Silver Perch Research Articles

Identification of neurogenic intestinal motility patterns in silver perch (Bidyanus bidyanus) that persist over wide temperature ranges.

Fish are increasingly being utilized as a model species for genetic manipulation studies related to gastrointestinal (GI) motility. Our aim was to identify whether patterns of GI motility in fish and the mechanisms underlying their generation are similar to those recorded from mammals (including humans). The entire intestine was removed from euthanized adult Silver Perch (n=11) and lesioned at the midway point to obtain two equal lengths. Proximal and distal segments were studied separately in organ baths with oxygenated Krebs solution, maintained at either 15°C (n=5) or 25°C (n=6). Motility was analyzed during rest, after oral infusion of Krebs solution, and after application of hexamethonium (100µM) and tetrodotoxin (TTX) (0.6µM). Antegrade and retrograde propagating contractions (PC) were recorded in all preparations. In the proximal intestine, at 15 and 25°C, retrograde PCs occurred at 2.7 [1.7-4.5] and 3.1 [1.6-6.5] times the frequency of antegrade PCs, respectively. Colder temperatures did not inhibit PC frequency. Hexamethonium did not inhibit PC, and however, TTX abolished all contractile activity. Both neurogenic antegrade and retrograde propagating contractions occur throughout the intestine of Silver Perch. However, unlike the mammalian colon, these motor patterns do not require enteric nicotinic transmission and they are not inhibited by cold temperatures (15°C). Therefore, while the GI motility patterns in Silver Perch resemble those recorded from the colon of mammals, there may be differences in the mechanisms that underlying their generation.

Alien pathogens and parasites impacting native freshwater fish of southern Australia: a scientific and historical review

ABSTRACT Native freshwater fish of Australia have a diverse but largely undescribed endemic pathogen and parasite fauna. However, due to long-shared evolutionary histories and virulence/transmissibility trade-offs, effects of these endemic pathogens and parasites appear to be subtle: significant impacts are rarely observed and epizootics have not been recorded. In contrast, a number of alien pathogens and parasites are now established across southern Australia, causing manifestly harmful effects to native fish species and known or suspected epizootics in native fish populations. Undetected and/or undescribed alien viral pathogens are also suspected of being present. Alien pathogens and parasites were introduced to Australia with imports of live alien fish or their fertilised eggs. A review of the scientific and historical evidence indicates that they have had, and continue to have, greater impacts on native fish species than previously realised—especially for freshwater species. This review also documents a previously unknown, Murray-Darling-Basin-wide epizootic of Murray cod Maccullochella peelii in 1929–30, which may have contributed to strong declines in the related eastern freshwater cod Maccullochella ikei. A serious Chilodonella epizootic of M. peelii in 1982 is also examined. In addition, a possible role for alien viruses of the family Iridoviridae (Ranavirus and Megalocytivirus) in the general decline of the critically endangered silver perch Bidyanus bidyanus, and the rapid collapse of two specific native fish populations—upper Murrumbidgee River B. bidyanus and Shoalhaven River Macquarie perch Macquaria australasica—is suggested. It is argued that the severity of the impact of the virulent alien oomycete Saprolegnia parasitica, both historical and present day, has been underestimated. Finally, action is recommended against emerging new pathogen and parasite threats, and the extreme risk current alien fish importations pose in introducing them. These will further threaten already stressed native fish populations in southern Australia, particularly across the Murray-Darling Basin.

Environmental influences on migration patterns and pathways of a threatened potamodromous fish in a regulated lowland river network

AbstractIn riverine ecosystems, movements of biota among different parts of the river network are fundamental to the functioning and sustainability of populations, especially for fish. Globally, human activities (e.g., dam construction and altered flow regimes) have impaired riverine fish movements through reduced river connectivity, leading to well‐documented population declines. This study examined the timing and extent of movement of a nationally threatened potamodromous fish, silver perch (Bidyanus bidyanus), among different parts of the mid‐Murray River network in south‐eastern Australia using acoustic telemetry to identify the relative importance of different environmental variables (including river discharge) influencing timing and extent of movements. A total of 203 silver perch (fork length range 120–415 mm) was tagged in the Murray River. Silver perch occupied extended reaches of river (hundreds of kilometres) and multiple habitats throughout the river network, ranging from a large mainstem river to inflowing tributaries, anabranches and floodplain habitats. Movements were associated with aspects of the flow regime, such as small to moderate mainstem discharge rises and elevated tributary discharge. These results emphasise the importance of managing silver perch populations within an integrated riverscape context rather than solely focusing on reach/river scales. The inter‐relationship between silver perch movement and river discharge highlights the susceptibility of the species to river regulation, which has previously been implicated in its decline. The information on migratory behaviours in this study provides empirical evidence to support refinement and implementation of targeted restoration strategies, such as environmental flows, to assist the management and conservation of an imperilled species.

Effects of suboptimal temperatures on larval and juvenile development and otolith morphology in three freshwater fishes: implications for cold water pollution in rivers

Temperature is a key determinant that governs fish survival, reproduction, growth and metabolism. In freshwater ecosystems, anthropogenic influences have resulted in acute and prolonged temperature changes which lead to lethal and sub-lethal impacts on the biota that occupy these environments. We assessed the effects of temperature on somatic and otolith growth and development of three species of native Australian freshwater fish (silver perch Bidyanus bidyanus, trout cod Maccullochella macquariensis and golden perch Macquaria ambigua) to simulate how thermal pollution from the release of unseasonably cold water from thermally stratified dams in Australian freshwater ecosystems may impact fish at critical life-history stages. Fish (31 days post-hatch) were exposed to four temperature treatments (13, 16, 20, 24 °C) for 30 days. Low temperatures resulted in reduced somatic growth, with no growth observed in silver perch and golden perch held at 13 °C over 30 days. Somatic growth was highest at the upper temperature of 24 °C. Morphological assessment of fish size reiterated that low water temperatures resulted in reduced body size, particularly in terms of body width and head size. Low temperatures were associated with reduced otolith growth in all species, however a somatic-otolith size relationship was maintained for all species in measures of otolith weight, area, length and perimeter. The sub-lethal impacts observed in our study are likely to manifest at the population level through a reduced capacity of larvae and juveniles to avoid size-dependent predation, a narrower range of prey sources due to extended gape-limited feeding and, ultimately, poorer survival and recruitment.

Thermal plasticity of the cardiorespiratory system provides cross-tolerance protection to fish exposed to elevated nitrate

Exposure to nitrate is toxic to aquatic animals due to the formation of methaemoglobin and a subsequent loss of blood-oxygen carrying capacity. Yet, nitrate toxicity can be modulated by other stressors in the environment, such as elevated temperatures. Acclimation to elevated temperatures has been shown to offset the negative effects of nitrate on whole animal performance in fish, but the mechanisms underlying this cross-tolerance interaction remain unclear. In this study, juvenile silver perch (Bidyanus bidyanus) were exposed to a factorial combination of temperature (28 °C or 32 °C) and nitrate concentrations (0, 50 or 100 mg NO3− L−1) treatments to test the hypothesis that thermal acclimation offsets the effects of nitrate via compensatory changes to the cardiorespiratory system (gills, ventricle and blood oxygen carrying capacity). Following 21 weeks of thermal acclimation, we found that fish acclimated to 32 °C experienced an expansion of gill surface area and an increase in ventricular thickness regardless of nitrate exposure concentration. Exposure to nitrate (both 50 and 100 mg NO3− L−1) reduced the blood oxygen carrying capacity of silver perch due to increases in methaemoglobin concentration and a right shift in oxygen-haemoglobin binding curves in fish from both thermal acclimation treatments. These results indicate that plasticity of the gills and ventricle of warm acclimated fish are potential mechanisms which may provide cross-tolerance protection to elevated nitrate concentrations despite nitrate induced reductions to oxygen transport.

Revealing and Revelling in the Floods on Country: Memory Poles within Toonooba

Revealing and Revelling in the Floods on Country: Memory Poles within Toonooba

Interactions between water depth, velocity and body size on fish swimming performance: Implications for culvert hydrodynamics

Understanding how fish traverse man-made barriers (e.g. road-crossings and culverts) ensures that engineering and design guidelines achieve positive outcomes for fish communities. Water velocity, depth and fish body size are interrelated factors that influence fish passage through culverts. Velocity barriers have been a major focus of culvert remediation efforts to improve fish passage, however wide culvert designs that aim to limit high water velocities, often create shallow depths in the culvert barrel that can potentially impede fish passage. Here, we quantified how water velocity, depth and body size interact to affect fish swimming performance and their ability to traverse a 12 m culvert-scale swimming channel. Juveniles and sub-adults of three large-bodied native Australian fish species, silver perch (Bidyanus bidyanus), Murray cod (Maccullochella peelii) and eel-tail catfish (Tandanus tandanus), were chosen to represent a range of body shapes, swimming styles and capabilities. Each species was swum in nine treatments, consisting of three velocities at three water depths, with their time to fatigue and traverse success rates over 8 m of flume length quantified. We found that B. bidyanus had an exceptional probability of traverse success, with larger individuals traversing faster than smaller sized fish, but they were physically hindered by shallow water depths. The interaction between velocity and depth was non-linear and highly affected the swimming performance and traverse success of M. peelii and T. tandanus, particularly for fish >250 mm and < 100 mm. Our results demonstrate the importance of considering size class and species-specific swimming capabilities in culvert design criteria.

Sound characterization and fine-scale spatial mapping of an estuarine soundscape in the southeastern USA

Estuaries are areas known for biological diversity, and their soundscapes reflect the acoustic signals used by organisms to communicate, defend territories, reproduce, and forage in an environment that has limited visibility. These biological sounds may be rhythmic in nature, spatially heterogeneous, and can provide information on habitat quality. The goal of our study was to investigate the temporal and spatial variability of sounds in Chechessee Creek (Stns CC1 and CC2) and an adjacent saltwater impoundment (Great Salt Pond, GSP) in South Carolina, USA, from April to November 2016. Fixed recording platforms revealed that sound pressure levels (SPLs) were significantly higher in CC1 and CC2 compared to GSP. We detected some biological sounds in GSP (snapping shrimp genera Alpheus and Synalpheus, silver perch Bairdiella chrysoura, oyster toadfish Opsanus tau, spotted seatrout Cynoscion nebulosus, Atlantic croaker Micropogonias undulatus, and American alligator Alligator mississippiensis); however, biological sound was much more prevalent in CC1 and CC2. In Chechessee Creek, snapping shrimp, oyster toadfish, and spotted seatrout sounds followed distinct temporal rhythms. Using these data, we conducted spatial passive acoustic surveys in Chechessee Creek. We discovered elevated high frequency SPLs (representing snapping shrimp acoustic activity) near an anti-erosion wall, as well as increased low frequency SPLs (indicating spotted seatrout spawning aggregations) near the anti-erosion wall and at the mouth of Chechessee Creek. This study has demonstrated the utility of combining stationary and mobile recording platforms to detect acoustic hotspots of biological sounds.

Hypoxic conditions interrupt flood‐response movements of three lowland river fish species: Implications for flow restoration in modified landscapes

AbstractMovement and migration of freshwater fish for the purpose of feeding, dispersal, and spawning are essential life history components often linked to river flows. Understanding movement responses to abiotic stimuli such as river flows is crucial to inform effective management, especially considering that river regulation is contributing to global population declines. We examined the flow‐related movements of three species of lowland river fish endemic to eastern Australia (golden perch Macquaria ambigua, Murray cod Maccullochella peelii, and silver perch Bidyanus bidyanus) over an 18‐month period in a large anabranch system of the Murray River, south‐eastern Australia. The telemetry study encompassed periods of extreme low river discharge, moderate discharge rates within the normal regulated operating range for this system, and a large overbank flood event that created hypoxic blackwater conditions and led to significant fish kill events. Movement of all three species was strongly affected by discharge rates, and although the direction and magnitude of responses varied by species, river discharge in general had a significant positive effect on the scale of movement. All species initiated large‐scale directional movements in response to flooding, and some emigration occurred; however, hypoxia‐induced mortality ultimately killed a large proportion of the tagged fish. The results of the current study highlight the significant challenges of managing flow restoration in highly modified landscapes that are prone to hypoxic conditions. We reiterate the value of connectivity, especially where it enables fish to access suitable refuges during periods of poor water quality and subsequently recolonize when conditions improve.

Can the impacts of cold-water pollution on fish be mitigated by thermal plasticity?

Increasingly, cold-water pollution (CWP) is being recognised as a significant threat to aquatic communities downstream of large, bottom-release dams. Cold water releases typically occur during summer when storage dams release unseasonably cold and anoxic hypolimnetic waters, which can decrease the temperature of downstream waters by up to 16°C. Depending on the release duration, these hypothermic conditions can persist for many months. The capacity of ectothermic species to tolerate or rapidly adjust to acute temperature changes may determine the nature and magnitude of the impact of CWP on affected species. This study assessed the impacts of an acute reduction in water temperature on the physiological function and locomotor performance of juvenile silver perch (Bidyanus bidyanus) and examined their capacity to thermally compensate for the depressive effects of low temperatures via phenotypic plasticity. Locomotor performance (Ucrit and Usprint) and energetic costs (routine and maximum metabolic rate) were measured at multiple points over a 10-week period following an abrupt 10°C drop in water temperature. We also measured the thermal sensitivity of metabolic enzymes from muscle samples taken from fish following the exposure period. Cold exposure had significant depressive effects on physiological traits, resulting in decreases in performance between 10% and 55%. Although there was partial acclimation of Ucrit (~35% increase in performance) and complete compensation of metabolic rate, this occurred late in the exposure period, meaning silver perch were unable to rapidly compensate for the depressive effects of thermal pollution. The results of this study have substantial implications for the management of cold water releases from large-scale dams and the conservation of native freshwater fish species, as this form of thermal pollution can act as a barrier to fish movement, cause reduced recruitment, ecological community shifts and disruptions to timing and success of reproduction.

The effects of cold shock on freshwater fish larvae and early-stage juveniles: implications for river management.

Temperature is essential to the maintenance of optimal physiological functioning in aquatic organisms. Fish can manage natural fluctuations in temperature; however, in freshwater ecosystems acute and rapid temperature changes can originate from sources such as large dams and industrial effluents. These rapid temperature changes may induce several physiological and behavioural responses that can result in lethal and sub-lethal consequences. The present study assessed immediate sub-lethal and short-term (10days) lethal responses of three species of Australian freshwater fish larvae and early-stage juveniles to a range of different 'field-relevant' cold shocks (-4, -6, -8 and -10°C). Murray cod (Maccullochella peelii), silver perch (Bidyanus bidyanus) and golden perch (Macquaria ambigua) were tested at two age groups to elucidate the interaction between ontogeny and sensitivity to cold shock. Cold shock caused mortality and reductions in swimming ability (time to exhaustion and lines crossed) in all species of fish at both age groups. Sensitivity was correlated to the magnitude of cold shock; a 10°C drop in temperature caused the highest mortalities. Ontogeny interacted with the severity of cold shock; the younger fish experienced higher mortalities and greater impairment to swimming ability. This study demonstrates the potential lethal and sub-lethal impacts of cold shock on freshwater fish at a critical life-history stage. Understanding the impacts of cold shock will aid management of freshwater ecosystems for the benefit of fish populations, with the current study identifying critical life stages to be considered in remediation and guiding thresholds necessary to reduce the impact of cold shock on native fish populations.

Thermal acclimation offsets the negative effects of nitrate on aerobic scope and performance.

Rising temperatures are set to imperil freshwater fishes as climate change ensues unless compensatory strategies are employed. However, the presence of additional stressors, such as elevated nitrate concentrations, may affect the efficacy of compensatory responses. Here, juvenile silver perch (Bidyanus bidyanus) were exposed to current-day summer temperatures (28°C) or a future climate-warming scenario (32°C) and simultaneously exposed to one of three ecologically relevant nitrate concentrations (0, 50 or 100 mg l-1). We measured indicators of fish performance (growth, swimming), aerobic scope (AS) and upper thermal tolerance (CTmax) to test the hypothesis that nitrate exposure would increase susceptibility to elevated temperatures and limit thermal compensatory responses. After 8 weeks of acclimation, the thermal sensitivity and plasticity of AS and swimming performance were tested at three test temperatures (28, 32, 36°C). The AS of 28°C-acclimated fish declined with increasing temperature, and the effect was more pronounced in nitrate-exposed individuals. In these fish, declines in AS corresponded with poorer swimming performance and a 0.8°C decrease in CTmax compared with unexposed fish. In contrast, acclimation to 32°C masked the effects of nitrate; fish acclimated to 32°C displayed a thermally insensitive phenotype whereby locomotor performance remained unchanged, AS was maintained and CTmax was increased by ∼1°C irrespective of nitrate treatment compared with fish acclimated to 28°C. However, growth was markedly reduced in 32°C-acclimated compared with 28°C-acclimated fish. Our results indicate that nitrate exposure increases the susceptibility of fish to acute high temperatures, but thermal compensation can override some of these potentially detrimental effects.

Habitat preferences and habitat restoration options for small‐bodied and juvenile fish species in the northern Murray–Darling Basin

SummaryDegradation of instream habitats in the northern Murray–Darling Basin has occurred through numerous stressors, including siltation, clearing of bankside vegetation, intrusion of livestock and impacts of pest species. A better understanding of habitat preferences of native fish species could help guide future instream habitat restoration actions. The habitat choices of seven native fish species, juvenile Murray Cod (Maccullochella peelii), juvenile Golden Perch (Macquaria ambigua ambigua), juvenile Silver Perch (Bidyanus bidyanus), adult Murray–Darling Rainbowfish (Melanotaenia fluviatilis), adult Olive Perchlet (Ambassis agassizii), adult Un‐specked Hardyhead (Craterocephalus stercusmuscarum fulvus) and adult carp gudgeons (Hypseleotris spp.) were tested in preference troughs to help inform potential habitat restoration actions in the Condamine catchment. Each species was given a choice between pair combinations of open sandy habitat, submerged macrophytes, emergent plants and rocky rubble. Habitat preferences varied between species. Murray Cod, Golden Perch, carp gudgeons and Olive Perchlets preferred structure over open sandy habitat, whilst juvenile Silver Perch, Un‐specked Hardyhead and Murray–Darling Rainbowfish did not avoid open sandy habitats. Juvenile Murray Cod preferred rocky rubble habitat over all other habitat choices. Use of complex rock piles to provide nursery habitat for Murray Cod populations is a potential restoration option. Introduction of rock could also benefit Golden Perch and carp gudgeons. Use of emergent plants, submerged macrophytes and rocky rubble for habitat restoration all appear to have merit for one or more species of small‐bodied fishes or juvenile stages of larger sized fishes. Rocky rubble or floating attached macrophytes could be viable restoration options in areas too turbid to establish submerged macrophytes. These habitat interventions would complement existing actions such as re‐snagging and provision of fish passage to assist with sustainable management of native fish populations.

Site Fidelity and Habitat Use by Young-of-the-Year Transient Fishes in Salt Marsh Intertidal Creeks

The extensive use of salt marsh creeks by nekton is widely recognized, yet few investigations have quantified the extent to which tidal migratory nekton make repeated visits to the same location within creek networks. An improved understanding of nekton movements and fidelity can improve insights into the nursery function and value of creek habitats. We conducted tag-recapture experiments with young fishes to determine movements among intertidal creeks in the North Inlet estuary, South Carolina. In 2008, 3246 fishes were tagged with coded microwire tags. In 2012, 577 fishes were tagged with passive integrated transponder (PIT) tags. Based on percentages of recaptures, creek-specific fidelity was determined for spot (Leiostomus xanthurus) (78%), silver perch (Bairdiella chrysoura) (71%), pinfish (Lagodon rhomboides) (80%), and mullets (Mugil curema and Mugil cephalus) (77%). Recaptures occurred up to 79 days after release. Rates of recapture of all tagged fishes combined varied among creeks (74% to 100%). Considering earlier research that demonstrated differences in habitat quality among local intertidal creeks, we suggest that differences in fidelity reflect differences in habitat quality among creeks. Greater rates of repeated use (higher fidelity) could help explain why some creeks consistently support more tidal migratory nekton than other creeks. Changes in the strength of fidelity over time could be a useful measure of changes in habitat quality. We suggest that site fidelity be considered as another metric to use in efforts by resource managers to identify and evaluate essential fish habitat.

Automatic detection, classification, and quantification of sciaenid fish calls in an estuarine soundscape in the Southeast United States.

In the Southeast USA, major contributors to estuarine soundscapes are the courtship calls produced by fish species belonging to the family Sciaenidae. Long-term monitoring of sciaenid courtship sounds may be valuable in understanding reproductive phenology, but this approach produces massive acoustic datasets. With this in mind, we designed a feature-based, signal detector for sciaenid fish calls and tested the efficacy of this detector against manually reviewed data. Acoustic recorders were deployed to collect sound samples for 2 min every 20 min at four stations in the May River estuary, South Carolina, USA from February to November, 2014. Manual analysis of acoustic files revealed that four fish species, belonging to the family Sciaenidae, were the major sound producers in this estuarine soundscape, and included black drum (Pogonias cromis), silver perch (Bairdiella chrysoura), spotted seatrout (Cynoscion nebulosus), and red drum (Sciaenops ocellatus). Recorded calls served as an acoustic library of signature features that were used to create a signal detector to automatically detect, classify, and quantify the number of calls in each acoustic file. Correlation between manual and automatic detection was significant and precision varied from 61% to 100%. Automatic detection provided quantitative data on calling rates for this long-term data set. Positive temperature anomalies increased calling rates of black drum, silver perch, and spotted seatrout, while negative anomalies increased calling rates of red drum. Acoustic monitoring combined with automatic detection could be an additional or alternative method for monitoring sciaenid spawning and changes in phenology associated with climate change.

Fish and flow management in the Murray–Darling Basin: Directions for research

SummaryEffective natural resource management requires knowledge exchange between researchers and managers to support evidence‐based decision‐making. To achieve this, there is a need to align research with management and policy needs. This project aimed to identify the flow‐related ecological knowledge needs for freshwater fish to better inform environmental water management in the Murray–Darling Basin, south‐eastern Australia. Our major objective was to provide an up‐to‐date assessment of scientific research and integrate this with the knowledge requirements of relevant managers to guide future research. We reviewed the contemporary scientific literature and engaged managers specifically responsible for delivering flows for fish outcomes via a questionnaire and workshop. Research on fishes of the MDB has generally evolved from single locations and/or times to larger spatio‐temporal scales, including multiple sites, rivers and catchments. There has also been a trend from single life stage studies to incorporation of multiple life stages and population processes. There remain, however, significant deficiencies in knowledge for most native species, many of which are threatened. Four agreed key knowledge gaps were derived from the literature review and managers’ suggestions: (i) population dynamics, (ii) movement, dispersal and connectivity, (iii) survival and recruitment to adults and (iv) recruitment drivers. To inform policy and management, managers desired timely advice, based on robust research and monitoring. Fish species of most relevance to managers were those highly regarded by community stakeholders and whose life histories and population dynamics are potentially influenced by flow. Populations of these mostly large‐bodied, angling species (e.g. Murray Cod, Golden Perch and Silver Perch) have declined, often due to river regulation and, in conjunction with managers’ priorities, are relevant candidates for research to support the management of flow to rehabilitate fish populations in the MDB.

Hydrology and water temperature influence recruitment dynamics of the threatened silver perch Bidyanus bidyanus in a regulated lowland river

Understanding the influence of extrinsic factors such as hydrology and hydraulics on recruitment provides essential insight to inform management of fish populations. The critically endangered silver perch Bidyanus bidyanus is a long-lived, potamodromous pelagophil endemic to Australia’s Murray–Darling Basin. Declines of this species are often attributed to river regulation, although quantitative studies linking hydrology and hydraulics to key aspects of its life cycle are sparse. We used a multidecadal age-structured dataset collected from a locally abundant population of silver perch to quantify the relative importance of different abiotic drivers on year-class strength (recruitment). Silver perch recruited across highly variable hydrological conditions. The strongest year classes were associated with a combination of low to average river discharge (i.e. within channel) and high water temperatures over the peak spawning period, followed in the next year by extended high flows and widespread flooding that promoted survival of age-1+ juvenile fish. We suggest that conditions affecting the growth and dispersal of juvenile fish, in addition to the spawning period, are critical in governing recruitment dynamics. This highlights the need for multiyear flow plans for freshwater fish populations.

Tolerable ranges of fluid shear for early life-stage fishes: implications for safe fish passage at hydropower and irrigation infrastructure

Egg and larval fish drifting downstream are likely to encounter river infrastructure leading to mortality. Elevated fluid shear is one likely cause. To confirm this and determine tolerable strain rates resulting from fluid shear, egg and larvae of three Australian species were exposed to a high-velocity, submerged jet in a laboratory flume. Mortality was modelled over a broad range of strain rates, allowing critical thresholds to be estimated. Eggs were very susceptible to mortality at low strain rates and 100% of golden and silver perch died once strain rate exceeded 629 and 148s–1 respectively. Larvae were less vulnerable than eggs, but mortality increased at higher strain rates and at younger ages. Most ages of larvae will be protected if strain rate does not exceed 600s–1, although a lower guideline of less than 400s–1 may be needed in areas where very early stage Murray cod larvae drift. Golden perch and silver perch were not susceptible to shear once maturity reached ~25 days post-hatch (nearing juvenile metamorphosis). The thresholds described here will prove useful when refining design and operational guidelines for hydropower and irrigation infrastructure to improve fish survival.

Effects of tag type, morphological location and tagger experience on tag retention rates in freshwater fishes

Mark–recapture of fish is often used to inform fisheries or conservation management. Given that variability in tagging efficiencies can affect estimates of population size, it is important that rates of tag rejection are quantified. We double tagged over 45000 large-bodied, long-lived fish in a turbid lowland river in south-eastern Australia. During yearly recapture fish surveys, data on tag rejection were collected. We found that the probability of tag rejection varied as a function of fish species, tag type and tagger experience. Floy tags were more likely to be rejected as dart tags in large-bodied golden perch Macquaria ambigua (6 and 3% respectively), silver perch (18 and 4% respectively) and trout cod (19 and 7% respectively), whereas rates of rejection in the large-bodied Murray cod Maccullochella peelii were similar for both tag types (12 and 10% respectively). More experienced taggers had lower rates of rejection, which varied between 1 and 48% for individual taggers. We trialled three morphological locations for passive integrated transponder (PIT) tagging of fish, with rejection rates lowest for tags implanted in the stomach cavity and highest for those implanted in the pectoral musculature. This study presents the first tag rejection rates for dart, floy and PIT tags for lowland Australian river fish, information that can help guide future monitoring programs.

Phylogenetic analysis of sooty grunter and other major freshwater fishes in the suborder Percoidei based on mitochondrial DNA

Perciformes is the largest order of fishes and vertebrates. Sooty grunter (Hephaestus fuliginosus) is an economic fish species in the Terapontidae family of Percoidei, a suborder within Perciformes. To conduct molecular-level analysis of the phylogenetic relationships between sooty grunter and major freshwater fishes in Percoidei, we analysed the entire sooty grunter mitochondrial genome sequence and obtained the mitochondrial genome information of 19 fishes from Terapontidae, Serranidae, and Centrarchidae families in Percoidei from GenBank. The complete length of the sooty grunter mitochondrial genome was 16,770 bp; it encoded 13 proteins, 2 rRNAs, 22 tRNAs, and a displacement loop (D-loop). Other than ND6 and eight tRNA genes that are encoded by the light strand, the majority of genes are encoded by the heavy strand. The sequence and distribution of sooty grunter mitochondrial-encoded genes and non-coding segment were similar to those of most vertebrates. The results of neighbour joining, maximum parsimony, and Bayesian inference analyses of the complete mitochondrial genome and six genes, including cytochrome oxidase I, cytochrome B, 12S rRNA, ND2, ND4, and ND5, were consistent. In the phylogenetic trees, fishes in Terapontidae and Centrarchidae formed monophyletic clades, whereas those in Serranidae were divided into two clades, each containing Lateolabrax and Siniperca species. Among the three freshwater fish species in Terapontidae, the freshwater Terapontidae were more closely related to jade perch than with silver perch, suggesting that freshwater Terapontidae fishes originate from marine fishes. In addition, the phylogenetic results indicated that Micropterus salmoides salmoides and Micropterus salmoides floridanus in Centrarchidae should be designated as two independent species, and Siniperca in Serranidae should be considered an independent family. The sooty grunter mitochondrial genome sequence obtained in this study could be used to conduct population genetic diversity and germplasm resource studies. Furthermore, the phylogenetic analysis results of freshwater fishes in Percoidei could provide a molecular basis for cross-breeding.