16,183 research outputs found
Detecting fish aggregations from reef habitats mapped with high resolution side scan sonar imagery
As part of a multibeam and side scan sonar (SSS) benthic survey of the Marine Conservation District (MCD) south of St. Thomas, USVI and the seasonal closed areas in St. Croix—Lang Bank (LB) for red hind (Epinephelus guttatus) and the Mutton Snapper (MS) (Lutjanus analis) area—we extracted signals from water column targets that represent individual
and aggregated fish over various benthic habitats encountered in the SSS imagery. The survey covered a total of 18 km2 throughout the federal jurisdiction fishery management areas. The complementary set of 28 habitat classification digital maps covered a total of 5,462.3 ha;
MCDW (West) accounted for 45% of that area, and MCDE (East) 26%, LB 17%, and MS the remaining 13%. With the exception
of MS, corals and gorgonians on consolidated habitats were significantly more abundant than submerged aquatic vegetation (SAV) on unconsolidated sediments or unconsolidated sediments. Continuous coral habitat was the most abundant consolidated habitat for both MCDW and MCDE (41% and 43% respectively). Consolidated habitats in LB and MS predominantly consisted of gorgonian plain habitat with 95% and 83% respectively. Coral limestone habitat was more abundant than coral patch habitat; it was found near the shelf break in MS, MCDW, and MCDE. Coral limestone and coral patch habitats only covered LB minimally. The high spatial resolution (0.15 m) of the acquired imagery allowed the detection of differing fish aggregation (FA) types. The
largest FA densities were located at MCDW and MCDE over coral communities that occupy up to 70% of the bottom cover.
Counts of unidentified swimming objects (USOs), likely representing individual fish, were similar among locations and occurred primarily over sand and shelf edge areas. Fish aggregation school sizes were significantly smaller at MS than the other three locations (MCDW, MCDE, and LB). This study shows the advantages of utilizing SSS in determining fish distributions and density
Manifestation of finite temperature size effects in nanogranular magnetic graphite
In addition to the double phase transition (with the Curie temperatures
T_C=300K and T_{Ct}=144K), a low-temperature anomaly in the dependence of the
magnetization is observed in the bulk magnetic graphite (with an average
granular size of L=10nm), which is attributed to manifestation of the size
effects below the quantum temperature. The best fits of the high-temperature
data (using the mean-field Curie-Weiss and Bloch expressions) produced
reasonable estimates for the model parameters, such as defects mediated
effective spin exchange energy J=12meV (which defines the intragranular Curie
temperature T_C) and proximity mediated interactions between neighboring grains
(through potential barriers created by thin layers of non-magnetic graphite)
with energy J_t=exp(-d/s)J=5.8meV (which defines the intergranular Curie
temperature T_{Ct}) with d=1.5nm and s=2nm being the intergranular distance and
characteristic length, respectively
Concepciones de los profesores : perspectivas para su estudio en contextos culturales diversos
Esta comunicación realiza una revisión, de diversos trabajos sobre concepciones de los profesores, para determinar tendencias que permitan abordar investigaciones sobre sus concepciones acerca del fenómeno de la diversidad cultural y sus implicaciones en la enseñanza. Se pudo establecer que en las tres perspectivas identificadas, las concepciones del conocimiento científico, profesional y escolar aportan referenciales valiosos. Este es el caso de posiciones que involucran en sus estudios y análisis aspectos históricos, sociales y políticos de actividad científica y docente. También cuentan las historias de vida, origen étnico de los participantes, como los aspectos situacionales que están involucradas en el desarrollo de la clase
Antiferromagnetic and Orbital Ordering on a Diamond Lattice Near Quantum Criticality
We present neutron scattering measurements on powder samples of the spinel
FeSc2S4 that reveal a previously unobserved magnetic ordering transition
occurring at 11.8(2)~K. Magnetic ordering occurs subsequent to a subtle
cubic-to-tetragonal structural transition which distorts Fe coordinating sulfur
tetrahedra lifting the orbital degeneracy. The application of 1~GPa hydrostatic
pressure appears to destabilize this N\'eel state, reducing the transition
temperature to 8.6(8)~K and redistributing magnetic spectral weight to higher
energies. The relative magnitudes of ordered
and fluctuating moments show that the
magnetically ordered ground state of FeSc2S4 is drastically renormalized and in
proximity to criticality.Comment: 16 pages, 12 figure
Impact of noise on a dynamical system: prediction and uncertainties from a swarm-optimized neural network
In this study, an artificial neural network (ANN) based on particle swarm
optimization (PSO) was developed for the time series prediction. The hybrid
ANN+PSO algorithm was applied on Mackey--Glass chaotic time series in the
short-term . The performance prediction was evaluated and compared with
another studies available in the literature. Also, we presented properties of
the dynamical system via the study of chaotic behaviour obtained from the
predicted time series. Next, the hybrid ANN+PSO algorithm was complemented with
a Gaussian stochastic procedure (called {\it stochastic} hybrid ANN+PSO) in
order to obtain a new estimator of the predictions, which also allowed us to
compute uncertainties of predictions for noisy Mackey--Glass chaotic time
series. Thus, we studied the impact of noise for several cases with a white
noise level () from 0.01 to 0.1.Comment: 11 pages, 8 figure
Evidence of secondary relaxations in the dielectric spectra of ionic liquids
We investigated the dynamics of a series of room temperature ionic liquids
based on the same 1-butyl-3-methyl imidazolium cation and different anions by
means of broadband dielectric spectroscopy covering 15 decades in frequency
(10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An
ionic conductivity is observed above the glass transition temperature T_{g}
with a relaxation in the electric modulus representation. Below T_{g}, two
relaxation processes appear, with the same features as the secondary
relaxations typically observed in molecular glasses. The activation energy of
the secondary processes and their dependence on the anion are different. The
slower process shows the characteristics of an intrinsic Johari-Goldstein
relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found,
as observed in molecular glasses.Comment: Major revision, submitted to Phys. Rev. Let
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