Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ulrm20 Lake and Reservoir Management ISSN: 1040-2381 (Print) 2151-5530 (Online) Journal homepage: http://www.tandfonline.com/loi/ulrm20 Vertical and seasonal distribution of fish in Três Marias reservoir Ivo Gavião Prado & Paulo Santos Pompeu To cite this article: Ivo Gavião Prado & Paulo Santos Pompeu (2014) Vertical and seasonal distribution of fish in Três Marias reservoir, Lake and Reservoir Management, 30:4, 393-404, DOI: 10.1080/10402381.2014.955221 To link to this article: https://doi.org/10.1080/10402381.2014.955221 Published online: 15 Sep 2014. Submit your article to this journal Article views: 97 View related articles View Crossmark data Citing articles: 1 View citing articles
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Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=ulrm20
Vertical and seasonal distribution of fish in TrêsMarias reservoir
Ivo Gavião Prado & Paulo Santos Pompeu
To cite this article: Ivo Gavião Prado & Paulo Santos Pompeu (2014) Vertical and seasonaldistribution of fish in Três Marias reservoir, Lake and Reservoir Management, 30:4, 393-404, DOI:10.1080/10402381.2014.955221
To link to this article: https://doi.org/10.1080/10402381.2014.955221
Vertical and seasonal distribution of fish in Tres Marias reservoir
Ivo Gaviao Prado∗ and Paulo Santos PompeuLaboratorio de Ecologia de Peixes, Setor de Ecologia, Departamento de Biologia, Universidade
Federal de Lavras, Campus Universitario, Lavras, Minas Gerais, Brasil 37200–000
Abstract
Prado IG, Pompeu PS. 2014. Vertical and seasonal distribution of fish in Tres Marias reservoir. Lake Reserv Manage.30:393–404.
Understanding fish community dynamics close to dams can provide critical information about the necessity andfeasibility of installing downstream fish passages. This understanding can also be used in reservoir operating plansto reduce the risk of fish being drawn into turbine intakes. To provide this background information for Tres Mariasdam we evaluated vertical and seasonal fish distribution immediately upstream from the Tres Marias dam usinghydroacoustics and gillnet sampling. We conducted hydroacoustic surveys and gillnetting on 18 separate trips. Oneach sampling trip, hydroacoustic surveys were done 4 times within a 24 h period at 6 h intervals. To validate theeffectiveness of hydroacoustics, gillnets were set for 24 h and checked at 12 and 24 h. The gonadal maturity stage ofmigratory fish species was evaluated. The greatest concentration of fish was found in the sampling area at night duringthe rainy season, suggesting that this might be the most appropriate time to open spillway gates for downstream fishpassage. This timing would also minimize the effect on reservoir storage. In addition, higher concentrations of fishand post spawning migratory species were captured during the rainy season. Our study results have the potential toenhance dam operation management plans to provide safer downstream passage for migrating fish.
Key words: downstream fish passage, fish behavior, hydroacoustics, management, tropical rivers
The construction of hydroelectric dams in Brazilian rivershas increased considerably in recent decades (ANEEL2008), leading to major, profound changes in watercourses(Agostinho et al. 2007a). Dams represent a physical barrierto species, which can be critical for migratory species, espe-cially if such a barrier impedes the connection between feed-ing and breeding (Godinho and Kynard 2009, Dugan et al.2010). Some migratory fish species reproduce in upstreamor headwater locations before returning to their downstreamfeeding sites.
Since the early 20th century there has been an effort toreduce this adverse effect on migratory fish populations byrequiring fishways for new dams and retrofitting existingones (Agostinho et al. 2011). The main purpose of thesefishways is to allow for fish passage upstream, however,which neglects the need for adults to return after spawningand the downstream movement of juveniles, eggs, and larvae(Pompeu et al. 2012).
Brazilian dams have no built-in mechanism to promotedownstream passage, and the existing technology for this
modality is not sufficiently advanced due to the belatedrecognition of the issue, the complexity in developing mech-anisms for this purpose (Larinier and Travade 2002), andthe low abundance of migratory fish at dam sites prior toconstruction (Pompeu et al. 2012). The lack of informa-tion about downstream fish movement in South Americamay bias against the importance of structural or operationalmanagement initiatives (Pompeu et al. 2012).
If fish passage around dams only promotes an upstream,one-way migration, stocks may be redistributed alongthe river, causing population imbalances including thesubtraction of stocks from downstream (Pompeu et al.2012). Thus, depending on the distribution of spawningareas, one-way fish passages can act as ecological traps(Pelicice and Agostinho 2008).
Fish passage from reservoirs to downstream free-flowingriver areas has been a major focus of recent studies onfish migration in North America and Europe (Larinier andTravade 2002). Research has included the impacts caused bypassage through turbines and over spillways, such as baro-trauma (Brown et al. 2014). Understanding the risk to fishdirectly affects development and implementation of man-agement tools (Cada 2001).
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Turbine passage is hazardous to fish and causes esti-mated mortality rates between 2 and 19% (Whitney et al.1997,Cada et al. 2006). Blade strikes and rapid pressurevariations are the main causes of death or injuries (Brownet al. 2009). Because of the challenge in facilitating effectivedownstream fish movements, some studies have suggestedthe use of spillways as an alternative means. Under appro-priate flows, spillways may represent a lower risk to fishthan passage through turbines. Use of spillways also hasthe obvious advantage of eliminating the need for structuralmodifications of the dam (Ruggles and Murray 1983, Skalskiet al. 1998, Larinier 2008). To be used effectively, we needto understand fish migration patterns and then coordinatedam operations to maximize fish passage; however, if wateris not available to spill during critical migration periods thenspillways will not be an option.
The goal of this study is to better understand fish commu-nity dynamics adjacent to the dam in Tres Marias reservoir,Brazil, to evaluate the necessity and feasibility of installinga fishway for downstream migrating fish, including an as-sessment of whether the dam’s spillway can be used for thispurpose. This study is being undertaken prior to an assess-ment of the need for an upstream fishway. Data on timing offish distribution along the dam can also be used to improvedam operation to reduce the risk of fish being drawn into theturbine intakes.
Hydroacoustics is the principal means for assessing fishdistribution within the water column (Busch and Mehner2009). It allows the sampling of large areas over relativelyshort periods of time and provides reliable high-resolutiondata through space and time (Brandt 1996). Hydroacousticresults provide a 3-dimensional picture of fish location anddensity (Simmonds and MacLennan 2005); however, simul-taneous active fish sampling is recommended to collect dataon species composition and fish size (Guillard 1998, Aglenet al. 1999, Yule 2000, Spinelli 2010, Jurvelius et al. 2011).
In this study we used hydroacoustics and gillnet samplingto evaluate vertical and seasonal fish distribution near thespillway area and water intake in the Tres Marias reservoirto access the feasibility of promoting downstream passageusing the available spillway.
Materials and methodsStudy area
The Tres Marias Hydropower Plant is located on theSao Francisco River, Minas Gerais, Brazil (18◦12′51′′S,45◦15′51′′W). The region experiences 2 distinct seasons:a rainy summer season (Oct–Mar) and a dry winter sea-son (Apr–Sep). The average annual rainfall ranges from
Figure 1. Spillway flow (m3/s.) at Tres Marias Power Plant betweenJan 2002 and Jan 2012.
1200 to 1300 mm, and the average annual temperature is21.9 C (Esteves et al. 1985). The wettest quarter (Nov–Jan)contributes about 55–60% of the annual total precipitation,while the driest quarter (Jun–Aug) contributes <5% of theannual precipitation (ANA 2009). The reservoir is classi-fied as warm monomictic (Esteves et al. 1985) with thermalstratification occurring during summer.
The Tres Marias Hydropower Plant was built during1957–1960 and began operation in 1962 to regulate theSao Francisco River, improve navigation, and provide floodcontrol, irrigation, and hydropower production. The damlength is 2.7 km and has a maximum height of 75 m. Atmaximum water level, the flood zone covers 1050 km2 andhas a water volume of 21 × 109 m3. Maximum depths are>60 m near the turbine intakes (Sampaio and Lopez 2003)and 18 m near the spillway gates. Reservoir depth usuallydoes not vary by more than 10 m within a year. Thereare at least 8 free-flowing tributaries upstream from thereservoir.
The installed capacity is 396 MW from 6 Kaplan turbineswith a maximum discharge of 150 m3/s each. There is norelevant dial or seasonal variation in the turbines operation.The dam has a controlled surface spillway with total dis-charge up to 8700 m3/s (CEMIG 2006). Despite the largereservoir size, spillover is common during the rainy season(Fig. 1).
There is no fish passage facility at this dam, but as partof a government agency requirement, there are currentlystudies being held to evaluate the need for an upstream fishpassage.
Hydroacoustic data
Hydroacoustic sampling was conducted immediately up-stream from the spillway gates and turbine intakes between
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Figure 2. Hydroacoustics and gillnet sample areas in the Tres Marias reservoir.
December 2010 and December 2011 (Fig. 2). Hydroacous-tic data were collected fortnightly during the rainy seasonand monthly during the dry season. Echo sounding transectswere always performed for a duration of 10–15 minutesat 6:00, 12:00, 18:00, and 24.00 h, covering crepuscularperiods, days, and nights at 6 h intervals between each as-sessment. Data for all 4 time periods were collected onthe same day, totaling 72 assessments over 18 samplingsdays.
Hydroacoustic sampling was conducted with a BioSonicsDT-X Digital Scientific echo sounder equipped with a DT-XSplitbeam Digital Transducer 6◦, 120 kHz. The echo sounderwas installed in a boat on a lateral structure 0.5 m below thewater surface with the sonar beam oriented vertically to-ward the bottom of the reservoir. This configuration allowedsampling of the entire water column (surface to bottom). ADifferential Global Positioning System (D-GPS) was cou-pled to the echo sounder with a high sensitivity Garmin17x NMEA 0183 HVS antenna. The sample areas were tra-versed with a vessel adapted for acquiring acoustic data. Weperformed 4 parallel transects, each with a length of 150 mand 50 m spacing, in the area near the spillway, and 8 par-allel transects, each with a length 150 m and 40 m spacing,in the area near the turbine intakes. While carrying out thetransects, the echo sounder emits and receives a number ofhydroacoustic signals that vary according to the expected
maximum depth (5 pulses per second in the spillway areaand 12 in the water intake area).
BioSonics Visual Acquisition 6.0 Software (BioSonics2010) was used for reception, viewing, and storage of data.The equipment was calibrated with a tungsten carbide sphereaccording to procedures recommended by Foote et al. (1987)and BioSonics (2004). Degree of coverage and coefficient ofvariation (values that certify the sampling design efficiency)and transect number, spacing, and length were evaluated andproposed according to Simmonds and MacLennan (2005)and Parker-Stetter et al. (2009). The values used to configurethe equipment for hydroacoustic data acquisition followedthe recommendations by Simmonds and MacLennan (2005)and Parker-Stetter et al. (2009) (Table 1). The hydroacousticdata collected were analyzed in Echoview software version4.8 Myriax. Fish abundance was determined by echo count-ing of fish tracks and echo integration.
Echo counting was used when individual fish echoes didnot overlap (Simmonds and MacLennan 2005). The soft-ware used the Blackman (1986) algorithm for detecting fishtracks. A fish trace requires at least 3 individual echoesof the same fish at a maximum distance of 3 pings fromeach other. Targets with strength values higher than −65 dBwere considered to be fish because values for these organ-isms usually range from −25 to −65 dB (Brandt 1996).
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Table 1. Sampling design and settings for data collection in the Tres Marias Hydropower Plant reservoir using hydroacoustics as a tool.
Values
Characteristics Spillway Water intake
Each transect length (m) 150 150Number of transects per sampling 4 8Spacing between transects (m) 50 40Degree of coverage 7.950 6.98Coefficient of variation 0.25 0.25Transect design ParallelPulse Duration 0.4 msAcoustic energy threshold −100 dBPings per second 5 12Beam width 3dB and −3dB 6◦ × 6◦
Mean values of target strength were calculated for each fishtrace.
When high fish agglomerations or schools were found, weadopted the echo integration technique (Taylor and Maxwell2007), which allows quantification when fish are concen-trated. Density per aggregation area was calculated accord-ing to the equation proposed by Parker-Stetter et al. (2009),which relates aggregation area and target strength mean inthe sampling (Loures 2011), allowing estimation of the num-ber of fish in the study region.
Gillnet sampling
The day following the hydroacoustic survey we conducteda fish collection survey using 3 sets of gillnets, each set withmesh sizes varying from 3 to 16 cm between opposite knotswith a total area per set of 160 m2. The gillnets were placedbetween the hydroacoustic sample sites (Fig. 2), supportedby buoys and fenders. Each gillnet set was located at 1 of3 depths <5 m, between 10 and 20 m, and >40 m. Gillnetswere set for 24 h and checked every 12 h at 18:00 and 6:00 h.
Catch per unit effort (CPUE) was calculated by dividing thenumber of captured individuals by the sampling effort (m2
gillnet) and multiplying by 100. The CPUE was calculatedfor each collection period, which allowed for both nighttimeand daytime estimates.
Fish were anesthetized with Indian clove oil, sacrificed,fixed in 10% formalin, and identified according to Britskiet al. (1984). Samples were later preserved in 70% alco-hol, and control samples were sent to the Federal Univer-sity of Lavras Ichthyologic Collection- CI-UFLA. Beforebeing preserved, migratory fish, including short-distancespecies, were macroscopically analyzed for maturationstages according to the scale proposed by Vazzoler (1996): 1
(immature or resting), 2 (initial maturation), 3 (advancedmaturation/mature), and 4 (spent).
Abiotic data
Water quality field measurements were collected during eachof the 18 sampling periods. We used a multiparameter probe(YSI 556 MPS) to measure surface water temperature, dis-solved oxygen, and pH in the Tres Marias Reservoir. Watertransparency values were obtained with a Secchi disk. Spillflow, turbine discharge, reservoir level, and rainfall data foreach sampling period were obtained at Cemig GT, the gen-eration and transmission unit of Brazil’s Minas Gerais stateintegrated power company.
Data analysis
We evaluated the relationship of the fish track depth fre-quency between the spillway and water intake samplingareas using Spearman correlation. The difference betweenaverage depth of fish tracks in surveys performed duringthe day (6:00 and 12:00 h) and night (18:00 and 24:00 h)was assessed with the nonparametric Wilcoxon 2-sampletest. The same test was used to assess the difference be-tween number of fish tracks during the day and night andbetween the dry and rainy seasons. The average fish trackdepth values in the spillway were plotted with the maximumdepth of the area immediately upstream from the spillwaygates. We used nonparametric tests because data were non-normal, even after using various data transformations (Sokaland Rohlf 1995).
The relationship between operating conditions at the TresMarias Hydropower Plant during sampling, limnologicalvariables, average depth of fish tracks during the day andnight, and fish track abundance was evaluated by backwardmultiple regression. Statistical analyses were performed
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Figure 3. Box plots showing data distribution of fish track depthsduring daytime and nighttime (transects at 18:00 and 24:00 h)surveys. Z = 56.72; P < 0.01.
with Statistica software 10.0 (StatSoft 2010), with a sig-nificance level of 0.05.
ResultsFish track abundance (r = 0.81, P < 0.01) at the spillwayand water intake areas were significantly correlated; this wasalso the case for depth (r = 0.51, P = 0.01). Both regionswere considered together for further analysis. During allhydroacoustic data collections we observed diel vertical mi-gration (Fig. 3). With the exception of January, March, May,and June, the daytime median depth of fish tracks was be-low the reservoir’s spillway gates maximum depth. At nightthe median fish track depth was always above the spillwaygate maximum depth (Fig. 4). The depth and number offish tracks during the day and night was not significantly re-lated to limnological variables (temperature, dissolved oxy-gen, and pH) or the operating conditions at Tres MariasHydropower Plant (P > 0.05).
We found a significantly greater number of fish tracks atnight (Z = 3.41; P < 0.01). There were a greater number offish tracks in the rainy season compared to the dry season, butnot significant (Z = 0.9; P = 0.45). May showed the highestconcentration of fish in the dry season, and December hadthe highest concentration of fish in the wet season (Fig. 5).Hydroacoustic target strength values for fish tracks rangedbetween −64.8 and −25.3 dB, indicating the study area hada large range of fish size.
Gillnet sampling yielded 520 individuals from 29 speciesthat belong to 11 families and 5 orders (Table 2). Ofthese species, Leporinus reinhardti, L. obtusidens, Salmi-nus hilarii, and Pimelodus maculatus are known for their
migratory movements during the breeding period, even ifonly for short distances (Godinho and Godinho 2003). Only2 individuals of Leporius obtusidens were captured, so theywere not included in the analysis.
CPUE values were low from April to July and increasedin the rainy months, but no correlation was found betweenCPUE and the number of fish tracks detected. During these2 months, nighttime CPUEs were greater (Fig. 6). No signif-icant relationships were found between fish abundance andlimnological variables (P > 0.05).
Migratory species L. reinhardti and P. maculatus wereamong the most frequently caught species at night (Fig. 7).During the rainy season, most of the individuals of thesespecies and S. hilarii were at the spent maturation stage.In addition, most immature or resting individuals were cap-tured after June (Fig. 8).
DiscussionOur study provides guidance that could maximize the TresMarias spillway usage timing to facilitate downstream fishpassage. After opening the reservoir spillway at Tres Marias,fish became trapped in the spillway chute (the area betweenthe gates and plunge pool). From April 2011 to March 2012,45 individuals of 6 species, most of them migratory, wererescued and released at least 20 days after being trapped. Thefishes had either minor bruises or minor scale loss, and nonewere dead or seriously injured (IG Prado, pers. observ.).
The high survival rate and low incidence of injuries indicatethe feasibility of fish passage through the spillway. At theSanta Clara dam on the Mucuri River, 93% of recapturesdownstream from the dam occurred when the spillway wasopen; only 7% of these individuals had injuries, and no deadfish were collected (Pompeu et al. 2004). Spinelli (2010)noted that about 9% of the fish stock estimated for a studyarea in the Hauser Reservoir, Montana, moved downstream.Ruggles and Murray (1983) reported no salmon deaths fromfish passage through smaller spillways, or ∼2% from struc-tures with a >90 m free fall. Agostinho et al. (2007a) suggestthat the survival rate of fish passing through spillways is gen-erally 97−100%. Other studies have found similar results(Skalski et al. 1998, Schilt 2007, Larinier 2008); however,Bickford and Skalski (2000) suggest that spillway designand the fish species present are important determinants ofsuccessful passage.
Site-specific analysis should be conducted to evaluate thepotential negative effects of spillways on fish passage,including the effect of barotrauma, because neotropicalspecies assemblages are different from those that occur inNorth America and Europe (Brown et al. 2014). Delayed
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Figure 4. Box plots of fish tracks depths during the day (6:00 and 12:00 h) and night (18.00 and 24:00 h) in the sampling months (1 = Jan,2 = Feb, etc.). The solid line shows the maximum depth immediately upstream from the spillway gates at Tres Marias Power Plant.
mortality related to collisions, notably with physical struc-tures, should be better evaluated. In addition to young andadult fish, some recent studies have reported the possibilityof eggs and larvae successfully passing through turbines orspillways. Their passage has been considered unfeasible inbig reservoirs because they seem to disappear in the lenticsection of the impoundment (Pompeu et al. 2004, Suzuki
et al. 2011). Further study on these initial fish stages couldyield important information on the actual role of these alter-native routes in maintaining environmental quality.
Although fish that pass upstream may be able to migratethrough reservoirs (Antonio et al. 2007), there are multiplebarriers preventing successful downstream movement. Fish
Figure 5. Fish track frequency on the surface (up to 15 m), at intermediate depth (between 15 and 30 m), and at the bottom (below 30 m)per collection time (6:00, 12:00, 18:00, and 24:00 h) during the months of hydroacoustics data collection at the Tres Marias reservoir.X-axis is by successive month, starting with January.
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Table 2. List of fish species sampled in Tres Marias reservoir, Sao Francisco River basin from December 2010 to December 2011, withpopular name, total number (n), relative abundance of individuals (%), and voucher number.
Figure 6. Catch per unit effort (CPUE) during the day and at nightin months of data collection with gillnets in Tres Marias reservoir.X-axis is by successive month, starting with January.
must travel across huge lentic environments, and rheophilicbehavior likely precludes downstream movements (Pompeuet al. 2012); however, fishes, including migratory species,were found throughout the year at the reservoir close to TresMarias Dam. The presence of several long, free-flowingtributaries upstream from the reservoir, with some draininginto the region near the dam, may explain the presence ofmigratory fish found in this study.
Downstream fish passage, which has been particularly ne-glected in neotropical countries (Pompeu et al. 2012), isessential to achieving the ecological goals of the fish passes(Pelicice and Agostinho 2008). The possibility of returningtransposed individuals, as well as eggs, larvae, and juve-niles, is a minimum requirement for the ecological successof these systems (Agostinho et al. 2007b).
Little information is known about downstream migrationpatterns. The few studies evaluating these migrations sug-gest that downstream passage of adult fish is either insignif-icant (Pompeu et al. 2004, Makrakis et al. 2007, Agostinhoet al. 2011) or absent (Agostinho et al. 2007b, Lopes et al.2007, Pelicice and Agostinho 2008). Assessing the magni-tude and efficiency of successful adult downstream migra-tion is a critical first step toward developing managementstrategies for minimizing dam impacts on fish (Pelicice andAgostinho 2008).
Spillways have been considered a possible route for down-stream fish passage, yet studies have suggested downstreampassage is low. For example, of the 15,000 individual fishesmarked and manually moved from the river upstream to thereservoir in east Brazil, only 29 were recaptured downstreamfrom a dam (Pompeu et al. 2004). With such a potentially
Figure 7. Number of individuals of the 10 most abundant species captured with gillnets in Tres Marias reservoir per period and depth.∗Indicates migratory species.
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Figure 8. Stage of gonadal maturation of the most abundant, short-distance migratory species collected in gillnets in the Tres Mariasreservoir. M = male and F = female. 1 (immature or resting), 2 (initial maturation), 3 (advanced maturation/mature), and 4 (spent).
low rate of downstream passage, any reservoir managementadjustments to enhance successful passage would be ben-eficial. High fish abundance in the rainy season may in-dicate that in Tres Marias success of downstream passagethrough spillway openings could be expected during thisperiod, which coincides with the breeding season of mostspecies of the Sao Francisco River Basin (Alves et al. 2011).After spawning, fish could migrate downstream. The diurnalchange in average fish track depth corresponds with estab-lished behavior of fish moving to shallow water at night andto greater depths during the day. The average depth of fishtracks was below the maximum depth of the spillway areaduring the day; therefore, daytime spillage is anticipated tohave little effect on successful downstream passage. Night-time use of the spillway may have greater success becausethe fish are already at the outlet depth.
The target strength (TS) values of the fish tracks representindividuals sized 2–200 cm long by the transformation pro-posed by Love (1971) and Loures (2011). The highest TSvalues indicate that spillage could possibly pass big fishes,
including post-spawning adults. The lowest may representboth small-species adults and large-species juveniles; how-ever, the passage of larvae and juveniles represents a majorchallenge to maintaining the proper functioning of fish com-munities (Suzuki et al. 2011) and could not be evaluated inour study.
Although fishes were captured by gillnets in all the surveys,in contrast to other studies (Emmrich et al. 2012) their abun-dance was not correlated with the fish track quantity. Thelack of correlation may be due to the greater number of fishtracks occurring at a depth where Emmrich et al. (2012)also noted a lower correlation between gillnet surveys andhydroacoustic based fish biomass estimates. A lack of corre-lation between gillnet biomass and hydroacoustics was alsofound by Loures (2011) in the shallower Tres Marias tailrace(maximum depth = 13 m).
Most of the migratory species of the upper Sao Franciscobasin were registered near Tres Marias dam. Pimelodus mac-ulatus, the second most abundant species captured in gill
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nets, is frequently a predominant species in South Americanfish passageways (Oldani and Baigun 2002, Silva 2008, Biz-zoto et al. 2009). Leporinus reinhardti, L. obtusidens, andS. hilarii were captured mostly at night, leading us to predicta greater spillway passage efficiency of these fish at night.Catching other migratory species (Prochilodus argenteus,P. costatus, Salminus franciscanus, and Brycon orthotaenia)is still common throughout the reservoir, especially duringthe rainy season (IG Prado, pers. observ.).
Tres Marias Hydropower Plant has a term of conduct ad-justment with the Public Prosecutors requiring that a studybe conducted to assess the feasibility of installing a fishway.Our results suggest that the opening of spillway gates maybe an alternative to increase downstream passage; however,we acknowledge that this alone is not sufficient to sustainmigratory fish populations.
The use of hydroacoustics for assessing fish movement, dis-tribution, and habitat preference seems to be highly efficient,mainly due to its low selectivity and possibility of evaluatingdepths and fish distribution throughout the water column. Inaddition, it is an easily applicable, noninvasive method. Dis-advantages are lack of accurate species identification andreal size and biomass assessment of fish, as well as lim-ited assessment of gender and gonadal stage; however, theseresults can be obtained by concurrently collecting fish.
The increasing demand for hydropower projects also in-creases the urgency and need for further studies on howbest to mitigate the impacts of these dams on the aquaticcommunity, including fish. For power plants already in op-eration without fish passes, the opening of spillway gatesmay initially be an economically and operationally feasi-ble alternative. While releasing water through the spillwayrather than the turbines represents a potential loss of powerrevenue, this must be balanced with the effectiveness of thespillway and additional cost of installing a fish pass. Sim-ilar studies to those conducted at Tres Marias Power Plantshould be performed at other dams to confirm the possibilityof using the spillway as a safe downstream passage. For thispurpose, hydroacoustics combined with active collections(cast nets, gill nets, and trawl nets) is a highly efficientmethod. We also recommend a well-designed and well-funded study to investigate the value of spilling for fishpassage, taking into account both immediate and delayedmortality and injuries due not just to physical damage, butalso diseases and predation related to disorientation.
AcknowledgmentsWe thank Igor Boratto for helping with the data collectionand Raquel Coelho Loures for support since the beginning ofthe project. We are thankful to the Brazilian Environmental
Ministry and the Federal University of Lavras for the satelliteimage of the sampled area and to Ludimilla P. Zambaldi fordrawing the map. We also thank the editors and reviewersfor their important comments on the manuscript.
FundingWe thank the Peixe Vivo Program of CEMIG for providingfinancial support. Also, PS Pompeu was awarded a researchproductivity grant (CNPq No. 306325/2011-0) and a Mi-nas Gerais researcher scholarship grant (FAPEMIG PPM-00237/13).
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