In lowland river systems, in-channel, slow-flowing or still-water areas (still-water patches, SWPs) are considered to be important habitats for many organisms, particularly the early-life stages of fish and shrimp. However, the distribution of the early life-stages of fish and shrimp among these habitats appears to be very patchy and studies suggest that the quality and diversity of microhabitat conditions within SWPs and the accessibility of SWPs to spawning adults and dispersing young may be important determinants of their suitability as nursery habitat. The aims of this thesis were to examine the utilisation of still-water patches by fish and shrimp in a lowland river in relation to habitat suitability and accessibility, with particular emphasis on early-life stages. To determine the factors influencing habitat selection among SWPs, the environmental variability in SWP habitat, and both the distribution and the movement patterns of fish and shrimp, were examined in the Broken River, a lowland river in south-eastern Australia. SWP habitat was found to be highly spatially and temporally variable in the Broken River. SWPs differed in relation to permanence, accessibility and microhabitat variables, and all life-stages of fish and shrimp were found to be significantly spatially aggregated among SWPs. This suggests that individual SWPs may differ in their suitability as habitat, and/or in their accessibility to dispersing organisms and indicates either differential rates of retention, movement into SWPs, spawning effort or survival among SWPs for these organisms. Significant associations were found for all species and life-stages in relation to the microhabitat characteristics of SWPs. The two introduced species, carp and gambusia, were found to have fewer associations, which suggests that these species are habitat generalists. Cover and SWP morphology variables were shown to be important for all native species. Significant, positive associations were found for most species and life-stages with large, deep, SWPs containing instream cover, however, the extent of cover preferred was variable. It was hypothesised that large, deep SWPs that contain instream cover are more environmentally stable and provide better foraging efficiency and reduced competition for space, whilst also providing refuge from predators and, that they may be easier to locate than smaller patches. Specific associations with microhabitat variables differed among all species and life-stages, and this was attributed to differences in diet and predation rates. Consequently, generalised microhabitat relationships for particular life-stages or species could not be identified and the results from this thesis suggest that a diversity of microhabitat conditions are required to meet the differing requirements of various life-stages and species. Significant associations were also found for most groups in relation to the accessibility characteristics of SWPs, indicating that the ability of individuals to access SWPs is an important factor in determining their distribution among SWPs. This further suggests that movement is an important factor in the distribution pattern of fish and shrimp among SWPs. Significant associations were found for most groups in relation to patch isolation, adjacent hydraulic habitat and entrance conditions, indicating that landscape composition and configuration as well as boundary conditions may be important determinants of organisms being able to locate suitable patches. Associations with accessibility variables differed among species and life-stages, and may be attributable to differences in movement capabilities. Field manipulations of instream cover and entrance depth were conducted to further examine the habitat associations found. The results confirmed a positive relationship between instream cover and fish and shrimp abundances. No species, however, responded consistently to the manipulation of entrance depths, and this was attributed to water level rises throughout the experiment and/or the correlation of entrance depth with SWP depth. However, the results from the field manipulations suggested that deeper habitats are able to be exploited by small-bodied adults and larvae when significant levels of instream cover are also available as refuge from predation. In order to confirm the importance of movement in the selection of SWP habitat by fish and shrimp, the movement patterns of fish and shrimp into and out of SWPs were investigated. Whilst the results from this aspect of the study were inconclusive for fish, the results for shrimp confirmed that adults and larvae moved routinely into and out of SWPs. However, for all shrimp species, movement appeared to be limited to a certain period of larval development, indicating that SWP quality and stability may be more important at particular stages of development than others. The results of this thesis have demonstrated the importance of SWP quality and stability for fish and shrimp in the Broken River and have shown that habitat preferences vary among individual species and life-stages. Consequently, in order to manage for multiple species and life-stages, consideration must be given not only to the availability of SWPs, but also to their stability over time and to the availability of a diverse range of microhabitats. In addition, consideration must also be given to the accessibility of SWPs and this will require a greater knowledge of the specific spawning and dispersal requirements of the organisms which utilise these patches, in combination with a greater understanding of the impacts of flow modification on riverine landscape composition and configuration.

Many aspects concerning the association of riverine fish with in-channel habitat remain poorly understood, greatly hindering the ability of researchers and managers to address declines in fish assemblages. Recent insights gained from landscape ecology suggest that small, uni-scalar approaches are unlikely to effectively determine those factors that influence riverine structure and function and mediate fish-habitat associations. There appears to be merit in using multiple-scale designs built upon a geomorphologically-derived hierarchy to bridge small, intermediate and large spatial scales in large rivers. This thesis employs a hierarchical design encompassing functional process zones (referred to hereafter as zones), reaches and mesohabitats to investigate fish-habitat associations as well as explore patterns of in-channel habitat structure in one of Australia's largest dryland river systems; the Barwon-Darling River. In this thesis, empirical evidence is presented showing that large dryland rivers are inherently complex in structure and different facets of existing conceptual models of landscape ecology must be refined when applied to these systems. In-channel habitat and fish exist within a hierarchical arrangement of spatial scales in the riverscape, displaying properties of discontinuities, longitudinal patterns and patch mosaics. During low flows that predominate for the majority of time in the Barwon-Darling River there is a significant difference in fish assemblage composition among mesohabitats. There is a strong association between large wood and golden perch, Murray cod and carp, but only a weak association with bony herring. Golden perch and Murray cod are large wood specialists, whereas carp are more general in there use of mesohabitats. Bony herring are strongly associated with smooth and irregular banks but are ubiquitous in most mesohabitats. Open water (mid-channel and deep pool) mesohabitats are characterised by relatively low abundances of all species and a particularly weak association with golden perch, Murray cod and carp. Murray cod are weakly associated with matted bank, whereas carp and bony herring associate with this mesohabitat patch in low abundance. Nocturnal sampling provided useful information on size-related use of habitat that was not evident from day sampling. Both bony herring and carp exhibited a variety of habitat use patterns throughout the die1 period and throughout their lifetime, with temporal partitioning of habitat use by juvenile bony herring and carp evident. Much of the strong association between bony herring and smooth and irregular banks was due to the abundance of juveniles (&lt100mm in length) in these mesohabitats. Adult bony herring (&gt100mm length) occupied large wood more than smooth and irregular banks. At night, juvenile bony herring were not captured, suggesting the use of deeper water habitats. Adult bony herring were captured at night and occupjed large wood, smooth bank and irregular bank. Juvenile carp (&lt200mm length) were more abundant at night and aggregated in smooth and irregular banks more than any other mesohabitat patch. Adult carp (&gt200mm length) occupied large wood during both day and night. There is a downstream pattern of change in the fish assemblage among river zones, with reaches in Zone 2 containing a larger proportion of introduced species (carp and goldfish) because of a significantly lower abundance of native species (bony herring, golden perch and Murray cod) than all other zones. In comparison, the fish assemblage of Zone 3 was characterised by a comparatively higher abundance of the native species bony herring, golden perch and Murray cod. A significant proportion of the amongreach variability in fish assemblage composition was explained at the zone scale, suggesting that geomorphological influences may impose some degree of top-down constraint over fish assemblage distribution. Although mesohabitat composition among reaches in the Barwon-Darling River also changed throughout the study area, this pattern explained very little of the large-scale distribution of the fish assemblage, with most of the variability in assemblage distribution remaining unexplained. Therefore, although mesohabitat patches strongly influence the distribution of species within reaches, they explain very little of assemblage composition at intermediate zone and larger river scales. These findings suggest that small scale mesohabitat rehabilitation projects within reaches are unlikely to produce measurable benefits for the fish assemblage over intermediate and large spatial scales in the Barwon-Darling River. This indicates the importance taking a holistic approach to river rehabilitation that correctly identifies and targets limiting processes at the correct scales. The variable nature of flow-pulse dynamics in the Barwon-Darling River creates a shifting habitat mosaic that serves to maintain an ever-changing arrangement of habitat patches. The inundation dynamics of large wood habitat described in this thesis highlights the fragmented nature of mesohabitat patches, with the largest proportion of total in-channel large wood remaining unavailable to fish for the majority of the time. At low flows there is a mosaic of large wood habitat and with increasing discharge more potential large wood habitat becomes available and does so in a complex spatial manner. What results in this dryland river is a dynamic pattern of spatio-temporal patchiness in large wood habitat availability that is seen both longitudinally among different river zones and vertically among different heights in the river channel. Water resource development impacts on this shifting habitat mosaic. Projects undertaking both fish habitat assessment and rehabilitation need to carefully consider spatial scale since the drivers of fish assemblage structure can occur at scales well beyond that of the reach. Fish-habitat associations occurring at small spatial scales can become decoupled by process occurring across large spatial scales, making responses in the fish assemblage hard to predict. As rivers become increasingly channelised, there is an urgent need to apply research such as that conducted in this thesis to better understand the role that in-channel habitats play in supporting fish and other ecosystem processes. Habitat rehabilitation projects need to be refined to consider the appropriate scales at which fish assemblages associate with habitat. Failure to do so risks wasting resources and forgoes valuable opportunities for addressing declines in native fish populations. Adopting multi-scalar approaches to understanding ecological processes in aquatic ecosystems, as developed in this thesis, should be a priority of research and management. To do so will enable more effective determination of those factors that influence riverine structure and function at the approariate scale.

Heavy metal pollutants enter watersheds because of deforestation, industrialization, and farming practices. Much research has been done on the Rio Tarcoles watershed, which includes Costa Rica’s capital city of San Jose, but much less has been performed on the Rio Parrita watershed, which lies adjacently South of the Tarcoles. This study aimed to examine the presence of metal pollutants in the Rio Parrita watershed and its possible implications on the wildlife that rely on it. Water and sediment samples were collected during the rainy season at the mouth of the Rio Tarcoles (high pollution control), Quebrada Terciopelo (low pollution control), and Rio Parrita watersheds and analyzed using ICP-MS to determine the concentrations of metallic elements along with the water’s pH and total dissolved solids (TDS). Statistical tests were used to analyze the differences in the availability of select metals in water and sediments that were frequently examined in relevant literature (Al, Fe, U, Cd, Tl, V, Ni, Cr). GIS maps were examined to delineate the watersheds and examine population densities in the Rio Parrita and Rio Tarcoles watersheds. Rio Parrita had significantly greater concentrations of Ni and Cr than either control river (p

abstract: A model is presented for real-time, river-reservoir operation systems. It epitomizes forward-thinking and efficient approaches to reservoir operations during flooding events. The optimization/simulation includes five major components. The components are a mix of hydrologic and hydraulic modeling, short-term rainfall forecasting, and optimization and reservoir operation models. The optimization/simulation model is designed for ultimate accessibility and efficiency. The optimization model uses the meta-heuristic approach, which has the capability to simultaneously search for multiple optimal solutions. The dynamics of the river are simulated by applying an unsteady flow-routing method. The rainfall-runoff simulation uses the National Weather Service NexRad gridded rainfall data, since it provides critical information regarding real storm events. The short-term rainfall-forecasting model utilizes a stochastic method. The reservoir-operation is simulated by a mass-balance approach. The optimization/simulation model offers more possible optimal solutions by using the Genetic Algorithm approach as opposed to traditional gradient methods that can only compute one optimal solution at a time. The optimization/simulation was developed for the 2010 flood event that occurred in the Cumberland River basin in Nashville, Tennessee. It revealed that the reservoir upstream of Nashville was more contained and that an optimal gate release schedule could have significantly decreased the floodwater levels in downtown Nashville. The model is for demonstrative purposes only but is perfectly suitable for real-world application. / Dissertation/Thesis / Doctoral Dissertation Civil Engineering 2015

M.Sc. (Zoology) / Please refer to full text to view abstract

The use of intestinal parasites as indicators of pollution has increased in Europe due to the fact that parasites accumulate more of the available metals than their hosts. Eighty Largemouth yellowfish, Labeobarbus kimberleyensis (Gilchrist and Thompson, 1913) were collected between April 2005 and February 2006 with gill nets at RAU Island (26° 52, 249’ S, 28 10, 249’ E) in the Vaal Dam. The fish were killed, weighed and their lengths determined. Muscle, liver and spinal cord tissues were collected from each fish and the intestines removed and opened to expose Bothriocephalus acheilognathi. The position of attachment of B. acheilognathi in the host’s intestine was recorded. Thereafter tapeworms were collected in glass bottles and frozen. Water and sediment samples were collected. Clossed vessel microwave digestion was used to digest liver, muscle and tapeworms with Suprapur Nitric acid, and spinal cord tissue with Suprapur Nitric acid and Suprapur Hydrochloric acid. Thereafter metal concentrations of twenty three elements (Lithium, Beryllium, Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, Copper, Zinc, Arsenic, Selenium, Molybdenum, Cadmium, Tin, Antimony, Tellurium, Barium, Mercury, Thallium, Lead and Uranium) were determined specifically for each tissue type with an ICP-MS. Certified dogfish liver (Dolt-3) was used as a quality control standard. Water quality data was provided by Rand Water and included. Statistical data were analyzed with ANOVA and post-hoc tests and parasite infection statistics were calculated. Seasonality and intensity of tapeworms were compared to the intestine and total lengths of the fish. All the tapeworms were identified as the Asian Tapeworm B. acheilognathi Yamaguti, 1934. Bothriocephalus acheilognathi attached between 10% and 20% from the anterior end of host intestine. A 100% prevalence of Asian tapeworm was recorded in all surveys and the highest mean intensity of 231.1 was recorded in autumn and the lowest mean intensity of 73.7 was recorded in summer. The fecundity of largemouth yellowfish was not influenced by the high infection rate of B. acheilognathi as young fish fry was collected during the summer survey and the fish condition was good. No correlation was observed between parasite intensity and total length of each fish. This indicates that intensity is not dependant on host size. Parasite numbers are higher and differ from a previous study. Water quality of Vaal Dam changed in summer, due to transferance of water from the Grootdraai Dam. Algal counts and secchi disk readings increased. This indicates that zooplanktonic intermediate hosts increased due to an increase in food availability and increased the chance of infection. Significant differences were recorded in one sample T-test comparisons, while ANOVA indicated that significant differences between tissue groups and seasons occurred. Bioaccumulation of metals occurred in tapeworms in Selenium, Mercury, and Lead during autumn, Copper, Zinc, Selenium, Cadmium, Antimony, Thallium and Mercury during winter, Lithium, Zinc, Selenium, Cadmium and Antimony during spring and Zinc during summer. The highest mean value was recorded in sediment, followed by water, tapeworms and then the host tissue. The general trend was sediment > water > tapeworms > liver > spinal cord > muscle. Die gebruik van intestinale parasiete as indikatore vir besoedeling het in Europa verhoog omdat die parasiete meer van die beskikbare metale as hul gashere akkumuleer. Tagtig Grootbekgeelvisse Labeobarbus kimberleyensis (Gilchrist en Thompson, 1913) was tussen April 2005 en Februarie 2006 met kieunette te RAU eiland (26° 52, 249’ S, 28 10, 249’ E) in die Vaaldam versamel. Die visse is doodgemaak, geweeg en hul lengtes bepaal. Spier, lewer en werwelkolomweefsel was versamel van elke vis en die intestinums verwyder en oopgemaak om Bothriocephalus acheilognathi bloot te lê. Die posisie van aanhegting van B. acheilognathi in die gasheer se intestinum is aangeteken. Daarna is die lintwurms in glasbottels versamel en gevries. Water en sediment was ook versamel. Geslote-flesmikrogolfvertering was gebruik om lewer, spier en lintwurms met Suprapur® Swaelsuur en werwelkolom met Suprapur® Swaelsuur en Suprapur® Soutsuur te verteer. Daarna is die metaalkonsentrasies van drie-en twintig elemente (Litium, Berillium, Titanium, Vanadium, Chroom, Mangaan, Yster, Kobalt, Nikkel, Kopper, Sink, Arseen, Selenium, Molibdenum, Cadmium, Tin, Antimoon, Tellurium, Barium, Kwik, Tallium, Lood and Uranium) spesifiek vir elke weefseltipe bepaal met ‘n IGP-MS. Gesertifiseerde haailewer (Dolt-3) was gebruik as ‘n standaard ter wille van kwaliteitskontrole. Waterkwaliteitsdata is deur Rand Water verskaf en word ingesluit. Statistiese data was geanaliseer met ANOVA en nagraadse toetse en parasiet infeksie statistieke was bereken. Seisonaliteit en intensiteit van die lintwurms was vergelyk met die totale lengtes van visse en intestinumlengte van die visse. Al die lintwurms was as Asiese lintwurms B. acheilognathi Yamaguti, 1934 geïdentifiseer. Bothriocephalus acheilognathi was tussen 10% en 20% vanaf die anteriorgedeelte van die gasheerintestinum geheg. ‘n 100% persentasiebesmetting was opgeteken in alle opnames met Asiese lintwurms en die hoogste gemiddelde besmettingsintensiteit van 231.1 was opgeteken in die herfs en laagste gemiddelde besmettingsintensiteit van 73.7 waargeneem in die somer. Die broeisukses van die Grootbekgeelvisse was nie beïnvloed deur die hoë infeksies met B. acheilognathi nie, soos blyk uit die feit dat jong vissies versamel is tydens die someropname en die viskondisie was ook goed. Daar was geen korrelasie waargeneem wanneer die parasietintensiteite en totale lengtes van die visse vergelyk is nie. Dit dui dat intensiteit nie afhanklik is van gasheergrootte nie. Die parasietgetalle is hoër en verskil van ‘n vorige studie. Die waterkwaliteit van die Vaaldam het verander gedurende somer a.g.v. die toevoer van water vanaf die Grootdraaidam. Die algtellings en die secchi-skyf lesings het dus verhoog. Dit dui dus aan dat die getal soöplanktoniese tussengashere verhoog het agv die verhoogde voedselbeskikbaarheid en ‘n hoër kans van infeksie vir visse. Betekenisvolle verskille was waargeneem wanneer enkel monster T-toets vergelykings gemaak is, terwyl ANOVA betekenisvolle verskille tussen groepe en seisoene gewys het. Bioakkumulasie van metale in lintwurms het in selenium, kwik en lood gedurende herfs, en in koper, sink, selenium, cadmium, antimoon, tallium en kwik in winter, en in litium, sink, selenium, cadmium en antimoon in lente en in sink gedurende somer voorgekom. Die hoogste gemiddelde waarde was waargeneem in sediment, gevolg deur water, lintwurms en gasheerweefsel. Die algehele akkumulasie verwantskap is sediment > water > lintwurms > lewer > werwelkolom > spier. / Prof. Annemarie Avenant-Oldewage

This study aimed to describe the distribution and relative abundance of the phytoplankton obtained during the two seasons (rainy and dry seasons) from the Red river system. The water and phytoplankton samples were monthly collected during the year 2012 at four sampling stations along the Red River (Yen Bai, Vu Quang Hoa Binh,and Ha Noi) . Environmental variables (e.g. temperature, dissolved oxygen, pH, suspended solids, conductivity, TDS, NO3­-N, NH4­-N, PO4-P, Total Phosphorus; and DOC) and phytoplankton (e.g. cell density and relative abundant species) were analyzed. Six phytoplankton classes were identified with the Bacillariophyceae dominating in the phytoplankton community. A distinct seasonal variation in phytoplankton structure was observed with high cells density in dry season and low values in rainy season. PCA (Principal Component Analysis) showed that suspended solid factor that governed the temporal and spatial distribution of phytoplankton structure in the Red River system. / Nghiên cứu này trình bày sự phân bố và độ phong phú tương đối của quần xã thực vật nổi vào mùa mưa và mùa khô trong hệ thống sông Hồng. Các mẫu nước và thực vật nổi được thu hàng tháng trong năm 2012 tại 4 điểm trên sông Hồng (Yên Bái, Vụ Quang, Hòa Bình và Hà Nội). Các thông số môi trường (nhiệt độ, lượng oxy hòa tan, pH, chất rắn lơ lửng, độ dẫn, TDS, NO3-N, NH4-N, PO4-P, T-P và DOC) và thực vật phù du (mật độ tế bào,độ phong phú tương đối ) đã được phân tích. Sáu lớph tảo được được xác định với tảo silíc chiếm ưu thế trong quần xã thực vật phù du. Sinh khối thực vật đạt giá trị cao vào mùa khô trong khi thấp vào mùa mưa. Phân tích hợp phần chính cho thấy yếu tố chất rắn lơ lửng đóng vai trò quan trọng việc xác định biến động thời gian và không gian cấu trúc quần xã thực vật nổi trong hệ thống sông Hồng.

In this thesis, we first develop a new method called penalty function with memory (PFM). PFM consists of a penalty parameter and a measure of constraint violation and it converts a discrete optimization via simulation (DOvS) problem with stochastic constraints into a series of DOvS problems without stochastic constraints. PFM determines a penalty of a visited solution based on past results of feasibility checks on the solution. Specifically, assuming a minimization problem, a penalty parameter of PFM, namely the penalty sequence, diverges to infinity for an infeasible solution but converges to zero almost surely for any strictly feasible solution under certain conditions. For a feasible solution located on the boundary of feasible and infeasible regions, the sequence converges to zero either with high probability or almost surely. As a result, a DOvS algorithm combined with PFM performs well even when optimal solutions are tight or nearly tight. Second, we design an optimal water quality monitoring network for river systems. The problem is to find the optimal location of a finite number of monitoring devices, minimizing the expected detection time of a contaminant spill event while guaranteeing good detection reliability. When uncertainties in spill and rain events are considered, both the expected detection time and detection reliability need to be estimated by stochastic simulation. This problem is formulated as a stochastic DOvS problem with the objective of minimizing expected detection time and with a stochastic constraint on the detection reliability; and it is solved by a DOvS algorithm combined with PFM. Finally, we improve PFM by combining it with an approximate budget allocation procedure. We revise an existing optimal budget allocation procedure so that it can handle active constraints and satisfy necessary conditions for the convergence of PFM.

The outlook for Nile crocodiles in the Olifants River does not look optimistic. Since the increase in capacity of the Loskop and Flag Boshielo Dams, the crocodile population was left with no basking or nesting sites and has declined over the past 30 years. Shortly after the Massingire Dam in Moçambique filled to full capacity an estimated 160 crocodiles died in the Olifants River Gorge, a couple of kilometres upstream from the dam. The Olifants River is acknowledged by many experts as one of the most polluted rivers in South Africa and acid mine drainage, industrial pollution and untreated sewage in the river are all contributing to the poor water quality of the river. Further, the Department of Water Affairs and Forestry acknowledge that water demand already exceeds their capacity to supply and that the situation will worsen considerably in the near future. Aerial surveys of Nile crocodiles in the Olifants River was carried out during December 2005 and November 2009. An average total population of 714 Nile crocodiles were counted and corrected to an estimated 1140 individual crocodiles to eliminate the effects of undercounting. The Kruger National Park and specifically the area of the Olifants River Gorge was found to be one of the preferred habitat areas for crocodiles in the Olifants River as was the Flag Boshielo Dam, the area between the Blyde River and the western boundary of the Kruger National Park and the Olifants River between the Loskop Dam and the Flag Boshielo Dam. Repeated nesting in areas such as the Kruger National Park, the Flag Boshielo Dam and the Olifants River between the Loskop Dam and the Flag Boshielo Dam confirmed that these areas are critically important to the nesting success of Nile crocodiles in the Olifants River. The Elands River was confirmed as an important refuge area for Nile crocodiles in the Groblersdal-Flag Boshielo Dam area of the Olifants River. Surveys revealed an estimated total of only 15 crocodiles in the Loskop Dam and confirmed that no crocodiles in the large (2.1 - 4.0m TL) and very large size class (>4.0m TL) are currently present in the population. Blood biochemistry results indicate that the Olifants River Nile crocodile population probably suffers from chronic inflammation (especially in the Loskop Dam and Olifants River Gorge populations), infectious disease (particularly in the Loskop Dam population but all other sites also showed elevated values), possible inadequate diet and malnutrition (especially during the pansteatitis outbreak of August/September 2008) and are suffering serious immune problems in the Olifants River Gorge. A conservation and management plan is suggested which identifies threats to the continued existence of a viable Nile crocodile population in the Olifants River. Finally, it is suggested that the conservation status and risk of extinction of Nile crocodiles in the Olifants River be upgraded to the Endangered category since it currently complies to the following criteria; EN A2abce; C2a(i) published in the IUCN Red List Categories and Criteria Version 3.1 (IUCN, 2001). / Thesis (PhD)--University of Pretoria, 2011. / Centre for Wildlife Management / unrestricted

Introduced species can become invasive and cause catastrophic alterations to the system they invade. Both zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) have caused significant ecosystem alterations wherever they have invaded. These Dreissena species have caused changes in water quality and biodiversity and have disrupted energy pathways which can have cascading effects on other trophic levels. Recently quagga mussels invaded Lake Powell, a reservoir located in the southwestern USA, creating the possibility of a trophic cascade that could alter energy flow in the reservoir and change the trophic niche structure of the fishes in the lake. However, due to Lake Powell’s large size, dynamic nature, and complex hydrological structure, the effects of quagga mussels on fish species is uncertain. To determine impacts of quagga mussels on Lake Powell fishes, we quantified trophic niches of five species of sport fish over three years (2017-2019) using stable isotopes of nitrogen, δ15N, and carbon, δ13C. We test the following hypothesis: quagga mussels will cause a shift in trophic niche in more pelagic fishes such that pelagic fishes decrease in trophic position and shift toward use of more littoral energy. In addition, we compare the trophic niche of these species with a previous study on the trophic niche of fish in Lake Powell prior to full colonization of the lake by quagga mussels (2014-2015). In general, fish in the southern region of the lake exhibited a trend of decreasing δ15N suggesting decreasing trophic position and an enrichment of δ13C indicating a littoral energy shift in some species. Fish in the northern region of the lake exhibited a slight increase in trophic position and a shift towards pelagic energy across the same time period. These shifts support the hypothesis with pelagic fish experiencing a trophic niche shift, in the direction predicted, but only in the southern region of Lake Powell. Additionally, this shift is not exclusive to pelagic fish, but happened in all five species. Sediment laden input from the Colorado River may offset the impact of quagga mussels in the northern region of the lake resulting in observed regional differences.