Behaviour and Locomotor Activity of a Migratory Catostomid during Fishway Passage

Fishways have been developed to restore longitudinal connectivity in rivers. Despite their potential for aiding fish passage, fishways may represent a source of significant energetic expenditure for fish as they are highly turbulent environments. Nonetheless, our understanding of the physiological mechanisms underpinning fishway passage of fish is still limited. We examined swimming behaviour and activity of silver redhorse (Moxostoma anisurum) during its upriver spawning migration in a vertical slot fishway. We used an accelerometer-derived instantaneous activity metric (overall dynamic body acceleration) to estimate location-specific swimming activity. Silver redhorse demonstrated progressive increases in activity during upstream fishway passage. Moreover, location-specific passage duration decreased with an increasing number of passage attempts. Turning basins and the most upstream basin were found to delay fish passage. No relationship was found between basin-specific passage duration and activity and the respective values from previous basins. The results demonstrate that successful fishway passage requires periods of high activity. The resultant energetic expenditure may affect fitness, foraging behaviour and increase susceptibility to predation, compromising population sustainability. This study highlights the need to understand the physiological mechanisms underpinning fishway passage to improve future designs and interpretation of biological evaluations.     

Phylogeography and the genetic structure of the land-locked freshwater prawn Macrobrachium asperulum (Crustacea: Decapoda: Palaemonidae) in Taiwan

Millions of fish ascend through fishways in South America every year, but there is no evidence that downstream passage is occurring in large dams. Based on this concern, this study monitored the fish ladder at Lajeado Dam, Tocantins River, a large Amazonian tributary. To investigate the extent of downstream movements through the fishway, ascending and descending movements were quantified between November 2007 and October 2008. Monitoring showed that the ladder is restrictive to many species in both directions, but downstream passage was much more limited: almost all fish captured in the ladder (99.5%) were ascending the facility. Consequently, the numerical ratio between ascending and descending fish (ind./m²) showed high positive values throughout the year (1508:1 on average); migratory species showed higher ratios. All results showed that downstream passage through the ladder is very limited, and that the facility allows only one-way movements (i.e., upstream). This is the first empirical evidence showing that a ladder is critically deficient for downstream passage of fishes in South America.     

Assessment of a vertical slot fishway in south‐eastern Australia designed to pass numerous species and size classes of fish

In the Barwon River, Australia, a tidal barrage formed a major impediment to fish movement so in 2013 a vertical slot fishway was installed. The assessment of fishways on tidal barriers is rare in Australia so to ensure the fishway was achieving its ecological objective (i.e. successfully passing the target size range of fish of 20–400 mm total length), fish were trapped at the entrance and exit on 12 occasions and the species composition, abundance and length of fish at the two locations were compared. Additionally, a section of the river downstream of the fishway was sampled to ensure fishway trapping accurately reflected the species composition wanting to use the fishway to move upstream. Eighteen species and 69,246 individual fish were caught in the fishway traps. Catch rates between locations did not differ for Common Galaxias (Galaxias maculatus) or Australian Smelt (Retropinna semoni), although species‐specific catch rates were lower at the exit for Flat‐headed Gudgeon (Philypnodon grandiceps), Tupong (Pseudaphritis urvillii) and Yellow‐eye Mullet (Aldrichetta forsteri). Length distribution between locations only differed for Australian Smelt with small fish under‐represented at the exit location (<25 mm total length). Eight species of fish were collected downstream of the fishway that were not collected in it; however, all of these were estuarine dependent except the non‐native Common Carp (Cyprinus carpio). Our results indicate that vertical slot fishways are a suitable design for improving river connectivity at a low head, tidal barrages in south‐eastern Australia. The study reiterates the importance of reinstating connectivity for species with obligate marine/freshwater migratory life history traits, and the indirect benefits of increased productivity made available to upstream areas.     

Evaluation of Adult White Sturgeon Swimming Capabilities and Applications to Fishway Design

Concern over passage of sturgeon barriers, has focused attention on fishway design that accommodates its swimming performance. In order to evaluate swimming performance, regarding fish ladder type partial barriers, wild adult sturgeons, Acipenser transmontanus; 121–76m fork length, were captured in the San Francisco Bay Estuary and Yolo Bypass toe drain. Hydrodynamic forces and kinematic parameters for swimming performance data were collected in a laboratory flume under three flow conditions through barriers and ramp. The experiments were conducted in a 24.4 m long, 2.1 m wide, and 1.62 m deep aluminum channel. Two geometric configurations of the laboratory model were designed based on channel characteristics that have been identified in natural river systems. At a given swimming speed and fish size, the highest guidance efficiencies of successful white sturgeon passage as a function of flow depth, flow velocity, turbulence intensity, Reynolds number, Froude number and shear velocity observed in the steady flow condition, tested with the horizontal ramp structure, occurred at an approach velocity of 0.33 ms^-1. The guidance efficiency of successful sturgeon passage increased both with increasing flow velocity and Froude number, and decreased both with the flow depth and the turbulence intensity. This study also provides evidence that tail beat frequency increases significantly with swimming speed, but tail beat frequency decreases with fish total length. Stride length increases both with swimming speed and fish total length. The importance of unsteady forces is expressed by the reduced frequency both with swimming speed and fish total length. Regression analysis indicates that swimming kinematic variables are explained by the swimming speed, the reduced frequency and the fish total length. The results emphasize the importance of fish ladder type patchiness when a fishway is designed for the passage of sturgeon.     

Sequential fishways reconnect a coastal river reflecting restored migratory pathways for an entire fish community

River regulation infrastructure has been implicated in worldwide aquatic biodiversity loss. In‐stream barriers such as weirs prevent fish migration and the impact can be particularly severe for diadromous species. Fishways are frequently installed on in‐stream barriers to reconnect migratory pathways and rehabilitate diadromous fish populations. Sequential fishways may be necessary in rivers where multiple barriers prevent migration. We monitored a coastal fish community's response to fish passage restoration at 10 predominantly low‐level weirs in the freshwater reaches of the Nepean River in southeastern Australia. Few studies have simultaneously assessed fish community passage at successive fishways within a river system. Prior to the installation of the fishways, there was a gradient of reduced species diversity in an upstream direction including the absence of many diadromous species, despite the regular inundation frequency of most weirs. Following the installation of the fishways, species diversity was still greater in the downstream monitoring sites; however, there was evidence of a positive change in fish community structure in middle and upstream sites. Most notably, three diadromous species rapidly expanded their distribution upstream and one amphidromous species expanded its downstream distribution. This study demonstrates appropriately designed successive fishways can successfully reconnect river systems for an entire fish community, encompassing species with a broad range of swimming abilities and diverse life histories.     

Physiological, energetic and behavioural correlates of successful fishway passage of adult sockeye salmon Oncorhynchus nerka in the Seton River, British Columbia

Electromyogram (EMG) radio telemetry was used in conjunction with physiological biopsy to relate prior physiological condition and subsequent swimming energetics and behaviours to passage success of 13 wild adult sockeye salmon Oncorhynchus nerka at a vertical-slot fishway on the Seton River, British Columbia. At the time of capture, plasma lactate, glucose and cortisol levels indicated that fish were not exhibiting unusually high levels of physiological stress. Very few differences existed between successful and unsuccessful fish in body size, initial plasma physiology and energy state and mean swim speed and energy use during passage. Generally, fish did not employ burst swimming during successful or failed attempts at passage, indicating that failure was probably not related to metabolic acidosis. Plasma Na⁺ concentration was significantly lower in unsuccessful fish ( P < 0·05), which is suggestive of a depressed ionic state or a possible stress component, although values in all fish were within an expected range for migrant adult O. nerka . Nevertheless, six of 13 fish failed to reascend the fishway and remained in the tailrace of the dam for more than a day on average before moving downstream and away from the dam. During this time, fish were observed actively seeking a means of passage, suggesting that there may have been other, undetermined causes of passage failure.     

The status of fishways in Canada: trends identified using the national CanFishPass database

The disruption of river connectivity through the construction of barriers used for hydropower development and water control purposes can severely damage river ecosystems, reduce the quality of fish habitat, and prevent the upstream migration of fishes. Fishways function as a means of passage around barriers for fish migrating both upstream and downstream. In 2009, the CanFishPass project was initiated in a partnership with Fisheries and Oceans Canada and Carleton University to create a searchable database containing specific information on fishways in Canada built to enable upstream passage. In this paper we evaluate the information gathered in the CanFishPass database to identify trends concerning fishways and fish passage in Canada, yielding, we believe, the first national-scale trend analysis related to fishways anywhere in the world. Although CanFishPass may not include all fishways in Canada, our analysis identified 211 which are primarily located along the coasts and along major rivers and water bodies such as the Great Lakes. British Columbia has the largest number of fishways in Canada (62) and Prince Edward Island has the fewest (2). The most popular type of fishway is the pool and weir fishway (85), followed by vertical slot (37) and Denil type fishways (23). Fishway construction has proceeded at a steady rate since the 1970’s, although there has been an increase in the number of nature-like fishways since the year 2000. The majority of fishways are installed to pass salmonids in Canada, although some fishways on warmwater systems pass large components of the fish community. Only 9 % of the fishways in Canada have been studied using methods that enable proper evaluation of biological effectiveness. We recommend that evaluations be carried out at new and existing fishways and that these evaluations enable the determination of attraction and passage efficiency. Additionally, we recommend that future fishway projects and evaluations in Canada be advised to submit details of their work to CanFishPass so that knowledge of these fishways is centralized. Similar efforts on a global scale could lead to opportunities to identify patterns in fishway design and biological effectiveness that would ultimately inform decision making and improve connectivity where deemed necessary.     

Diel pattern of fish presence at the Changzhou fishway (Pearl River,China) during the flood season

Restoration of fish populations is required in China with the use of fishways at dammed rivers, but scant biological data is available for their design and operation. The Changzhou fishway (CFW) on the Pearl River is the largest in China, providing a channel for multispecies upstream migrations. This preliminary study used a combination of fish traps and hydroacoustic detection to document the diurnal presence of fish in the CFW, with the purpose of providing essential data for a future, solid performance of this fishway. Eighteen species from a total of 1362 fish individuals were collected in the traps. Three species were dominant, namely, Squaliobarbus curriculus, Pseudobagrus vachellii and Cirrhinus molitorella, altogether sharing proportions of 74.70 and 76.26% in the daytime and nighttime, respectively. Balanced by the up‐/downward movements of each fish, they entered the fishway with net numbers of 339.9 ± 113.4 ind.h^?1, and exited with net numbers of 234.54 ± 26.66 ind.h^?1. In diel fishway use, no significant differences were observed in either species composition or individual size during a diurnal period. Fish presence at the monitored sites within the fishway was recorded primarily during the daytime, showing a particular trend towards crepuscular use at the entrance and an uninterrupted use at the exit. This initial study showed a significant fish presence in the CFW during the flood season; the outcome of the study will be the basis of future research aimed at evaluating the passage efficiency of the CFW and the use thereof in other seasons.     

Effect of flow regime hydraulics on passage performance of Iberian chub (Squalius pyrenaicus) (Günther, 1868) in an experimental pool-and-weir fishway

Dams and weirs strongly affect or interrupt the longitudinal connectivity of rivers, by limiting the movements of several fish species while altering and fragmenting habitats. Fishways, especially pool-type fishways, are used to diminish this impact by enabling fish to negotiate barriers. The aim of this work is to study the behaviour of a small-sized resident cyprinid fish, the Iberian chub—Squalius pyrenaicus (Günther, 1868)—within a prototype of a pool-type fishway, equipped with both surface notches and bottom orifices, and experiencing two different flow regimes, plunging and streaming. Results show that the fish tended to be more successful in their upstream movements under streaming flow and that, for this regime, the surface notch was the preferred route for upstream movements. Flow velocity patterns corroborated the results, showing that the plunging flow regime may have driven fish downstream, while the streaming flow regime may have attracted fish upstream. The plunging flow also prevented access of this surface-oriented species to the surface notch, effectively reducing the area available for transport. Thus, the streaming flow regime can be a sound option for this and other small-sized species, as it enhances fish movements through the fishway.     

基于视频跟踪的竖缝式鱼道内鱼类运动行为分析

竖缝式鱼道过鱼对象运动行为与鱼道池室内水力条件是否相适应是进行鱼道设计的关键。研究通过视频跟踪法对竖缝式鱼道中目标鱼的运动轨迹进行实时跟踪, 获取鱼的运动加速度、运动速度, 并和人工手动跟踪的鱼类运动轨迹进行对比, 证明基于视频跟踪法的鱼类运动分析程序既能较好的应用于竖缝式鱼道中, 获取鱼类运动行为, 又可减少大量的人工操作, 有助于为竖缝式鱼道设计提供重要基础数据。     

Selection of Iberian barbel Barbus bocagei (Steindachner, 1864) for orifices and notches upon different hydraulic configurations in an experimental pool-type fishway

Trends in fish passage studies are increasingly moving towards a holistic approach in considering the movements of a wide range of fish species and other aquatic fauna. In this context, it is important to consider not only salmonids and other game species, but also coarse species such as cyprinids. Moreover, knowledge of their behaviour when confronted with fishways is limited, particularly their upstream movements with regard to different hydraulic configurations. To address this deficit, research was conducted in an indoor full-scale pool-type fishway to assess the cyprinid Iberian barbel Barbus bocagei (Steindachner, 1864) response to the simultaneous presence of submerged orifices and surface notches with adjustable dimensions in association with two different flow regimes over the notches, i.e. plunging and streaming. Overall, orifices (76%) were the clear choice to negotiate the fishway. When the flow was in the plunging mode, a significantly higher proportion of fish selected the orifices to move upstream, but the proportions were equal during the streaming mode. The time taken to enter the fishway differed significantly according to the chosen discharge device and was lower for submerged orifices (52 s to 35 min.). The present study identified key factors in Iberian barbel upstream movements, which provide direct applications to future fishway designs for this species.     

亚成体巨须裂腹鱼游泳能力及活动代谢研究

以野生雅鲁藏布江巨须裂腹鱼(Schizothorax macropogon)为对象,通过自制的鱼类游泳实验装置,测定了4个温度(5、10、15和18℃)梯度下亚成体巨须裂腹鱼的临界游泳速度(Ucrit)及流速变化对耗氧率的影响,并通过摄像记录分析了不同游泳速度下的游泳行为。野生亚成体巨须裂腹鱼的临界游速随着温度的变化呈近似线性的递增趋势(P<0.001),4个温度下的绝对临界游速(Ucrit-a)分别为(0.88±0.07)、(1.09±0.07)、(1.24±0.15)和(1.49±0.15)m/s;若以单位时间内游过的体长倍数(BL/s)表示,相对临界游速(Ucrit-r)分别为(3.96±0.21)、(4.4±0.16)、(4.9±0.18)和(5.35±0.14)BL/s。根据不同温度及流速下耗氧率的变化情况,采用非线性拟合得到了4个温度梯度下耗氧率与游泳速度关系的幂函数模型(P<0.05)。模型表明耗氧率随游泳速度的增大而增加,且温度越高耗氧率随游泳速度的变化越显著。4个温度下的速度指数分别为2.4、2.6,2.8及3.1,表明有氧运动的效率随温度升高有所降低。在自然水温条件下(5—9℃),摆尾频率(TBF)与流速的关系呈线性正相关(P<0.001),而运动步长(Ls)的变化与流速没有显著关系,出现由高至低再升高的三个阶段。录像分析表明在流速逐渐增加的过程中,巨须裂腹鱼采用了三种不同的游泳方式,以实现降低能量消耗的目的。研究可为鱼道等过鱼设施的设计提供参考,对数量日益减少的巨须裂腹鱼保护具有较大的意义。     

White sturgeon (Acipenser transmontanus) passage at the Dalles Dam, Columbia River, USA

White sturgeon (Acipenser transmontanus) ≥95 cm TL were monitored using acoustic and radio telemetry at a large hydroelectric dam (the Dalles Dam) on the Columbia River, during March 2004 through November 2005 to determine timing and routes of passage and to characterize general movements. Transmitters were surgically implanted into 148 fish during the study; 90 were released into the tailrace and 58 into the forebay. We documented 26 passage events by 19 tagged fish: eight upstream via fish ladders and 18 downstream, mostly through open spill gates. During the study 17 fish entered the two ladders one or more times; 11 entered only the east ladder, three entered only the north ladder, and three entered both ladders at sometime. Residence time within the ladders by individual fish was variable, ranging from about 1 min to nearly 6 months (median = 7.7 h). Only six fish successfully ascended the east ladder, one fish twice. We could not unequivocally determine which fish ladder one fish used to pass upstream. Differences in construction between the north and east fish ladders may account for the greater success of the east fish ladder in passing sturgeon upstream. Changes to operations at hydroelectric dams to benefit migrating anadromous salmonids may influence upstream or downstream passage by white sturgeon. Altering patterns and timing of spill discharge, altering fish ladder entrance attraction flows, and the use of lights, sound, and partial barriers to direct other species of fish to preferred passage routes have unknown effects on sturgeon passage. A better understanding of the consequences to the metapopulation of increasing or precluding upstream or downstream passage is needed.     

Experimental quantification of the swimming performance and behaviour of spawning run river lamprey Lampetra fluviatilis and European eel Anguilla anguilla

Using a large-scale open-channel flume, the swimming ability and behaviour of individual adult European eel Anguilla anguilla and river lamprey Lampetra fluviatilis, species that exhibit anguilliform locomotion, were quantified under complex hydraulic conditions created by a 0·2-0·3 m high under- or overshot weir during four discharge regimes. Fishes were allowed to approach the weirs from both up- and downstream. All fishes passed the undershot weir, independent of discharge and direction of movement, and under high flow (mean ±S.E. 194·63 ± 6·48 l s(-1)) moved upstream against velocities that ranged between 1·75 and 2·12 m s(-1), suggesting greater maximum swimming capability than previously reported. In comparison, passage efficiency during upstream movement was lower for the overshot weir for both L. fluviatilis and A. anguilla. Downstream moving A. anguilla took longer to pass the over- than undershot weir. This study describes a methodology to attain realistic measures of swimming ability and behavioural performance required to develop multispecies fish passage criteria.     

鱼类通过鱼道内水流速度障碍能力的评估方法

鱼类通过鱼道内水流速度障碍能力的量化对鱼道设计有重要理论和实际价值,其基础是鱼类游泳能力的测定.首先对鱼类游泳能力的研究方法进行了概述总结,指出了鱼类游泳能力经典测试方法存在测定流场与自然情况相差较大的不足;分析了关键要素如鱼类行为特征、生理耗能规律及水力特性对鱼类通过水流速度障碍能力的影响;提出了分析鱼类游泳行为和能力与特征流场的关系,探讨鱼类通过水流障碍行为规律和生理疲劳恢复特征,通过研究仿自然流态下的鱼类自由游泳行为、水力计算及生理耗能的关系,构建多因素鱼类游泳能力关系式,定量评价鱼类通过鱼道内水流速度障碍的发展方向.     

基于Matlab的鱼类游泳动力学分析

鱼类游泳动力学分析研究对解决鱼道等工程应用中水力学设计方面的关键问题有着重要的意义,利用计算机技术对鱼类游泳动力学进行分析有助于研究目标鱼类的生理特性、游泳能力及其与水力环境因子的响应关系。基于MATLAB软件对我国特有鱼类鲢幼鱼进行游泳动力学分析,借助鲢幼鱼游泳时的摆尾行为,得到不同水流速度下鲢幼鱼的摆尾频率、摆尾幅度、游泳速度和加速度;对比人工计数和手动跟踪分析方法,从实际操作复杂程度和实验数据准确性的角度,分析各数据采集方法的优劣性。结果表明基于Matlab软件采用跟踪鱼的身体中线的思路能更高效的获取大量的运动参数,比如摆尾频率、摆尾幅度、游泳速度和加速度等指标。文章介绍了一种基于Matlab开发的鱼类游泳动力学分析方法,有助于为以后鱼类游泳动力学研究提供依据。     

Assessment of lake sturgeon spawning stocks using fixed-location, split-beam sonar technology

Fixed‐location, split‐beam sonar technology was used successfully to identify adult lake sturgeon Acipenser fulvescens as they moved upstream and downstream for spawning in the Sturgeon River, Michigan, May–June 2004. A Hydroacoustic Technology Inc. Model 241 Split‐Beam Echo Sounder operating at 200 kHz and a single 4 × 10° elliptical‐beam transducer with a near field range of 1.7 m set perpendicular to the river flow was used. Data collected from migrating lake sturgeon included direction of movement, swimming speed, range from transducer, time and date of passage, and target strength. The spawning population of lake sturgeon was estimated to be at 350–400 fish, with almost equal numbers of fish seen moving upstream as downstream. Most fish were recorded moving within the mid‐section of the river, 1.5–1.65 m deep, and swimming speeds upstream were slower than those for downstream moving fish. These results show that spilt‐beam sonar can be applied to lake sturgeon assessments, without the stress of actually handling these large, pre‐spawning fish.     

Three-dimensional migratory behaviour of European silver eels (Anguilla anguilla) approaching a hydropower plant

The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.     

Diet and feeding habits of predatory fishes upstream and downstream of a low-level weir

A study investigating the influence of a low-level weir on the diets of three migratory percithyid species was undertaken on the Murrumbidgee River, Australia. A combination of fish community sampling and stomach flushing determined that Yanco Weir substantially impacted upon the abundance and feeding habits of golden perch Macquaria ambigua , Murray cod Maccullochella peelii peelii and trout cod Maccullochella macquariensis . The relative abundance of all three species was significantly greater downstream of the weir, where individuals attempting upstream migrations were obstructed. In areas of fish aggregations, some species displayed altered feeding strategies and exploited different prey taxa upstream and downstream of the weir. Diet overlap, and the proportion of individuals with empty stomachs, was also substantially greater from downstream zones. These results suggest that competition among species may be greater in areas of increased predator abundance and some species could be partitioning resources to minimize competitive interactions. Reducing accumulations of predatory species, by providing suitable fish passage facilities, would be an effective means to prevent such trophic interactions occurring at other low-level weirs.     

Water velocity shapes fish movement behavior

Stream and river ecosystems present fluvial fishes with a dynamic energy landscape because moving water generates heterogeneous flow fields that are rarely static in space and time. Fish movement behavior should be consistent with conserving energy in these dynamic flowing environments, but little evidence supporting this hypothesis exists. Here, we tested experimentally whether three general movement behaviors—against the current, with the current, or holding position (i.e., staying in one position and location)—were performed in a way consistent with minimizing the cost of swimming in a heterogeneous flow field. We tested the effects of water velocity on movement behavior across three age classes (0, 1, and 5 years) of two different fluvial specialist fishes, the pallid sturgeon (Scaphirhynchus albus) and shovelnose sturgeon (Scaphirhynchus platorynchus). Individuals from the three age classes were exposed to a continuous and dynamic velocity field ranging from 0.02 to 0.53 m s⁻¹, which represented natural benthic flow regimes occupied by these species in rivers. Both sturgeon species exhibited the same pattern with regard to their tendency to hold position, move upstream, or move downstream. Moving downstream was positively associated with velocity for all age groups. Moving upstream was inversely related to velocity for young fish, but as the fish aged, moving upstream was not related to water velocity. The oldest fish (age 5) moved upstream more frequently compared to the younger age classes. Holding position within a water current was the most frequent behavior and occurred with similar probability across the range of experimental velocity for youngest fish (age 0), but was inversely related to velocity in older fish. Our experiment across age classes suggests that the suite of swimming behaviors exhibited by fluvial specialists might have evolved to mitigate the energetic costs of complex energy landscapes generated by moving water to ultimately maximize net energy gain.