通江湖泊石臼湖鱼类群落结构现状及其季节变化

石臼湖是长江下游唯一的自然通江湖泊, 为掌握其鱼类资源状况, 本研究于 2017 年 1 月至 2018 年 3 月采用多目刺网和定制串联笼壶采集石臼湖渔获物, 分析了石臼湖鱼类群落结构及其季节变化。结果显示, 共采集到鱼类 36 种, 隶属于 6 目 7 科 28 属, 其中鲤科鱼类最多, 占总物种数的 69.44%; 鱼类物种组成以定居性、杂食性、中上层鱼类为主, 小型鱼类达 25 种, 占总物种数的 69.44%。似鱎(Toxabramis swinhonis)、刀鲚(Coilia nasus)和鲫 (Carassius auratus)为石臼湖鱼类全年优势种; 除全年优势种外, 季节特有优势种有春季的麦穗鱼(Pseudorasbora parva)、夏季的翘嘴鲌(Culter alburnus)和?(Hemiculter leucisculus)、秋季的似鳊(Pseudobrama simoni)、冬季的贝氏?(Hemiculter bleekeri)。鱼类相对密度、相对生物量有较大的季节变化, 但其变化趋势一致, 均按照冬季、春季、 秋季、夏季由高到低排列。Margalef 丰富度指数 R 的变动范围为 2.28~3.17, 基于个体数量的 Shannon-Wiener 多样性指数 H_N′ 、Pielou 均匀度指数 J_N′ 变动范围分别为 1.24~2.14、0.14~0.34, 基于生物量的 Shannon-Wiener 多样性指数 H_W′ 、Pielou 均匀度指数 J_W′ 变动范围分别为 1.64~2.51、0.21~0.49, 各指数除 H_W′ 在夏秋季间差异不显著外, 其余指数夏季都显著高于其他季节(P<0.05)。研究结果表明, 石臼湖鱼类种类较少, 多样性偏低。本研究补充了通江湖泊鱼类群落研究的基础资料, 对长江中下游鱼类多样性保护具有一定的意义。     

Seasonal migration and fine‐scale movement of invasive round goby (Neogobius melanostomus) in a Great Lakes tributary

To determine whether invasive round goby (Neogobius melanostomus) from Lake Ontario were establishing a year‐round population in a tributary stream or migrating to the lake, we assessed population and individual movement patterns using mark–recapture assessment generated from weekly backpack electrofishing from May until November 2016. Round goby abundance was low in spring, peaked in summer and decreased again in autumn, suggesting seasonal inward stream migration and outward migration back to the lake. Adult round goby movement patterns were positively associated with changes in water temperature, but this was not the case for juveniles. Juveniles displayed a preference for shallow, upstream habitats. Observations of reproductive individuals coupled with a peak in juvenile abundance following the peak in adult abundance indicate that the tributary was used for reproduction and recruitment. The individual movement was primarily upstream in spring, and there was little net movement in summer, likely during reproduction. Downstream movement occurred in autumn over a short time period, suggesting rapid outmigration to the lake. The combined observations of seasonal population structure and individual movement suggest that tributary streams connected to large, infested waterbodies can be used for round goby reproduction and recruitment, rather than year‐round residence. This study provides evidence of round goby seasonal migration and their individual movement patterns within tributary streams, which complements an earlier study in Lake Erie tributaries and may be a common occurrence in other Great Lakes tributaries.     

Mitigating the impacts of stream and lake regulation in the flathead river catchment,Montana, USA: An ecosystem perspective

1. Seventy-two per cent of the Flathead River catchment (22 241 km²) is federally designated and protected as wilderness or national park. Thus, the catchment remains one of the more pristine areas of its size in the temperate latitudes of the world. 2. Discharge in the downstream reaches of the river system outside the protected areas is regulated by three dams for flood control and hydropower production. These dams have blocked natural migration of native fish from Flathead Lake (496 km²) and isolated populations in sub-catchments. Temperature and erratic flow fluctuations have altered phenologies of river zoobenthos and fish, and in dam tailwaters aquatic biodiversity is drastically reduced in comparison to unregulated segments. 3. Ecological problems caused by changing water quality conditions, altered land-use patterns and introductions of non-native biota are interactive with the impacts of stream and lake level regulation, thereby emphasizing the complexity of this river–lake ecosystem. 4. Mitigation of the effects of regulation is compromised by differing management priorities and regulatory mandates of County, State, Tribal, and Federal agencies responsible for natural resource management within the catchment. Moreover, economic and ecological interests outside the Flathead influence the way flows are regulated within the catchment. 5. The most pervasive influences of stream and lake regulation can be ameliorated by retrofitting the hypolimnial release dam with a selective depth outlet structure to allow temperature control, and by controlling changes in flow rates to create a more natural hydrograph in the tailwaters of the large dams. Allowing fish passage by construction of fish ladders is problematic because upstream passage will commingle native species that were isolated upstream by construction of the dams with non-native species that were introduced subsequently below the dams. Cascading food web interactions elicited by invasions of non-native biota may offset any advantage to native stocks gained by passage and/or augmentation with hatchery stocks. 6. Mitigation must be adaptive in the sense that unanticipated effects and interactions with other management objectives can be documented and alternative action can be implemented. 7. This case history of the effects of stream and lake level regulation, and the approaches to management reviewed in this paper, should serve as a lesson in river conservation.     

Resource use of native and stocked brown trout Salmo trutta L., in a subarctic lake

Habitat use, food composition and growth of stocked and native brown trout, Salmo trutta L., were studied in the subarctic Lake Muddusjärvi in northern Finland. Stocked brown trout and native brown trout preferred littoral and pelagic areas. Trout were stocked in October. In June stocked trout fed primarily on invertebrates while native fish were piscivorous. From July onwards the composition of the diet of both stocked and native trout was similar and consisted almost entirely of small‐sized whitefish. Brown trout were already piscivorous at a length of about 20 cm. The mean length of prey consumed was about 12 cm. Mean length‐at‐age was similar from the second year in the lake despite of the larger size of stocked fish during the first year in the lake.     

The relationship between the snowmelt flood and the establishment of non‐native brown trout (Salmo trutta) in streams of the Chitose River,Hokkaido, northern Japan

Flow regime is one of the major determinants of establishment success for non‐native aquatic organisms. Here, we examine the influence of flow variability associated with snowmelt flood on the establishment success of non‐native brown trout in 10 streams in northern Japan. We regarded the presence of Age‐0 brown trout as the index of the successful establishment. The emergence of Age‐0 brown trout in our study region begins in May, a time that overlaps with the occurrence of snowmelt flood. The presence of Age‐0 brown trout was negatively associated with flow variability, and it was also negatively associated with summer water temperature. Our results indicate that the non‐native brown trout tends to establish in the streams with smaller snowmelt floods and lower summer water temperatures. Brown trout is an invasive, non‐native species that is problematic all over the world, and effective management strategies for preventing their further expansion are urgently needed. This study suggests that river managers should recognise that stable streams such as spring‐fed streams (i.e., low flow and summer water temperature) and flow‐regulated streams, have a higher potential risk of brown trout invasion.     

Seasonal spatial distribution of juvenile lake sturgeon in Muskegon Lake,Michigan, USA

We examined seasonal spatial distribution and diel movements of juvenile lake sturgeon Acipenser fulvescens in Muskegon Lake, Michigan (a protected, drowned river mouth lake that links the Muskegon River to Lake Michigan). We surgically implanted ultrasonic tags in 20 juveniles (age 1–7) captured in gill nets to track their locations during August–December 2008/2009 and September 2010–October 2011. Most juveniles were observed ≤1.5 km from the mouth of the Muskegon River in Muskegon Lake at a mean depth of 7.5 m (SE = 1.3 m) during summer. In fall, juveniles moved away from the river mouth to the deepest part of Muskegon Lake and were observed at a mean depth of 15.8 m (SE = 1.3 m) during winter. The shift in spatial distribution coincided with fall turnover (i.e., loss of thermal stratification) and with changes in dissolved oxygen (DO) concentrations in the hypolimnion. During summer, DO concentrations in the hypolimnion were typically <4 mg·l^?1 in the deepest part of Muskegon Lake and DO concentrations at locations of tagged lake sturgeon were >7 mg·l^?1 in 94% of instances. Tracking in 2009 revealed no significant change in depth distribution or movement over the diel cycle. We only observed two tagged juveniles immigrating to Lake Michigan, suggesting that juveniles use Muskegon Lake for multiple years. Our results suggest that: (i) Muskegon Lake serves as an important nursery habitat for juvenile lake sturgeon that hatched in the Muskegon River before they enter Lake Michigan and (ii) seasonal changes in DO concentration in the hypolimnion likely affect the spatial distribution of juveniles in Muskegon Lake.     

Diel and seasonal variations in brown trout (Salmo trutta) feeding patterns and relationship with invertebrate drift under natural and hydropeaking conditions in a mountain stream

The abundance of prey ingested by brown trout (Salmo trutta) were compared with the abundance of drifting invertebrates in a mountain stream managed by hydropeaking, upstream (site A) and downstream (site B) of a hydroelectric power plant. During power generation, flow and temperature were the two main environmental factors modified. The natural flow in the river below the outlet may be enhanced several times a day from 1 to 11 m³.s^–1 in summer and winter, and from 5 to 15 m³.s^–1 during spring spates. During hydropeaking, the water was cooled by an average of 6 °C in summer and warmed by an average of 2 °C in winter. Overall drift density was higher at site B than at site A. There was a clear diel pattern of drift at site A, with low drift density during the day and high drift density at night, whereas no clear diel pattern was observed at site B. Below the plant, at site B, drift pattern appeared to be influenced most by hydropeaking. The flushing action of peaking flows caused a catastrophic drift, which was highest in autumn when the difference between natural and peak flows was greatest. Juvenile trout were adversely affected by hydropeaking conditions and subsequently their density and biomass were reduced by 30% from site A to site B, whereas no significant difference was noticed for adults. Gut contents analysis showed that brown trout chiefly fed on the most available prey items at both sites. Fish did not seem to feed in response to diel drift patterns above the plant, whereas they chiefly used drift pulses generated by peaking flows below the outlet. Under natural conditions, fullness indices increased from autumn to summer, suggesting they may be related to prey availability and changes in water temperature. In the regulated section, fullness indices were the lowest in spring, i.e. the season when peak flows added to snowmelt floods, suggesting a prominent role of high current velocities through habitat suitability, position maintenance, and ability to capture preys. Although hydropeaking is known to disturb trout population dynamics in this and other rivers, this kind of river regulation (natural discharge except during periods of power generation, and intermittent hydropeaking from a separate reservoir) allowed the maintain of brown trout below the outlet, probably because the river returned to natural conditions when the plant was inoperative, and because daily artificial fluctuations in flow and temperature remained within the limits of natural seasonal variations.     

网箱养殖对千岛湖水环境的影响

通过对千岛湖网箱养殖区内外水域的四季水质调查,分析得出网箱养殖对千岛湖局部水域会产生明显的影响。造成养殖区水域的DO、pH下降,TN升高,不同季节和不同的水层影响存在差别。冬季对浅层的DO、pH产生明显下降,影响区域小;春季对表层的DO、pH产生明显下降,影响区域大;夏季受水温分层,影响不明显;秋季造成各层水体DO、pH下降,TN上升,影响区域大。千岛湖网箱养殖在春季和秋季对养殖区局部水域影响明显,但分散的网箱养殖对水域影响范围较小,对千岛湖水体短期内影响不明显。     

Seasonal cycle of body composition and energy of brown trout (Salmo trutta) in a temperate zone lake

Abstract – Seasonal changes in protein and fat were investigated in brown trout ( Salmo trutta L.) of a stunted population from a small, temperate zone lake. The lake was ice-covered during winter (about 200 days), and hypolimnic water temperature during winter was about 4°C. During winter there was an increase in specific fat, in particular among the larger sized fish, while there was a general decrease in specific protein content among both small (<5 winters) and large (age 5 or older) trout. During winter (end of October to mid-May) an average trout gained 32 kJ of fat, but protein energy content decreased by 14 kJ, yielding a net energy increase of 18 kJ or a daily energy gain of 0.09 kJ  ·  day⁻¹. During the ice-free season (mid-May to the end of October) the trout increased mainly in protein content with daily energy gains of about 1.4 kJ  ·  day⁻¹, a value about 14 times higher than the corresponding winter value. Trout living in lakes may store considerable amounts of fat during wintertime in contrast to the depletion of fat reserves found among stream-living trout in the same area, which face lower water temperatures in the winter period. ^Note     

Movements by adult cutthroat trout in a lotic system: implications for watershed-scale management

Abstract  Movements by adult cutthroat trout, Oncorhynchus clarkii (Richardson), were assessed from autumn to summer in the Salt River watershed, Wyoming-Idaho, USA by radio telemetry. Adult cutthroat trout were captured during September and October 2005 in the main stem of the Salt River, surgically implanted with radio transmitters, and tracked through to August 2006. Adult cutthroat trout were relatively sedentary and resided primarily in pools from October to March, but their movement rates increased during April. Higher movement rates were observed among tagged fish during May and early June. Among 43 fish residing in the Salt River during April 2006, 44% remained in the river, 37% moved into mountain tributaries and 19% moved into spring streams during the spawning season. Fish did not use segments of mountain tributaries or the upstream Salt River where fish passage was blocked by anthropogenic barriers or the channel was dewatered during summer. Almost all the fish that moved into spring streams used spring streams where pools and gravel–cobble riffles had been constructed by landowners. The results suggest that adult Snake River cutthroat move widely during May and early June to use spawning habitat in mountain tributaries and improved spring streams. Maintaining the ability of adult fish to move into mountain streams with spawning habitat, preserving spawning habitat in accessible mountain tributaries and removing barriers to upstream movements, and re-establishing summer stream flows in mountain tributaries affected by dams appear to be habitat management alternatives to preserve the Snake River cutthroat trout fishery in the Salt River.     

Investigations into the temporal development of epitheliocystis infections in brown trout: a histological study

Epitheliocystis in Swiss brown trout (Salmo trutta) is a chlamydial infection, mainly caused by Candidatus Piscichlamydia salmonis and Candidatus Clavichlamydia salmonicola. To gain a better understanding of the temporal development of infections in wild brown trout, we investigated epitheliocystis infections during the course of the summer and autumn months of a single year (2015), and compared this to sampling points over the span of the years 2012–2014. The survey focused on tributaries (Venoge and Boiron) of the Rhone flowing in to Lake Geneva. When evaluated histologically, epitheliocystis infections were found throughout the period of investigation with the exception of the month of June. Fifty to 86 animals per sampling were investigated. Highest prevalence and infection intensities were seen in September. A correlation between epitheliocystis infection and water temperatures was not evident. Interyear comparison revealed consistent levels of prevalence and infection intensities in late summer. The absence of infections in June, combined with the consistent interyear results, indicates seasonal fluctuation of epitheliocystis infections in brown trout with a reservoir persisting during winter months from which infections can re‐initiate each year. This could either be at levels below detection limits within the brown trout population itself or in an alternative host.     

Seasonal movement patterns of Credit River brown trout (Salmo trutta)

Zimmer M, Schreer JF, Power M. Seasonal movement patterns of Credit River brown trout (Salmo trutta).
Ecology of Freshwater Fish 2010: 19: 290–299. © 2010 John Wiley & Sons A/S Abstract – Movement habits in riverine populations of brown trout vary among watersheds. Thus it is important to identify factors influencing differences in individual behaviour so as to improve the information resource base available for the design of river‐specific management strategies. Such information is particularly needed in the rapidly urbanising watersheds of eastern North America where relatively little is known about anthropogenic influences on brown trout populations. In this study, we examined the influence of water temperature on brown trout behaviour in the Credit River in south‐central Ontario, Canada with respect to seasonal movement patterns. Observed patterns of movement were also correlated with variations in river discharge and habitat quality. Forty‐three radio‐tagged, adult brown trout were tracked in a confined 39.8 km portion of the Credit River from 15 May 2002 to 28 July 2003. Fish were captured in three sections of the river that differed in distance downstream and habitat quality. Fish size had little impact on movement patterns. However, there was considerable variation in seasonal movement with upstream movements to summer positions, maintenance of summer positions, downstream and often extensive movements in fall. Also observed was maintenance of winter positions and repeated upstream movements in late spring‐early summer to previously used summer positions. The elaborate movement behaviour in the Credit River population was attributed to seasonal changes in thermal habitat quality. Fish tagged in less suitable thermal habitats moved significantly more than fish from more suitable thermal habitats.     

Factors influencing recruitment patterns of Golden Perch (Macquaria ambigua oriens) within a hydrologically variable and regulated Australian tropical river system

Abstract – The links between river flows, water temperature, river regulation and recruitment variability of Golden Perch, Macquaria ambigua oriens, were investigated from the Fitzroy Basin, Queensland. The dominant age classes determined by interpreting growth marks observed in otoliths were variable among rivers. Positive correlations between the age frequency and monthly river flow volumes were greatest in summer (December–March) conversely, autumn, winter and spring river flows correlated poorly for most rivers. Water temperature exhibits strong seasonality across the basin with mean monthly temperatures >23 °C generally occurring between October and April. These data indicate the combination of water temperature and increased river flows are important factors for recruitment. The degree of river regulation is also suspected of contributing to increased variability in dominant age classes within rivers. This assessment highlights the importance of timing of river flows in mitigating the negative effects of river regulation on golden perch populations.     

Habitat use,home range,movements and interactions of introduced Lepomis gibbosus and native Salmo trutta in a small stream of Southern England

Radio telemetry data were analysed to assess the microhabitat use, movement patterns, home range overlap and interspecific interactions of non‐native pumpkinseed Lepomis gibbosus (L.) and native brown trout Salmo trutta (L.) in a small English stream located immediately below a commercial angling lake from which pumpkinseed escaped. Although both species favoured pool habitats, brown trout preferred higher velocities and coarser substrata compared with pumpkinseed. Also, some individual brown trout preferred deeper waters than did pumpkinseed. Home range area of brown trout was substantially larger than that of pumpkinseed in spring and summer, and for both species, home range area in autumn was significantly smaller than in the other seasons. Range centre distribution analysis revealed that both species were distributed significantly nonrandomly within the stream during all seasons. There was substantial home range overlap between the two species in all seasons, the greatest being in spring. Overall distances moved were greatest during spring for both species, with brown trout moving greater distances relative to pumpkinseed. However, the absence of mutual attraction or avoidance between the species, as well as the lack of cohesion in preferred habitats and strong territorial fidelity, suggests little or no impact of introduced pumpkinseed on resident brown trout.     

Overwintering dormancy behaviour of the European eel (Anguilla anguilla L.) in a large lake

Overwintering dormancy behaviour was studied in female silver eels in Lake Mälaren in Sweden between 2008 and 2010. Depth choices and movements in relation to temperature were analysed from pressure and temperature records for 13 eels with implanted data storage tags, covering 17 overwintering periods and three intervening summer periods. Dormancy commenced in October–November as temperatures fell below 4–12 °C. Eels tended to remain motionless throughout the winter, with some short periods of activity signalled by small changes in depth distributions. During dormancy, the eel shows a clear avoidance of shallow areas <5 m in favour of the 10–25‐m‐depth interval. Activity tended to resume 4–6 months later in April–May as temperatures rose above 3–7 °C and ice cover broke, and eels spent more time at shallower depths of <5–10 m. The majority of the eels were assessed as being in the silver eel stage at the time of tagging. During the autumn months, the diving behaviour, with frequent and large vertical excursions and periods at the surface, was similar to that seen in migrating eels in the Baltic and Atlantic Ocean. In spring and summer, the behaviour differed, being dominated by more gradual depth variations, implying that the eels reverted from silver eel migration behaviour to yellow eel foraging behaviour. Body weight declined during dormancy, but other studies of starvation over comparable time periods showed significantly higher average specific weight losses, implying that the Mälaren silver eels must have fed between the end of dormancy and recapture.     

Lake trout growth is sensitive to spring temperature in southwest Alaska lakes

In high‐latitude lakes, air temperature is an important driver of ice cover thickness and duration, which in turn influence water temperature and primary production supporting lake consumers and predators. In lieu of multidecadal observational records necessary to assess the response of lakes to long‐term warming, we used otolith‐based growth records from a long‐lived resident lake fish, lake trout (Salvelinus namaycush), as a proxy for production. Lake trout were collected from seven deep, oligotrophic lakes in Lake Clark National Park and Preserve on in southwest Alaska that varied in the presence of marine‐derived nutrients (MDN) from anadromous sockeye salmon (Oncorhynchus nerka). Linear mixed‐effects models were used to partition variation in lake trout growth by age and calendar‐year and model comparisons tested for a mean increase in lake trout growth with sockeye salmon presence. Year effects from the best mixed‐effects model were subsequently compared to indices of temperature, lake ice, and regional indices of sockeye salmon escapement. A strong positive correlation between annual lake trout growth and temperature suggested that warmer springs, earlier lake ice break‐up, and a longer ice‐free growing season increase lake trout growth via previously identified bottom‐up increases in production with warming. Accounting for differences in the presence or annual escapement of sockeye salmon with available data did not improve model fit. Collectively with other studies, the results suggest that productivity of subarctic lakes has benefitted from warming spring temperatures and that temperature can synchronise otolith growth across lakes with and without sockeye salmon MDN.     

Lake trout (Salvelinus namaycush) otoliths indicate effects of climate and lake morphology on growth patterns in Arctic lakes

Climate change is occurring rapidly in the Arctic, and an improved understanding of the response of aquatic biota and ecosystems will be important for this data-limited region. Here, we applied biochronology techniques and mixed-effects modelling to assess relationships among growth increments found on lake trout (Salvelinus namaycush) otoliths (N = 49) captured from 13 lakes on the Arctic Coastal Plain of northern Alaska, observed and modelled climate patterns, and individual-level fish and lake characteristics. We found that annual growth varied by year, fish growth slowed significantly as individuals aged, and females grew faster than males. Lake trout had higher growth in flow-through lakes relative to lakes that were perennially or seasonally connected. Annual growth was positively correlated with observed air temperature measurements from a local weather station for the period 1998–2014, but no clear warming trend was evident for this period. Modelled August air temperatures from 1978–2014 predicted lake trout annual growth (root mean squared error = 0.045 mm) and indicated increasing temperatures and annual lake trout growth over the period 1950–2014. This study demonstrated that biochronology techniques can reconstruct recent climate patterns and provide a better understanding of trends in Arctic lake ecosystems under a changing climate.     

太湖不同区域浮游植物群落结构特征及其与环境因子的关系

为研究太湖不同区域浮游植物群落结构特征及其与环境因子之间的关系,对水质状况进行调查,于2013年10月至2015年7月对太湖9个湖区的33个采样点进行了季节性浮游植物群落调查和水质监测,共鉴定出浮游植物121种,分别属于7门、74属,其中绿藻门(Chlorophyta)的种类最多,共28属、59种。春季和夏季浮游植物总密度最高值分别出现在2014年竺山湖0.59×10~8个/L和西部沿岸区5.90×10~8个/L,秋季和冬季浮游植物总密度最高值分别出现在2015年的西部沿岸区1.09×10~8个/L和湖心区3.28×10~8个/L。综合分析2年的浮游植物总密度监测数据,西部沿岸区梅梁湖竺山湖五里湖湖心区贡湖南部沿岸区东太湖东部沿岸区。研究表明,太湖9个区域污染情况不尽相同,主要超标指标为总氮。SPSS 22.0相关性分析表明,水温、DO、硝态氮、BOD5是影响太湖浮游植物群落结构的主要环境因子。2015年秋冬季浮游植物总密度较2014年有所增长,春季各湖区之间浮游植物密度的年际差异与其余季节不同,可能是由于水温变化所致。     

巢湖后生浮游动物群落及不同湖区营养状态评价

为了给巢湖水质管理及富营养化防治提供科学依据，根据水系特征，在巢湖东、西湖区湖心、裕溪河出湖口、丰乐河、派河和南淝河共设置8个采样点，2013年12月至2014年11月共采样12次，分别代表冬季(12月-次年2月)、春季(3-5月)、夏季(6-8月)和秋季(9-11月)，对巢湖水体理化因子和后生浮游动物（枝角类、桡足类、轮虫）群落进行逐月周年调查。基于叶绿素(Chl-a)、总磷(TP)、总氮(TN)、化学需氧量(CODMn)、透明度(SD)等水体理化指标，运用综合营养状态指数(TLI)和生物多样性指数，对不同湖区受污染程度和营养状态进行综合评价。结果显示，巢湖共检出后生浮游动物42种（属），优势种矩形龟甲轮虫(Keratella quadrata)、萼花臂尾轮虫(Brachionus calyciflorus)为富营养化水体或有机质较多水体的常见种，不同季节的优势种存在较大差异；总体上，后生浮游动物全年平均丰度371.6 个/L（191.5~600.0 个/L）和全年平均生物量3.0 mg/L（2.0~4.6 mg/L）均呈现西半湖高于东半湖、夏秋季高于冬春季的分布特点；其群落结构也存在显著的季节差异和空间差异，矩形龟甲轮虫、萼花臂尾轮虫等是造成差异的主要种类。水温(WT)和Chl-a是影响枝角类丰度的重要因子，桡足类丰度与WT呈显著正相关，而轮虫丰度受TLI(∑)指数的显著影响。TLI(∑)指数均值为60.33（52.18~66.28），指示巢湖总体处于轻度至中度富营养化状态。Shannon-Wiener多样性指数和Margalef丰富度指数表明，巢湖总体处于中度污染状态，不同湖区水体具有明显的不均衡性。     

Characterization of the quality of the gametes and sex steroid levels during spring and summer spawning events in pejerrey Odontesthes bonariensis reared under natural conditions of temperature and photoperiod

Pejerrey Odontesthes bonariensis spawning period takes place during the end of winter and spring and, depending on temperature conditions, during late summer and autumn. Nevertheless, the occurrence and quality of the summer–autumn spawning period have not been well documented and are not considered for aquaculture production. In this context, this study aimed to characterize the reproductive activity of pejerrey reared in captivity under natural conditions of temperature and photoperiod throughout a year in order to summarize the spawning quality of the two spawning periods. The present results showed that pejerrey might spawn from late winter to the end of the spring with a peak in October (mid‐spring) and, also during summer and early autumn, interrupted by a period of 56 days without spawning activity in between. The extension of both spawning periods was almost equal (67 and 69 days). Nevertheless, higher relative fecundity and larger egg size were observed during the first spawning period, associated with higher levels of oestradiol measured in females demonstrating that it is better than the second spawning. The fertilization rate was not affected throughout the reproductive periods. These differences in the reproductive parameters are discussed in order to design specific strategies to increase seed production.