.8%)(表3, 图2)。

稳定同位素标准椭圆面积重叠度结果显示,在各研究区域,3种鲌的生态位总空间(TA)和核心生态位(SEAC)均有重叠,但重叠度存在空间差异。蒙古鲌与达氏鲌的稳定同位素椭圆面积在库区重叠度最高(0.321),重叠面积分别占达氏鲌和蒙古鲌SEAC的78.5%和35.3%。而蒙古鲌与翘嘴鲌的核心营养生态位重叠度在中游最高(0.522),重叠面积分别占蒙古鲌和翘嘴鲌SEAC的62.4%和76.3%。达氏鲌与翘嘴鲌的核心生态位重叠度较低,在上游和中游仅为0.1左右,库区则无重叠(0.000)(表3, 图3)。

图2  长江中上游3种鲌的贝叶斯标准椭圆面积(SEAB)黑点代表SEAB的众数值(mode),红叉是小样本量修正的SEA (SEAC),方框表示50%、75%和95%置信区间. CuD:达氏鲌;CuM:蒙古鲌;CuA:翘嘴鲌. Fig. 2  Bayesian standard elliptical area (SEAB) of three culters in the middle and upper reaches of the Yangtze RiverThe black dot represents mode values of SEAB, the red cross is the small sample size modified SEA (SEAC), and the box represents the 50%, 75%, and 95% confidence intervals. CuD: Culter dabryi; CuM: Culter mongolicus; CuA: Culter alburnus.
表3  长江中上游3种鲌的SEAC重叠度(OA)及贝叶斯标准椭圆面积(SEAB)比较 Tab. 3  Comparison of SEAC overlap area (OA) and Bayesian standard ellipse area (SEAB) of three culters in the middle and upper reaches of the Yangtze River
2.2.2 营养多样性

3种鲌营养生态位指标的比较结果随区域变化而变化,在上游江段,蒙古鲌的摄食范围(CR)、营养生态位总面积(TA)以及种群营养生态位分布范围(SDNND)参数指标均高于达氏鲌和翘嘴鲌,但达氏鲌的营养跨度(NR)和营养多样性(CD)指标值较高。在库区江段,除表征营养密度(MNND)的指标外,蒙古鲌的各种营养生态位指标值均为最大。在中游江段,除NR和TA指标外,蒙古鲌的各项营养生态位度量指标值也较另两种鲌更高(表4)。

基于以上3种鲌的营养生态位指标估算值在各江段的比较结果,本研究以不同区域的同一物种作为一个整体进行5种营养生态位指标的贝叶斯模型估算,比较分析3种鲌之间的营养结构差异。贝叶斯结果显示,3种鲌中,达氏鲌的营养跨度最大(NR),翘嘴鲌次之,蒙古鲌最低。与分区域的结果类似,蒙古鲌在长江中上游的CR、CD以及MNND指标的贝叶斯结果均最高(表4, 图4)。

图3  基于碳氮稳定同位素的3种鲌营养框架图a. 上游;b. 库区;c. 中游. CuD:达氏鲌;CuM:蒙古鲌;CuA:翘嘴鲌. Fig. 3  Trophic framework of three culters based on stable carbon and nitrogen isotopesa. The upstream; b. The reservoir; c. The midstream. CuD: Culter dabryi; CuM: Culter mongolicus; CuA: Culter alburnus.
表4  长江中上游3种鲌的营养生态位指标 Tab. 4  Trophic niche indexes of three culters in the middle and upper reaches of the Yangtze River
图4  3种鲌营养生态位指标的贝叶斯结果NR: δ15N值变幅;CR: δ13C值变幅;CD:平均离心距离;MNND:平均最邻近距离;SDNND:最邻近距离标准差;CuD:达氏鲌;CuM:蒙古鲌;CuA:翘嘴鲌. 黑点为模式值,方框表示50%、75%和95%置信区间. Fig. 4  Bayesian results of nutritional niche indexes of three cultersNR: δ15N range; CR: δ13C range; CD: mean distance to centroid; MNND: mean nearest neighbor distance; SDNND: standard deviation of nearest neighbor distance; CuD: Culter dabryi; CuM: Culter mongolicus; CuA: Culter alburnus. The black dot is the mode value, and the box represents 50%, 75% and 95% confidence intervals.
3 讨论 3.1 3种鲌的碳氮稳定同位素特征

鲌属鱼类属于肉食性鱼类,在生长发育过程中涉及到食性转化过程[15]。根据李斌等[26]、林明利等[27]的研究结果显示,当蒙古鲌体长大于200 mm,其食性已能转化为肉食性。本研究试验样本经筛选后,目标样本体长均值都大于 200 mm,且3种鲌的体长和体重与其δ13C和δ15N值均无显著相关关系,因此本研究3种鲌的碳氮稳定同位素特征不受体长体重影响。

由于生物的碳同位素与其食物相近,故食物检测大多采用碳同位素进行,δ13C值则用于指示生物的摄食来源[28]。而氮同位素会随着能量流动在食物链中逐步富集,故δ15N值常被用于确定生物的营养级位置[29-30]。本研究中3种鲌的δ13C值在各区域均有显著性差异(P<0.05),表明3种鲌摄食的饵料来源或食物组成不同。同时,使用传统的肠含物分析法得到的鲌类在各区域摄食的饵料差异也印证了上述分析结果(未发表数据),即鲌类在长江中上游的饵料生物共有5类(包括鱼类、虾类、浮游动物、水生昆虫、螺类),虽然3种鱼饵料种类存在相似性,但其所占比例不同,从而造成三者的δ13C值具有差异,与宋业晖等[31]研究3种肉食性的石首鱼生态位的结果类似,3种鲌类之间可以通过改变摄食食物的比例来产生营养生态位的交错,缓和对有限饵料资源的竞争,实现共存。3种鲌的δ15N值仅在上游江段存在显著性差异(P<0.05),这可能与三者在上游江段饵料生物的组成差异较大有关,三者的饵料差异在上游江段体现得更明显。3种鲌的δ15N值在库区和中游差异均不显著(P>0.05),表明三者所处营养级位置相差不大,这可能是因为3种鲌的饵料种类相似,且饵料生物营养级相近导致,该结果与邓华堂等[32]、何春等[33]在该区域的调查研究结果一致。

3.2 营养生态位

Layman等[23]提出的6个量化稳定同位素的指标,从不同角度度量种群营养生态位的大小。种群营养结构的指标NR、CR、CD、TA常用来表征种群营养结构的多样性;MNND、SDNND用于量化种群营养冗余程度。本研究中3种鲌的δ13C值变幅(CR)和平均离心距离(CD)指标的贝叶斯结果均显示,无论是在各个区域或整体考虑,蒙古鲌的摄食范围都最广泛,营养多样性最高。鱼类的摄食选择除了受环境中饵料生物本身的丰度及适口性影响,更与消费者摄食器官的偏好密切相关。从觅食策略看,凶猛性鱼类更偏向于摄食大小适合其口裂及消化器官结构特点的种类[34]。相关研究结果显示[11,14,16], 3种鲌的摄食器官形态存在差异,翘嘴鲌口上位,达氏鲌口亚上位,二者口裂均较小,鳃耙长且多,口咽腔无齿等辅助摄食器官;而蒙古鲌口端位,口裂宽大且鳃耙稀疏更利于食物通过,因此蒙古鲌的饵料选择范围较其他两种鲌更为广泛。3种鲌的δ15N值变幅(NR)在不同区域不同,并无明显的规律性,可能是由于不同区域河段的生物和非生物因子差异较大导致。据沙永翠等[2]的研究结果显示,水环境因子的波动虽不会直接影响高营养级鱼类的营养生态位,但会通过改变水体中饵料资源的丰度与分布而影响鱼类的食物组成。由于不同区域内3种鲌摄食饵料的种类或比例不同,使得它们富集15N的途径产生差异,15N富集在不同途径中的占比不同最终导致了三者δ15N值变幅在空间上产生了差异。但总体上看,蒙古鲌的δ15N值变幅(NR)最小,说明摄食范围广泛并未影响蒙古鲌营养级位置结构的相对稳定性。当种群中的个体营养特征相似时,MNND值越大,种群的营养冗余程度越低[8]。本研究中,除库区外,蒙古鲌在各区域的群体营养聚集密度(MNND)均为最高,表明蒙古鲌虽摄食广泛,但营养冗余度较少,饵料资源利用较充分。

生态位宽度代表生物对栖息地和资源利用及竞争能力的强弱[35-36],在资源供应不足时,物种的生态位宽度越大,竞争能力越强[37-39]。本研究中,3种鲌的贝叶斯标准椭圆面积比较结果显示,在各江段,蒙古鲌的稳定同位素生态位宽度均最大,表明其对饵料资源的利用能力最强,食物来源更为多样广泛。通过比较3种鲌SEAB值大小概率,结果表明在长江中上游蒙古鲌的生态位宽度在多数情况下是高于达氏鲌和翘嘴鲌的,在资源有限时,这可能会增加蒙古鲌的种间竞争能力。生态位的重叠代表了“两个物种在同一资源位上的相遇频率”,也可以用来反映物种间的潜在竞争关系[40]。根据Schoener食性重叠指数[41]及Jackson等[24]的研究结果,在生态学意义上,重叠度大于0.6代表重叠显著。本研究中SEA分析结果显示,3种鲌在各研究区域的重叠度均小于0.6,表明三者的生态位出现一定程度分化。其中,达氏鲌和翘嘴鲌的生态位分化较明显,尤其是在库区,二者生态位重叠度为零。由于达氏鲌的饵料来源较少(CR值低),翘嘴鲌也因其特殊的口位,饵料选择范围偏向于中上层,而长江中上游尤其是库区生境多样饵料资源丰富,水流较缓,较适合鲌类的索饵和生长[42],在这种饵料相对充裕的条件下,鲌类易出现“专食者”特性[43],摄食偏好性变强,这也体现在翘嘴鲌和达氏鲌的δ13C值在大部分区域内差异显著,因此在食物资源维度上产生生态位分化的现象。由于蒙古鲌摄食范围较广,易与另两种鲌发生食物竞争,因此蒙古鲌与二者的生态位重叠度相对较高,其中,蒙古鲌与翘嘴鲌的核心营养生态位面积重叠度在中游最高(0.522),与达氏鲌的重叠度在库区最高(0.321),而3种鲌的δ13C值差异性比较结果显示,蒙古鲌的δ13C值与翘嘴鲌和达氏鲌的δ13C值分别在中游和库区无显著性差异(P>0.05),这说明在库区和中游蒙古鲌与它们摄食了相似的饵料并产生了一定的竞争,但总体上长江中上游的3种鲌摄食竞争并不明显。

本研究结果表明,在长江中上游,蒙古鲌的摄食范围最广,营养多样性较高,但营养级位置相对稳定,易于与翘嘴鲌和达氏鲌在中游、库区江段发生食物竞争。而达氏鲌与翘嘴鲌的生态位重叠度相对较小甚至无重叠,尤其是在库区出现了较明显的生态位分化。3种鲌类在长江中上游不同区域通过不同程度的生态位分化来缓解对有限资源的竞争,但鱼类的种间竞争不仅取决于营养生态位重叠程度,也与栖息地环境、饵料生物的丰度及捕食者的生物量等因素息息相关。因此,有关鱼类种间食物关系的影响因素还有待进一步研究。

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