Epizootiology of Eustrongylides ignotus in Florida: transmission and development of larvae in intermediate hosts

Under laboratory conditions, 2 modes of transmission of Eustrongylides ignotus (Nematoda: Dioctophymatoidea) to fish were identified. Eastern mosquitofish (Gambusia holbrooki) became infected after ingestion of either eggs of E. ignotus containing first-stage larvae or aquatic oligochaetes (Limnodrilus hoffmeisteri) containing third-stage larvae of E. ignotus. After removal from the uterus of gravid E. ignotus females and incubation for 17-28 days, depending on temperature, it was found that parasite eggs contained first-stage larvae that were infective to fish and oligochaetes. Larvae developed to the third stage in oligochaetes and were infective to fish 35-77 days postinfection (PI) and when fed to fish, developed to the fourth stage between 127 and 184 days PI. Eggs containing first-stage larvae fed directly to fish developed to the fourth stage between 84 and 105 days PI. The amount of time for development from the undifferentiated egg to the fourth-stage larva was 78-156 days shorter when fish ingested eggs containing first-stage larvae than when fish ingested oligochaetes containing third-stage larvae. Three species of large piscivorous fish, including black crappie (Pomoxis nigromaculatus), largemouth bass (Micropterus salmoides), and warmouth (Lepomis gulosus), were fed mosquitofish containing fourth-stage larvae. At necropsy, live E. ignotus larvae were recovered from all 3 species. Several fish had multiple infections after ingesting > 1 larva, indicating that bioaccumulation of the parasite in the food chain may occur.     

Altered predation susceptibility of mosquitofish infected with Eustrongylides ignotus

Eustrongylides ignotus is a parasitic nematode whose definitive hosts are often piscivorous wading birds (Ciconiiformes). Several species of small fishes are intermediate hosts, while larger predatory fish may be paratenic (transport) hosts. We examined predation susceptibility of infected mosquitofish (Gambusia holbrooki) to three species of predatory fishes, including juvenile largemouth bass (Micropterus salminoides), warmouth (Lepomis gulosus), and bluegill (Lepomis macrochirus). A 250 L aquarium with removable plexiglass divider and remote observation windows was constructed. Aquatic macrophytes were placed in the tank to provide refuge for the fishes. Predatory fish were allowed to acclimate to one half of the tank, while one infected and one uninfected mosquitofish were placed in the other. The divider was removed and an observer recorded the number of capture attempts and time required for capture. Predators were observed for behavioral alterations for 4 days after ingestion of infected mosquitofish, then examined at necropsy. Infected prey were selected preferentially in 31 of 38 (82%) trials. The number of capture attempts was 2.7+/-0.2 (x +/- SE) for infected fish and 3.9+/-0.4 for uninfected fish. Mean time of capture was 12.4+/-1.6 min for infected fish and 21.7+/-2.9 for uninfected fish. Because of these differences, infected mosquitofish were more susceptible to predation (P < 0.01) than uninfected fish. Aberrant behavior including lethargy, convulsions, and buoyancy abnormalities was observed in eight (67%) predatory fish. At necropsy, larvae of E. ignotus were found in the coelomic cavity, viscera, and swim bladders of predators. Parasite-induced behavior modification of intermediate hosts may predispose them to predation by wading birds and thereby facilitate the transmission of this nematode in natural populations.     

Larval Anisakid Infections in Marine Fish from Three Sea Areas of the Republic of Korea

The present study was performed to determine the infection status of anisakid larvae in marine fish collected from 3 sea areas of the Republic of Korea. Total 86 marine fish (8 species) collected from the East Sea (Goseong-gun, Gangwon-do), 171 fish (10 species) from the South Sea (Sacheon-si, Gyeongsangnam-do), and 92 fish (7 species) from the Yellow Sea (Incheon Metropolitan City) were examined by both naked eyes and artificial digestion method. Among the total of 349 fish examined, 213 (61.0%) were infected with 8 species of anisakid larvae, i.e., Anisakis simplex, 6 types of Contracaecum spp., and Raphidascaris sp., and the mean larval density was 13.8 per infected fish. Anisakid larvae were detected in 45 fish (52.3%) from the East Sea, 131 fish (76.6%) from the South Sea, and 37 fish (40.2%) from the Yellow Sea. The average numbers of larvae detected were 4.0, 16.6, and 15.9, respectively. Anisakis simplex larvae were detected in 149 fish (42.7%), and the mean larval density was 9.0 per infected fish. They were found in 26 fish (30.2%) collected from the East Sea, 96 fish (56.1%) from the South Sea, and 27 fish (29.3%) from the Yellow Sea. The average numbers of larvae detected were 2.9, 10.3, and 10.5, respectively. Conclusively, the present study suggests that the infection rate and density of anisakid larvae are more or less higher in the fish from the South Sea than those from the East Sea or the Yellow Sea.     

Sticklebacks from Greenland

Greenland freshwater sticklebacks Gasterosteus aculeatus were low-plated. Length distribution of 1 + and 2+ fish peaked at 33 and 46 mm, in early July. Pre-reproductive fish including 1 + fish were infected with Schistocephalus solidus . The diet consisted of chironomid larvae and pupae. Infected individuals ate smaller prey than non-infected ones, on one occasion where competition for food was likely to occur. Infected fish had lower stomach fullness, and the parasite compressed the stomach in heavily infected fish. The other fish species was charr, and the stickleback seemed to occupy a wide range of habitats.     

Composition, structure and distribution of the ichthyoplankton in a Mediterranean coastal lagoon

The species composition and assemblage structure of the ichthyoplankton from the Mar Menor Lagoon in south‐east Spain are given. The fish larvae were sorted from zooplankton samples collected at 20 stations with a plankton net (50 cm mouth diameter and 500 μm mesh) during 36 surveys between February and December 1997. A total of 39 575 fish larvae representing 14 families, 22 genera and 36 species were identified. Gobiidae was the most dominant family (77·0%) followed by Blenniidae (19·4%) and Atherinidae (1·3%). The most abundant species were Gobius niger and Gobius paganellus , which accounted for 42·7 and 19·3% of the total respectively. These species were followed in order of relative abundance by Pomatoschistus marmoratus (13·9%), Parablennius pilicornis (9·4%), Lipophrys pavo (7·7%), Atherina boyeri (1·3%) and Parablennius tentacularis (1·3%). The high species diversity (2·0–2·8 bits individual⁻¹ for the annual diversity spectra at each sampling station) reflected a diverse assemblage of species. The main commercial species in the lagoon (Sparidae and Mugilidae) were poorly represented among the ichthyoplankton and they probably enter the lagoon on the bottom as recruits. Chlorophyll a concentration in the water column was the main factor explaining the seasonal variation in larval abundance. Spatial distribution of larvae was related to hydrographical circulation patterns in the lagoon and the movement of marine‐spawned larvae through the channels connecting the lagoon with the Mediterranean Sea.     

Romanomermis culicivorax parasitism and the development, growth, and feeding rates of two mosquito species

The dry weight, development, and feeding rates of Culex pipiens and Toxorhynchites amboinensis larvae infected with the nematode, Romanomermis culicivorax, were measured and gross conversion efficiences were calculated. The weight of C. pipiens larvae infected at two different inoculum levels did not differ significantly from controls until day 6 postinfection (PI). Infected larvae of T. amboinensis were significantly lighter than controls at days 2, 4, and 6 PI. The rate of larval mosquito development was slowed after day 3 in parasitized individuals of both mosquito species. Infection significantly retarded the interval feeding rate of the filter-feeding C. pipiens throughout development. Infected T. amboinensis larvae consumed significantly fewer prey larvae of C. pipiens than controls. Calculation of gross conversion efficiency (GCE) showed that lightly infected C. pipiens larvae had an elevated GCE early in the infection but were less efficient relative to controls after 4 days PI. Lightly parasitized T. amboinensis had a lower total GCE than controls.     

COMPOSITION,ABUNDANCE AND SEASONALITY OF LARVAL FISH IN RICHARDS BAY HARBOUR,KWAZULU-NATAL,SOUTH AFRICA

Summary

The present study examined the composition, abundance and seasonality of fish larvae occurring in Richards Bay Harbour, over an 18-month period from June 1991 to November 1992. Ichthyoplankton samples were collected every six weeks at top, mid and bottom depths in the dredged channel 100 to 200 m from the harbour entrance. A total of 7 163 fish larvae, representing 105 taxa and 53 families, was collected throughout the study period. The dominant families were the Engraulidae and Gobiidae representing 50% and 37% of the total catch, respectively. The most abundant larvae were those of the thorny anchovy, Stolephorus holodon, and an unidentified goby, Gobiid 12 (32% and 30% of the total catch, respectively). Estuarine-associated taxa dominated the total catch in terms of density (61%), with 11 out of the 28 taxa being totally dependent on estuaries at some stage in their life cycle. Turbidity and temperature were significant variables accounting for 27% of the variation in larval densities of estuarine-dependent species. The main peak in larval density occurred in January 1992 (summer) with a mean larval density of 160 larvae per 100 m³. Larvae of estuarine-associated taxa were predominantly at the flexion and postflexion developmental stages. Since larval densities of certain estuarine-associated species were significantly greater in bottom samples and on flood tides, it is suggested that selective tidal stream transport is a recruitment mechanism used by these species. The present study concluded that it is important to maintain and preserve the estuarine component of Richards Bay Harbour so that the system can function as an estuarine nursery for many of the fish species depending on it at certain stages in their life cycle.     

Habitat rehabilitation for juvenile fish in urban waterways: A case study from Berlin,Germany

This study examines the improvement of juvenile fish recruitment by technical rehabilitation measures implemented in two urban waterways in Berlin, Germany: the River Spree and the Teltow Canal. Rehabilitation sites provide new littoral habitats protected from navigation‐induced flow and wake wash and are intended to improve habitat quality for juvenile fish and to support urban fish diversity. Both sites and four control sites with common embankments have been sampled monthly by point electric fishing (875 dips in total) from May to September 2009. In total, 6,243 fish were caught representing 12 species. Two of the species, roach (Rutilus rutilus) and perch (Perca fluviatilis), dominated the fish community, comprising 81.5% and 16.1% of the total catch. Nine species were rare, with relative abundances <1%, which hampered more detailed species‐specific analyses. Of the dips 33% contained no fish, causing zero‐inflation of the dataset. A hurdle regression model confirmed positive effects of reduced hydraulic disturbances and resulting vegetation cover on juvenile fish occurrence. However, it also showed low oxygen concentrations limiting habitat quality, as observed at the older, densely vegetated rehabilitation site in the River Spree. The general improvement of the juvenile fish assemblage by the rehabilitation measure appeared rather low, as partially vegetated, rectangle trapeze and trapeze profile banks had even higher fish abundance. The study also revealed a need for adaptive maintenance to improve the long‐term performance of this type of rehabilitation structure. With the recovery of aquatic vegetation, the technical bank protection should be adaptively removed to retain the connectivity with the main channel for a sufficient oxygen supply for the long‐term functionality as a fish habitat.     

Larval supply to a marine reserve and adjacent fished area in the Soufrière Marine Management Area, St Lucia, West Indies

A total of 76 reef fish species from 31 families was collected at two coral reef sites, one in a marine reserve and the other in an adjacent fished area of the Soufrière Marine Management Area (SMMA) in St. Lucia. Five families (Scaridae, Pomacentridae, Synodontidae, Apogonidae and Blennidae) dominated the collections at both sites while species of high commercial value were rare. Monthly patterns of larval supply differed among selected species, but overall trends were similar between the two sites for most species. However, despite the geographical proximity of the two sites, the fished area received a consistently higher abundance and diversity of larvae than the marine reserve throughout the study period. Patterns in larval supply generally were reflected in the settlement patterns of Stegastes partitus. Results suggest that local-scale variation in hydrodynamic and or biological features is influencing the arrival and hence settlement of larvae at the reef.     

青岛崂山青山湾人工鱼礁区及附近海域鱼卵仔稚鱼种类组成与数量分布

为研究人工鱼礁对产卵鱼类的诱集效果和庇护效应,2014—2015年对青岛崂山青山湾海域人工鱼礁区及附近海域鱼卵和仔稚鱼的种类组成和数量分布进行了水平和垂直拖网调查. 2014年春、夏、秋3个季节的7个航次调查中,共采集鱼卵7306粒、仔稚鱼52尾,隶属于4目9科11属12种;2015年同期进行的13个航次调查中,共采集鱼卵10373粒、仔稚鱼159尾,隶属于6目11科14属15种.2年间鱼卵和仔稚鱼样品均以鲈形目数量最多,鲽形目次之,其中鱼卵样品中仅有少鳞鱚和短吻红舌鳎构成优势种;仔稚鱼样品以少鳞鱚优势度最高,但不存在仔稚鱼优势种.鱼卵和稚鱼的高优势度物种的季节间演替明显.春、夏、秋季鱼类浮游生物群落Margalef丰富度指数、Shannon多样性指数和Pielou均匀度指数平均值均较低,说明该海域鱼类浮游生物群落结构稳定性较低.2年间所采集物种对应的成鱼平均营养级分别为3.71和3.78,均属第三营养级中的低级肉食性鱼类,绝大多数为暖温种,无冷温性种类,表现出明显的暖温带特点.综合分析认为,人工鱼礁区鱼类浮游生物群落的多样性相对较高,这与鱼礁区流速和流场特征及复杂的空间异质性有关.     

Terrestrial gastropod hosts of Parelaphostrongylus tenuis at the National Zoological Park's Conservation and Research Center, Virginia

Terrestrial gastropods at the National Zoological Park's Conservation and Research Center (CRC) near Front Royal, Virginia, were surveyed from June 1985 to May 1986, to assess their role in transmission of the meningeal worm, Parelaphostrongylus tenuis. A total of 670 gastropods representing 9 families and 18 species was collected. The slug Deroceras laeve accounted for 50.4% of the specimens collected. Parelaphostrongylus tenuis infections were found in 15 gastropods (2.2% prevalence) representing 5 species; 5 of the infected gastropods were D. laeve. New host records for this nematode are Ventridens collisella and Philomycus carolinianus. Infected gastropods were recovered from June through October 1985 and in May 1986. All 6 sampling locations yielded infected gastropods, suggesting this parasite is widely distributed at the CRC, and is therefore likely to continue to pose a serious threat to exotic ungulates maintained there for conservation and research purposes.     

Spatial and Temporal Ecology of Native and Introduced Fish Larvae in Lower Putah Creek,California

For two years we studied the distribution and abundance of native and introduced fish larvae in Putah Creek (Yolo County, CA), a low elevation regulated stream. We used light traps and conical drift nets to sample the fish larvae at two spatially separated sites from March through July 1997 and at four sites from February through August 1998. Native larvae occurred both earlier in the year and in higher abundance than those of introduced species. Both native larvae and overall numbers of larvae were more abundant at upstream sites in both years. Sampling of larvae appeared to be sensitive to the detection of rare species. Drift nets and light traps collected similar numbers of larvae, but each method tended to select for different taxa. There were significant trends in diel patterns of abundance, with more fish larvae being found during the hours of darkness. We suggest that differences between the sites were due to habitat changes resulting from an upstream dam that has created a refuge of diverse habitat and cool flowing water for native taxa.     

Parasitism of Cichlasoma paranaense Kullander, 1983 and Gymnotus carapo Linnaeus, 1814 by Clinostomum complanatum (Rudolphi, 1814) metacercariae in the Taquari river.

The infection, known as yellow spot disease, produced by metacercariae of Clinostomum spp. was studied in fishes of the Taquari river, located in Jataizinho, Paraná State, Brazil. A total of 1,582 specimens, belonging to 36 species, were collected between March 1999 and April 2001. Yellow spot disease was observed only in Gymnotus carapo Linnaeus, 1814 (Gymnotiformes, Gymnotidae) and Cichlasoma paranaense Kullander, 1983 (Perciformes, Cichlidae). This parasitism was generated by metacercariae of Clinostomum complanatum (Rudolphi, 1814) (Digenea, Clinostomidae). Among the 88 specimens of G. carapo examined, 7 (prevalence = 8%) had cysts of the parasite. Four of them were captured in July 1999 and three in October 1999. In the other months, no specimen found was infected. Using relative condition factor (Kn) analysis, it was determined that both infected and non-infected specimens had a total weight equal to the theoretically expected value for each total length (Kn = 1.0). Among 56 individuals of Cichlasoma paranaense, 6 (prevalence = 10.7%) had between 1 and 27 metacercariae of C. complanatum (mean intensity of infection = 9.3 +/- 9.6). In March 1999 and April of both 2000 and 2001, the specimens examined were not infected. The infected fish had a total weight higher than the expected value (Kn > 1.0), while the non-infected fish had a weight equal to the expected value (Kn = 1.0).     

Behavioral and ecological correlates of foureye butterflyfish, Chaetodon capistratus, (Perciformes: Chaetodontidae) infected with Anilocra chaetodontis (Isopoda: Cymothoidae)

We observed the behavior and ecology of Chaetodon capistratus infected and uninfected with the ectoparasitic isopod Anilocra chaetodontis to assess whether there may be parasite induced alterations in host biology, host defenses against infection, and/or pathology related to infection. We also examined habitat related differences in infection rates. Infected fish had higher rates of interaction with conspecifics and spent more time in low flow environments (which might improve transmission of juvenile parasites to new hosts). Butterfly fish without isopods were chased more frequently by damselfishes, fed more, and had larger territories. Time spent near conspecifics, and fish condition and gonadosomatic index did not vary between infected and uninfected fish. These results suggest that foureye butterfly fish behavior is altered by the isopod parasite in order for the isopods to more easily gain mates or transmit offspring to new hosts.     

城镇化背景下深圳典型流域鱼类群落特征及驱动因子

理解城镇的快速发展对河流鱼类群落结构的影响,是城镇河流科学管理和生物多样性保护的关键基础。本研究于2019年丰水期（8月）和枯水期（11月）,选取我国城镇化典型城市-深圳域内两个处于不同城镇化程度的代表性流域,应用多重统计方法比较分析了流域间鱼类群落结构的差异,并探讨了驱动鱼类群落变异的关键环境要素。结果发现,城镇化程度高的观澜河流域其鱼类种类组成、优势类群、生物多样性指数与城镇化程度低的坪山河流域有明显差别。 具体表现为：城镇化程度高的流域土著敏感种类如异鱲、吸鳅等几近消失,优势类群为外来入侵耐受种类,其物种多样性显著低于城镇化程度低的流域（P<0.05）。同时,外来鱼类在城镇河段其数量占比平均达92.5%,广泛分布于深圳城镇河流中。在环境因素方面,城镇化程度高的观澜河流域水体理化指标总氮、总磷、氨氮、化学需氧量、生化需氧量、高锰酸盐指数均显著性高于城镇化程度低的坪山河流域（P<0.05）。基于Bray-Curtis距离的冗余分析显示：城镇用地占比和总氮是影响观澜河和坪山河流域鱼类群落差异的主要因素。城镇化进程中河流生境的改变已影响到土著鱼类的生物多样性。因此,推动以恢复土著鱼类生物多样性的河流生态治理与保护是今后水生态目标管理的重要方向。     

Actinosporean infections in oligochaetes in a river system in southwest Ireland with descriptions of three new forms

Sediment samples were collected in April, June and August 1998 from a river in southwest Ireland. A total of 1,836 oligochaete specimens were collected from these sediments, and examined for actinosporean infections. Actinosporean-infected oligochaetes have been determined, symptoms and course of infection have been considered and the spores released from them have been described. Spore characteristics for each actinosporean were defined and the prevalence of infection in each oligochaete species was also determined. The overall prevalence of infection with actinosporeans within the oligochaete sample was approximately 2%. Seven types were distinguished, belonging to echinactinomyxon, aurantiactinomyxon and neoactinomyxum forms, from Tubifex ignotus, Lumbriculus variegatus and Limnodrilus hoffmeisteri, and compared with actinosporean types previously reported in the literature. Three of these forms have not been previously described, while the other 4 are morphologically and morphometrically similar to known forms. These actinosporeans may be stages in myxozoan life cycles.     

Molecular phylogeny of tubificid oligochaetes with special emphasis on Tubifex tubifex (Tubificidae)

Tubifex tubifex is a cosmopolitan freshwater oligochaete whose presence has been studied as a health indicator of the aquatic environment and as a host for several myxozoan parasites of fish. Unfortunately, current morphological criteria used to distinguish Tubifex spp. (Tubificidae) are inadequate. We therefore developed mitochondrial 16S ribosomal DNA markers to examine phylogenetic relationships among aquatic oligochaetes and to distinguish species of Tubifex that might serve as hosts for a particular myxozoan parasite, Myxobolus cerebralis. Our phylogenetic analyses of oligochaetes based on a 378-bp segment yielded one most parsimonious tree with three major groups that corresponded to the families Lumbricidae, Sparganophilidae, and Tubificidae. T. tubifex and T. ignotus formed a monophyletic assemblage, and a sister relationship between the genera Tubifex and Limnodrilus was strongly supported. A second analysis of the relationship within the genus Tubifex identified six genetically distinct lineages of T. tubifex from North America and Europe that were separated by genetic distances comparable to those found for "well-defined" species of Limnodrilus. Therefore, the existence of several morphologically indistinguishable, thus cryptic, species of Tubifex in North America and Europe is suggested.     

Oligochaetes and eutrophication; an experiment over four years in outdoor mesocosms

Eight experimental ditch mesocosms were used to study the effect of eutrophication over four years. The experimental ditches had a sand or clay bottom. The ditches were treated with additions of phosphorus, phosphorus and nitrogen, or without additions (controls). Oligochaetes were sampled by deploying trays with substratum for colonization over twenty weeks. Both the important variables phosphorus, nitrogen and oxygen as well as the oligochaete species and numbers are presented. The effects of nutrient additions on phosphorus, nitrogen and oxygen concentrations were described together with changes in oligochaete species composition and numbers. The results were further analyzed by redundancy analysis (RDA). In the clay-lined ditches nutrient addition coincided with fluctuation in oxygen concentration. The higher the nutrient addition levels the longer the period of oxygen depletion became. During oxygen depletion the number of oligochaetes was strongly reduced or even became zero. The low nutrient status of the sandy bed in the sand-lined ditches slowed down the rate of colonization. Only a few tubificids were collected. Eutrophication effects were only observed at the highest nutrient addition level. Considerable variation is attributed to stochastic factors in the sand-lined ditches. Whether oligochaete species were present was related to the length of the colonization period. The substratum composition and food together with oxygen regime decided whether they become more or less abundant in ditches. Large-scale mesocosm experiments require time to develop. Only after the first colonization period variables of species presences and abundances can be employed to detect changes associated with eutrophication. Oligochaetes can be used to measure colonization as well as eutrophication processes.     

中街山列岛海域鱼卵、仔稚鱼群落结构特征及其与环境因子的关系

为了解中街山列岛海域鱼卵仔稚鱼空间分布、种类组成及与环境因子的关系,于2010年5月—2012年2月在中街山列岛海域利用水平和垂直拖网开展了8个航次的鱼卵、仔稚鱼调查。运用非度量多维标度排序、聚类分析和典范对应分析(CCA)等方法对该海域鱼卵、仔稚鱼的空间分布进行了综合分析。结果表明:两年水平拖网共采集到鱼卵1783粒,仔稚鱼67尾;垂直拖网共采集到鱼卵576粒,仔稚鱼59尾。2010年平拖优势种为短吻红舌鳎(Cynoglossus joyneri),垂拖重要种为鲉形目未定种(Scorpaeniformes sp.)和扁舵鰹(Auxis thazard)。2011年平拖重要种为短吻红舌鳎、鰏属未定种(Leiognathuss sp.)、龙头鱼(Harpodon nehereus)和鳀(Engraulis japonicus);垂拖重要种为短吻红舌鳎、龙头鱼和中华小公鱼(Stolephorus chinensis)。经单因子相似性分析(ANOSIM),鱼卵仔稚鱼的优势种和重要种群落物种组成存在显著差异(P<0.01)。研究结果表明,温度和盐度对鱼卵仔稚鱼的影响较大,夏季是中街山列岛...     

Distribution and abundance of fish eggs and larvae in three tributaries of the Upper Uruguay River (Brazil)

The present study aimed to investigate the distribution and abundance of fish eggs and larvae in three important tributaries (Chapecó, Peixe, and Ligeiro rivers) of the Upper Uruguay River. The spatial and temporal distribution of fish eggs and larvae were studied as well as the correlation between environmental parameters and the abundance of ichthyoplankton species. The study was conducted between October 2005 and September 2006. Ichthyoplankton samples were collected at night with cylindroconical 0.5-mm mesh plankton nets every 5 days. Of the 591 samples collected, 170 contained ichthyoplankton organisms, resulting in the capture of 12,847 fish eggs and 962 fish larvae. Twenty-seven fish species were observed, of which 69% were Characiforms and 27% were Siluriforms. Among the fish species captured, the representatives were predominantly young forms of small and medium size fishes, with rheophilic species occurring infrequently. Eggs occurred exclusively between October and January, while the highest larvae occurrence was observed between November and December. Fish larvae assemblage structure was shown to be related to some environmental variables. There was a tendency of higher values of water temperature and velocity at the lower sampling sites than at the upper ones, as well an increase number of eggs and larvae. The study tributaries serve as reproduction sites and nursery areas for several fish species of the Upper Uruguay River; thus, the maintenance of their integrity is important for the preservation of diversity and enhancement of fisheries in the region.