Weakfish Research Articles

Seasonal residency of adult weakfish (Cynoscion regalis) in a small temperate estuary based on acoustic telemetry: a local perspective of a coast wide phenomenon

Weakfish (Cynoscion regalis, Bloch and Schneider), an economically important species, enter estuaries during migration along inner-continental shelf habitat for use as nursery, feeding, and spawning habitat. The residency and therefore seasonal turnover of individuals in estuarine habitat is unknown. Aspects of seasonal residency in the relatively unaltered Mullica River-Great Bay estuary in southern New Jersey were evaluated during 2007 and 2008. Acoustic telemetry revealed estuarine residency of adult weakfish (273–864 mm TL) on the scale of days to several months (mean: 55 days). Egress from the estuary occurred throughout the study period but was lowest during July and highest during the final month of detection in November corresponding to seasonally declining water temperatures. Individuals maintained a constrained distribution within the estuary relative to the available range of water quality parameters with average encountered temperature (22.8 °C), salinity (30.1), dissolved oxygen (6.9 mg/L), and pH (7.8) all being higher than the annual estuary-wide mean value. The long residence time indicated the importance of the estuary as a seasonal destination, rather than intermittent resting area for some individuals and points to a diversity of individual migration patterns typical of other species from this region. Given that weakfish used small estuarine habitats for extended periods in both years of this study, estuaries such as this one may serve as important sites for spawning, feeding and other functions.

Habitat utilization patterns determine the physiological condition of Cynoscion regalis during estuarine residency

For many species, quantifying nursery function of estuarine habitats, including tidal salt marshes, for early life stages is complicated by multiple habitat shifts over their period of estu- arine residency. Our study relates the physiological condition of young of the year (YOY) of a mar- ine transient species (weakfish Cynoscion regalis) to the quality of habitats frequented during their period of estuarine residency by examining their physiological condition (energetic reserves in the form of triacylglycerol, normalized to the expected changes with length) as a function of habitat use patterns and corresponding stable isotope signatures. In addition, for C. regalis preparing to emi- grate offshore to wintering grounds, we evaluated the benefit of energetic reserves towards miti- gating over-winter starvation in the context of individual metabolic rate. We found significant spa- tial distribution in the physiological condition of YOY C. regalis over their period of estuarine residency, with those utilizing polyhaline salt marshes consistently having comparable or higher condition across all years of the study. For C. regalis preparing to emigrate to offshore, we found inter-annual variability both in the relative utilization of marsh versus open bay regions and in their associated energetic reserves. However, variability in reserves was not reflected in the potential benefit of reserves in mitigating over-winter starvation, suggesting plasticity in energy allocation for YOY C. regalis. These results demonstrate that both the habitats utilized over the period of estu- arine residency and larger-scale inter-annual variability can affect the condition and preparedness of YOY marine transients for the rigors of migration and over-wintering.

Elevated prenatal methylmercury exposure in Nigeria: Evidence from maternal and cord blood

Methylmercury is a neurodevelopmental toxicant that is globally distributed though little is known about prenatal exposures in sub-Saharan Africa. The objective of the current study was to measure total mercury levels in cord blood and maternal blood from 95 mother-newborn pairs recruited from hospitals in Nnewi, Nigeria. The secondary aims of the study were to explore if demographic and dietary factors were associated with blood mercury levels, and to explore if mercury levels were associated with any self-reported health outcome and childbirth outcome. Maternal blood mercury levels averaged 3.6μgL−1 and ranged from 1.1μgL−1 to 9.5μgL−1. Cord blood mercury averaged 5.1μgL−1 and ranged from 1.2μgL−1 to 10.6μgL−1. The mean ratio of mercury in paired cord blood to maternal blood was 1.5 and it ranged from 0.4 to 3.2. Mercury in maternal and cord blood were significantly correlated (r=0.471). More than one-third of mothers reported eating fish at least once per day, and a weak (p=0.08) fish consumption-related increase in blood mercury was found. Cord blood mercury was positively and significantly associated with birth weight and length, and head and chest circumference. Mercury levels in 36% of the participants exceeded the biomonitoring guideline associated with the United States Environmental Protection Agency (U.S. EPA) reference dose for mercury. The study shows that pregnant women and their newborns are exposed to methylmercury and that their exposures are higher compared to general populations sampled from other regions of the world.

Denitrification by cystic fibrosis pathogens – Stenotrophomonas maltophilia is dormant in sputum

ObjectiveChronic Pseudomonas aeruginosa lung infection is the most severe complication for cystic fibrosis (CF) patients. Infected endobronchial mucus of CF patients contains anaerobic zones mainly due to the respiratory burst of polymorphonuclear leukocytes. We have recently demonstrated ongoing denitrification in sputum from patients infected with P. aeruginosa. Therefore we aimed to investigate, whether the pathogenicity of several known CF pathogens is correlated to their ability to perform denitrification. MethodsWe measured denitrification with N2O microsensors in concert with anaerobic growth measurements by absorbance changes and colony counting in isolates from 32 CF patients chronically infected with the highly pathogenic bacteria P. aeruginosa, Achromobacter xylosoxidans, Burkholderia multivorans or the less pathogenic bacterium Stenotrophomonas maltophilia. Consumption of NO3− and NO2− was estimated by the Griess Assay. All isolates were assayed during 2 days of incubation in anaerobic LB broth with NO3− or NO2−. PNA FISH staining of 16S rRNA was used to estimate the amount of ribosomes per bacterial cells and thereby the in situ growth rate of S. maltophilia in sputum. ResultsSupplemental NO3− caused increased production of N2O by P. aeruginosa, A. xylosoxidans and B. multivorans and increased growth for all pathogens. Growth was, however, lowest for S. maltophilia. NO3− was metabolized by all pathogens, but only P. aeruginosa was able to remove NO2−. S. maltophilia had limited growth in sputum as seen by the weak PNA FISH staining. ConclusionsAll four pathogens were able to grow anaerobically by NO3− reduction. Denitrification as demonstrated by N2O production was, however, not found in S. maltophilia isolates. The ability to perform denitrification may contribute to the pathogenicity of the infectious isolates since complete denitrification promotes faster anaerobic growth. The inability of S. maltophilia to proliferate by denitrification and therefore grow in the anaerobic CF sputum may explain its low pathogenicity in CF patients.

Managing a Marine Stock Portfolio: Stock Identification, Structure, and Management of 25 Fishery Species along the Atlantic Coast of the United States

AbstractIn this review, stock identification methods used, resulting stock numbers and boundaries, and assessment and management context were explored for all 25 species managed by the Atlantic States Marine Fisheries Commission (ASMFC). This included invertebrates and vertebrates distributed between Maine and Florida, with a few species ranging across all these states and some ranging into the Gulf of Mexico and the Canadian Maritimes. The effects of larval dispersal or mixing of adults in the marine environment were evident. Marine and catadromous spawners were recognized and treated as a unit stock (e.g., northern shrimp Pandalus borealis, American Eel Anguilla rostrata, Atlantic menhaden Brevoortia tyrannus, Bluefish Pomatomus saltatrix, Tautog Tautoga onitis), a metapopulation (American lobster Homarus americanus, Atlantic Herring Clupea harengus), or two stocks, north and south of Cape Hatteras, a major biogeographic boundary, (Black Sea Bass Centropristis striata, Scup Stenotomus chrysops, Red Drum Sciaenops ocellatus, Summer Flounder Paralichthys dentatus). Estuarine and anadromous spawners were structured and managed at a finer spatial scale (horseshoe crab Limulus polyphemus, Atlantic Sturgeon Acipenser oxyrinchus, American Shad Alosa sapidissima and the river herrings Blueback Herring A. aestivalis and Alewife A. pseudoharengus, and Spotted Seatrout Cynoscion nebulosus). A broad suite of stock identification methods have been applied to ASMFC species and reviewed here in five categories: life history traits, other phenotypic traits, genetic traits, natural marks, and applied marks. An interdisciplinary mix of methods has been achieved for a few species (Striped Bass Morone saxatilis, Winter Flounder Pseudopleuronectes americanus), but only a few or no stock identification methods have been applied to others (Spiny Dogfish Squalus acanthias, Hickory Shad A. mediocris, Spot Leiostomus xanthurus, Spanish Mackerel Scomberomorus maculatus). Clinal phenotypic variation has contributed to several long‐standing debates about stock structure; some of these have been recently reevaluated as a unit stock (Atlantic Croaker Micropogonias undulatus, Weakfish Cynoscion regalis), and others are still debated. For some ASMFC species, other priorities (e.g., bycatch) dominate the uncertainty of the assessment or management process. Otherwise, stock identification remains a research priority for most of these species. Continued research of this subject should consider (1) research priorities tabulated by ASMFC review panels, (2) strategic use of interdisciplinary stock identification methods, (3) use of experiments or reaction norms to separate phenotypes from genotypes, (4) genetic surveys at a seascape scale, (5) demonstration of contingent (nongenetic) structure and its implications for management, and (6) simulation modeling. Obstacles to adopting finer‐scale structure into assessments or management of ASMFC fisheries include: (1) multiple stock units are apparent but boundaries are not clear, (2) monitoring requirements for smaller areas or for mixed‐stock catches are not cost effective, or (3) mixing rates within a metapopulation or across biogeographic boundaries are poorly described.Received May 8, 2013; accepted March 3, 2014

Trends in Abundance Indices of Fishes in Maryland’s Coastal Bays During 1972–2009

Maryland’s coastal bays provide habitat for juveniles of many commercially and recreationally important species of shellfish and finfish. Since 1972, the Maryland Department of Natural Resources has conducted the Maryland Coastal Bays Trawl and Seine Survey to monitor the populations of key species. The survey has undergone substantial spatial and methodological changes affecting the interpretation of simple indices of abundance. We developed generalized linear models to standardize the indices of abundance of five commonly caught fish species: Atlantic menhaden Brevoortia tyrannus, weakfish Cynoscion regalis, spot Leiostomus xanthurus, bay anchovy Anchoa mitchilli, and summer flounder Paralichthys dentatus. Density declined significantly since 1972 for menhaden, bay anchovy, and spot in at least one region within the coastal bays. The northern bays had significantly higher densities than the southern bays for all species. Changes in abundance indices of the five species examined were not related to sea grass coverage, temperature, salinity, nitrogen-to-phosphorus ratios, and other habitat variables but were likely a result of stock-wide recruitment processes.

Movement of juvenile weakfish Cynoscion regalis and spot Leiostomus xanthurus in relation to diel-cycling hypoxia in an estuarine tidal tributary

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 491:199-219 (2013) - DOI: https://doi.org/10.3354/meps10466 Movement of juvenile weakfish Cynoscion regalis and spot Leiostomus xanthurus in relation to diel-cycling hypoxia in an estuarine tidal tributary Damian C. Brady1,2,*, Timothy E. Targett1 1University of Delaware, School of Marine Science and Policy, Lewes, Delaware 19958, USA 2Present address: University of Maine, School of Marine Sciences, Walpole, Maine 043573, USA *Email: damian.brady@maine.edu ABSTRACT: Fish movement and the spatial and temporal dynamics of hypoxia determine hypoxia exposure and the effect of poor water quality on nursery habitat function. Although water quality criteria for dissolved oxygen (DO) are well defined, hypoxia exposure of juvenile estuary-dependent fishes in situ is largely unknown. Thirty-one juvenile weakfish Cynoscion regalis and spot Leiostomus xanthurus were implanted with acoustic tags. Fish were acclimated for 5 d to either DO saturation or diel-cycling hypoxia (cycling between 11.0 and 2.0 mg O2 l-1). Fish were released during summer into Pepper Creek, Delaware, an estuarine tributary. A logistic generalized additive model with generalized estimating equations was used to determine which environmental covariates significantly discriminated between movement types. Individual fish tracks were overlain on spatiotemporal contour plots of DO that highlight behavioral avoidance thresholds determined in the laboratory. Most models showed that DO, tide, and the spatial DO gradient were significant predictors of movement. Saturation-acclimated fish generally avoided DO <2.8 mg O2 l-1 by increasing swimming speed and using tidal flow to facilitate escape. An exception to tidally aided movement occurred when a flooding tide appeared to trap smaller fish in spatially extensive hypoxia. Diel-cycling hypoxia acclimated weakfish appeared to have a lower avoidance threshold of ~1.4 mg O2 l-1. Downstream movements were far more frequent than upstream movements, possibly because late stage juveniles were moving out of the estuary. The relationship between tributary residency and the spatial extent of hypoxia suggests that there is a tradeoff between risk of hypoxic exposure and energetic benefits of remaining in productive areas. KEY WORDS: Diel-cycling hypoxia · Movement · Weakfish · Spot · Acoustic telemetry · Acclimation Full text in pdf format PreviousNextCite this article as: Brady DC, Targett TE (2013) Movement of juvenile weakfish Cynoscion regalis and spot Leiostomus xanthurus in relation to diel-cycling hypoxia in an estuarine tidal tributary. Mar Ecol Prog Ser 491:199-219. https://doi.org/10.3354/meps10466 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 491. Online publication date: October 02, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

P327 Accuracy of prediction rules and risk factors for mortality in patients with hospital-acquired pneumonia

Chronic Pseudomonas aeruginosa lung infection is the most severe complication for cystic fibrosis (CF) patients. Infected endobronchial mucus of CF patients contains anaerobic zones mainly due to the respiratory burst of polymorphonuclear leukocytes. We have recently demonstrated ongoing denitrification in sputum from patients infected with P. aeruginosa. Therefore we aimed to investigate, whether the pathogenicity of several known CF pathogens is correlated to their ability to perform denitrification.We measured denitrification with N2O microsensors in concert with anaerobic growth measurements by absorbance changes and colony counting in isolates from 32 CF patients chronically infected with the highly pathogenic bacteria P. aeruginosa, Achromobacter xylosoxidans, Burkholderia multivorans or the less pathogenic bacterium Stenotrophomonas maltophilia. Consumption of NO3− and NO2− was estimated by the Griess Assay. All isolates were assayed during 2 days of incubation in anaerobic LB broth with NO3− or NO2−. PNA FISH staining of 16S rRNA was used to estimate the amount of ribosomes per bacterial cells and thereby the in situ growth rate of S. maltophilia in sputum.Supplemental NO3− caused increased production of N2O by P. aeruginosa, A. xylosoxidans and B. multivorans and increased growth for all pathogens. Growth was, however, lowest for S. maltophilia. NO3− was metabolized by all pathogens, but only P. aeruginosa was able to remove NO2−. S. maltophilia had limited growth in sputum as seen by the weak PNA FISH staining.All four pathogens were able to grow anaerobically by NO3− reduction. Denitrification as demonstrated by N2O production was, however, not found in S. maltophilia isolates. The ability to perform denitrification may contribute to the pathogenicity of the infectious isolates since complete denitrification promotes faster anaerobic growth. The inability of S. maltophilia to proliferate by denitrification and therefore grow in the anaerobic CF sputum may explain its low pathogenicity in CF patients.

Controls of plankton production by pelagic fish predation and resource availability in the Alboran and Balearic Seas

A one-dimensional coupled physical and intermediate-complexity biochemical model comprising large and small phytoplankton and zooplankton groups, particulate organic nitrogen, ammonium and nitrate was developed to study the physical–biogeochemical interactions and parameters that control plankton production in the Alboran and Balearic Sea ecosystems. The model findings suggest that pelagic fish predation and resource availability through lateral and vertical nutrient inputs jointly characterize the plankton community structures. In agreement with previous observations, a typical annual plankton structure of the mesotrophic systems involves a vertically homogeneous biomass of large groups of phytoplankton and zooplankton within the upper 50-to-100m layer from mid-November to April and a subsurface biomass accumulation distributed roughly within 25–75m depths in the following months. Their light and temperature limitations constrain the smaller groups into the thermocline zone (25–50m) during late spring and summer. These obtained results were dependent on the zooplankton actively switching between preys (i.e., the food preference coefficients dependent on prey biomass). In the case of no switching, spurious dynamic equilibrium solutions may arise in the case of a constant and weak fish predation rate and using the quadratic predation formulation. The choice of a Holling Type II (i.e., hyperbolic) predation function may, however avoid ambiguous representation of the annual plankton structure in the case of a constant food preference choice under relatively weak predation pressures.

Phylogenetic Relationships Among Four Western Atlantic Cynoscion Species Based on DNA Sequences From 11 Nuclear Introns, Two Mitochondrial Genes, and Genotypes From 32 Microsatellite Markers

Four species of Cynoscion occur in the waters off the Atlantic and Gulf coasts of North America, where they are targeted by commercial and recreational fisheries. Previous studies have not resolved the phylogenetic relationships of the four species, largely due to uncertainty as to whether the spotted seatrout, Cynoscion nebulosus, or silver seatrout, Cynoscion nothus, is the most divergent member of the North American assemblage. This study used DNA sequences from the nuclear and mitochondrial genes and multilocus genotypes from microsatellite markers to infer relationships among these species. Together, these three techniques strongly suggest that the weakfish, Cynoscion regalis, and the sand seatrout, Cynoscion arenarius, are the most closely related species, and that C. nothus is the most divergent from all the others.

Sinuolinea infections in the urinary system of Cynoscion species (Sciaenidae) and phylogenetic position of the type species of Sinuolinea Davis, 1917 (Myxozoa: Myxosporea)

Myxosporean infections that we diagnosed frequently in the urinary tract of Cynoscion nebulosus (Cuvier, 1830) and Cynoscion regalis (Bloch and Schneider, 1801) (Sciaenidae) collected in the estuarine systems of SC, USA, are described together with their etiological agent. Based on the morphology of spores and plasmodial stages, we identified the agent, in both fish host species, as Sinuolinea dimorpha (Davis, 1916), which is the type species of the genus. Based on sequences of SSU rDNA generated in this study from type host material, this species of SinuolineaDavis, 1917 has found its place in the current phylogenetic reconstruction of Myxozoa and enlarged the limited number of myxosporean genera represented in phylogenetic analyses by sequences of type species. Sequences of SSU rDNA of S. dimorpha from Cynoscion host species formed two clusters, irrespective of their host species, and also revealed differences within each cluster. These findings contribute to the acknowledgement of myxosporean cryptic species diversity, an important topic that emphasizes the general necessity of species delimitation and of continued effort to improve our knowledge of Myxosporea based on both morphology of spores and molecular data.

Fish stocking creates an ecological trap for an avian predator via effects on prey availability

In anthropogenic landscapes animals may be lured into low‐quality habitats where they survive or reproduce poorly (‘ecological traps’). I investigated breeding habitat selection in relation to intra‐seasonal changes in food availability and reproductive output in red‐necked grebes Podiceps grisegena, a size‐limited predator, of common carp Cyprinus carpio ponds. Carp farms constitute highly heterogeneous habitat mosaics due to separate stocking of different age/size fish. Pond features significant for grebe settling decisions, i.e. hydroperiod and emergent vegetation cover, had no obvious effects on prey abundance for chicks and on fledging success. Breeding grebes avoided ponds containing fish too large for them to ingest but exhibited little preference between ponds with medium‐sized one‐year‐old carp that could be exploited by pre‐laying birds, and ponds designated for young‐of‐the‐year carp, where only invertebrates and amphibians were available as prey in early spring. Red‐necked grebes settling on ponds with medium‐sized fish failed to predict future shifts in interactions with carp stocks; carp exceeded the prey‐size threshold of chicks and adversely affected their non‐fish prey levels. The resulting food shortage led to severe egg‐to‐fledging mortality rates compared to fishless ponds or those containing young‐of‐the‐year fish. This study shows that waterbirds vulnerable to competition from fish can risk maladaptive habitat selection due to unrecognised spatial and temporal variation in food resources caused by fish stocking practices. Ecological traps created by perturbations to trophic interactions may be common but difficult to detect because altered dynamics of trophic resources can affect wildlife indirectly. As with other types of ecological traps, manipulation of habitat features identified as attractive cues for settling animals, but not related to critical food resources, may help to reduce perceptual pitfalls. For example, wetland management to mitigate trap effects driven by commercially stocked fish should preserve abundant emergent vegetation in habitats with weak fish impact and extend their hydroperiod.

Modelling non-stationary natural mortality in catch-at-age models

AbstractJiao, Y., Smith, E. P., O'Reilly, R., and Orth, D. J. 2012. Modelling non-stationary natural mortality in catch-at-age models. – ICES Journal of Marine Science, 69: 105–118. Non-stationarity in the natural mortality of many species has been discussed among research scientists, but no generally applicable models/methods have been developed where a statistical catch-at-age (SCA) model framework is used. Using the Atlantic weakfish (Cynoscion regalis) fishery as an example, several SCA models are developed to assess the population dynamics, then compared. Models used included (i) an SCA with constant natural mortality, (ii) an SCA with unknown natural mortality, but with a hierarchical prior distribution from a mixture of distributions based on life-history information, (iii) an SCA with age-specific unknown natural mortality, (iv) an SCA with time-varying natural mortality, following a random-walk process, and (v) an SCA with age-specific time-varying natural mortality. The last two models imply that the population dynamics are not stationary. A Bayesian approach was used to estimate parameters, and performance of the models was compared through goodness-of-fit and the retrospective patterns of the models. A simulation study was then conducted to test the uncertainty resulting from model selection, the efficiency of using the best model selected based on deviance information criterion, and whether changes in natural mortality could be detected. An SCA with time-varying natural mortality, following a random-walk process, is proposed for the example fishery here. The estimated non-stationary temporal patterns in natural mortality were linked further to climate–ocean oscillation indices, to diagnose possible mechanisms/linkages to these temporal variations in natural mortality.

Comparing the nursery role of inner continental shelf and estuarine habitats for temperate marine fishes

Abstract The marine-estuarine transition represents an important biogeographic boundary, yet juvenile marine finfish have been observed in both temperate inner continental shelf (ICS) and estuarine habitats during the summer nursery period. In a direct comparison of ICS and estuary nurseries, spatial and temporal patterns in species composition, biodiversity, size structure, and relative abundance of age-0 fishes were tested using contemporaneous data from multiyear (2004–2006) trawl surveys of the Maryland ICS near Assateague Island, MD, and lower Chesapeake Bay, VA (estuary). Survey data from both habitats showed similar seasonal progression of assemblage structure, biodiversity phenologies, and dominant species identities. Late summer densities for four of five numerically dominant species varied by habitat. Densities of bay anchovy Anchoa mitchilli , weakfish Cynoscion regalis , and Atlantic butterfish Peprilus triacanthus were higher in the ICS; whereas summer flounder Paralichthys dentatus density was higher in the estuary. Density of spot Leiostomus xanthurus did not differ between habitats. Apparent daily growth rates of these five species, as estimated by modal length progression, were not significantly different between the two habitats. Although individual species displayed varying affinities for ICS versus estuarine habitats, this study provides strong evidence that the ICS of the Middle Atlantic Bight is capable of functioning interchangeably with polyhaline estuarine regions as nursery habitat for a diverse group of marine finfish.

Epizootic Nocardia infection in cultured weakfish, Cynoscion regalis (Bloch and Schneider)

Epizootic Nocardia infection in cultured weakfish, Cynoscion regalis (Bloch and Schneider)

A Dispersal-Dependent Zone of Introgressive Hybridization between Weakfish, Cynoscion regalis, and Sand Seatrout, C. arenarius, (Sciaenidae) in the Florida Atlantic

Five diagnostic codominant nuclear DNA markers and a diagnostic mitochondrial DNA marker were used to survey weakfish (Cynoscion regalis) and sand seatrout (C. arenarius), with particular focus on heretofore uncharacterized juvenile populations along the Florida (FL) Atlantic coast. Geographic and reproductive ranges of weakfish and sand seatrout were shown to overlap on the Atlantic coast along north and central FL. An active bidirectional zone of introgressive hybridization exists between these taxa, centered in the St Johns River, FL. Strong patterns of Hardy-Weinberg, linkage, and cytonuclear disequilibrium and a bimodal hybrid index distribution were observed for juvenile cohorts in the zone center, coupled with narrow (∼240 km) concordant clines. Parental forms had disparate habitat preferences; hybrid forms occurred predominantly in intermediate habitats. All genetic data were consistent with the hypothesis that the C. arenarius-C. regalis hybrid zone is maintained by a dynamic equilibrium between continued interspecific gene flow and one or more opposing forces. Cytonuclear analyses indicated that parental forms mate assortatively in the zone but that mate recognition was imperfect. Ethological mating dynamics are likely stabilized by some form of endogenous or exogenous postfertilization selection against hybrids such that parental taxa will likely continue to evolve independently.

Lipid class dynamics and storage depots in juvenile weakfish Cynoscion regalis and their application to condition assessment

Lipid class dynamics, the pattern of change in the primary form and location of lipid stores and their relationship with standard length (L(S) ), were investigated in collections of young-of-the-year weakfish Cynoscion regalis for the purpose of determining the utility of this analysis as an indication of condition. The separation of total lipids into individual classes and the analysis of potential storage depots revealed the general patterns of lipid class dynamics and energy storage in C. regalis during their period of juvenile estuarine residency. Phospholipid and cholesterol exhibited moderate but variable (8·1-40·0 and 1·3-21·5 mg g(-1) , respectively) concentrations across the entire juvenile period and were the predominant lipid classes in juveniles <100 mm L(S) , while wax ester concentrations were low (c. 1 mg g(-1) ) and exhibited the least amount of variability among lipid classes. Triacylglycerols (TAG) and free fatty acids (FFA) exhibited similar dynamics, with relatively low concentrations (<15 mg g(-1) ) in individuals ≤100 mm L(S) . In larger juveniles both TAG and FFA concentrations generally increased rapidly, though there was considerable variability in both measures (0·0-199·7 and 0·0-49·7 mg g(-1) , respectively). Increasing levels of lipids, primarily in the form of TAG, with size indicated an accumulation of energy reserves with growth, thus providing an indication of individual condition for larger juveniles. Separate analysis of liver, viscera and the remaining carcass indicated that liver and viscera did not represent a significant depot of TAG reserves. Analysis of samples derived from whole juvenile C. regalis thus provided an accurate estimate of energy reserves.

Modeling fish aggregation sounds in very shallow water to estimate numbers of calling fish in aggregations.

Many fishes in the Family Sciaenidae produce courtship sounds associated with spawning. Some estuarine sciaenid species such as weakfish Cynoscion regalis, spotted seatrout C. nebulosus, and silver perch Bairdiella chrysoura occur in very shallow water (water depths 3–10 m) aggregations so large that individual calls are no longer audible above the combined sound of calling fish in the aggregation. We have measured sound levels in these aggregations that are 30 dB greater than the estimated received level of an individual fish at a distance of 1 m. To estimate the number of individual calling fish that would result in these sound levels, we have produced a model of virtual fish aggregations based on echosounder data. We used a finite difference time domain (FDTD) model to calculate the pulsed sounds from the individual fish in randomized aggregations at our virtual hydrophone location. We produced many randomized instances of fish aggregations to obtain statistical distributions of sound levels produced by different densities of calling fish. The characteristics of aggregation sounds will be presented for different densities of calling fish for these species. These calculations could lead to better estimations of fish densities from single‐hydrophone measurements.

Flow seasonality and fish assemblage in a tropical river, French Guiana, South America

The objective of this study is to verify the existence of a seasonal pattern of variation in the fish assemblages of a tropical river using taxonomic and functional descriptors. Fish were sampled using gillnets at two sites on the Comté River, a large-sized river 254.8 km long, flowing entirely through rainforest areas of French Guiana. Samplings were conducted every other month from August 1998 to July 2000. Four types of fish assemblage descriptors were used: the species descriptor (number of individual fish of each species in the sample); the family descriptor (number of individual fish of each family in the sample); the trophic descriptor (distribution of the fish biomass in each feeding guild) and the specific maximum observed size - MOS (number of individual fish in each of four classes of MOS: &lt; 100 mm; 101-200 mm; 201-300 mm; &gt; 300 mm). Results point out that changes in the fish assemblage are related to water level oscillations. The role of migration seems to be weak and is limited to trophic displacements characteristic of few species. In the low-water season, characterized by weak water level oscillation, fish species and families belonging to piscivorous or aquatic invertivorous guilds were predominant, whereas in the high-water season the environment is submitted to strong variations caused by fast and large water level oscillations, and the fish assemblage was characterized by species or families with an opportunistic omnivorous diet.

Hypoxia tolerance of juvenile weakfish ( Cynoscion regalis): Laboratory assessment of growth and behavioral avoidance responses

Laboratory experiments were conducted to examine the growth rate and avoidance behavior of juvenile weakfish ( Cynoscion regalis) exposed to hypoxia (dissolved oxygen (DO) below saturation). Growth rates were measured under constant (2.0, 3.5, 5.0 and 7.0 mg O 2 L − 1 ) and diel-cycling (2.0–11.0 mg O 2 L − 1 ) DO conditions at three temperatures (20, 25, and 30 °C) over 7 days. Two-way DO choice trials were conducted over the same temperature range to 1) determine the DO level at which juvenile weakfish exhibit hypoxia avoidance and 2) examine whether temperature influences hypoxia avoidance behavior. There was no significant effect of constant hypoxia, or diel-cycling hypoxia, on growth rates of weakfish at any temperature. In the DO choice trials, weakfish avoided 1 mg O 2 L − 1 but showed no preference for DO when the low choice was ≥ 2 mg O 2 L − 1 . Temperature had no significant effect on growth or avoidance responses of juvenile weakfish. Lack of mortality during seven days exposure to hypoxia as low as 2 mg O 2 L − 1 , moderate to high growth rates, and lack of hypoxia avoidance of DO concentrations as low as 2 mg O 2 L − 1 indicates that juvenile weakfish are physiologically capable of tolerating levels of hypoxia commonly encountered in estuarine nurseries during summer months. There was no indication that hypoxia avoidance occurs at a DO level higher than that which negatively impacts growth rate in juvenile weakfish.