The Second International Symposium on Tilapia in Aquaculture

Proceedings of the symposium held in 1987 in Bangkok, Thailand, by tilapia scientists to discuss strategies for future research and development in the tilapia industry worldwide. Contains 82 full papers, 17 poster abstracts and author and species indexes. The full papers were presented under 7 sessions: culture systems, management and production; pathology; genetics and reproduction; nutrition, physiology; biology and ecology; and economics and socioeconomics.Tilapia culture, Conferences

Study of the helical aerial

1. The theory of electromagnetic wave propagation along an infinite helical conductor using (a) the Sheath Helix model and (b) the Tape Helix model has been applied to the helical aerial a result the following predictions have been made for she first time, for any pitch angle .(i) the upper frequency limit of the Endfire Helical Aerial by means of the Sheath Helix Model(ii) the upper and lower frequency limits of the Endfire Helical Aerial by means of the 'ape Helix Model.(iii) the upper and lower frequency limits of the Broadside Helical Aerial with Coaxial Conducting Cylinder using the Sheath Helix model.2. As a result of the above theoretical investigations it has been predicted and confined experimentally that the upper frequency limit of the helical aerial is not independent of length as had previously been reported. she form of this variation can be computed and the theoretical values agree with experiment to an accuracy or better than 10%.3. The prediction has been further made and confirmed that what was previously believed to be a lower limit of pitch angle of 5° for the Endfire Helical Aerial does not in fact exist. Satisfactory experiments have been carried out with pitch angles as lo',: as 1.8 °, which was the lowest physically possible at the frequency of operation used. It is believed now that there is no lower limit of pitch angle.4. It has been predicted and confirmed that the free space circumferential length C equal to unity is not the centre frequency of operation of the helical aerial in general. It is the centre frequency only for short aerials using a medium helical pitch angle. for low pitch angle helices Cλ equal to unity is well above the upper frequency limit of the aerial, and for long helices this may also be the case even for a medium pitch angle.5. It has been found that it is not always possible to neglect the effect of the ground screen on the radiation pattern of the helix. Specifically when the ground screen is (a) several wavelengths in diameter or (b) constructed of n radial wires the pattern is profoundly affected, adversely. Experiment has been limited to n≤8.6. A Tchebycheff type of current distribution has been proposed for the helical aerial. It has beer shown theoretically, using the assumption of a constant amplitude travelling wave, that the prospect of increased directivity from this distribution is severely restricted because of the linking up turns necessary. Reduced side-lobe level has been obtained however, in agreement with the above simplified theory, over the small frequency range determined by the narrow pitch turns. The measured current distribution shows large fluctuations by comparison with the simple uniform helix.7. General theoretical solutions which in principle enable the ::base velocity to be calculated, and hence the frequency limits of the aerial to be predicted, have been obtained for the first time for each of the following cases: - (i) The Sheath and Tape Helix with a Coaxial Conducting Cylinder (ii) The Tape Helix wound on a dielectric tube (iii) The Sheath Helix embedded in a dielectric medium, with a hollow coaxial tube. (iv) Two Coaxial Sheath Helices. Calculations have been carried out only in Case (i) for the Sheath Helix and approximate calculation for the Tape Helix.8. What is believed to be a new type of helical aerial - he linearly polarised Conora--Round Helix is proposed. Some measurements have been made, and the travelling-wave analysis for the simple helix has been extended to include this case.9. A critical review has been made of the theory that all End-Fire Travelling Wave aerials intrinsically possess the same pattern bandwidth

Optimal capital growth with convex shortfall penalties

The optimal capital growth strategy or Kelly strategy, has many desirable properties such as maximizing the asympotic long run growth of capital. However, it has considerable short run risk since the utility is logarithmic, with essentially zero Arrow-Pratt risk aversion. Most investors favor a smooth wealth path with high growth. In this paper we provide a method to obtain the maximum growth while staying above a predetermined ex-ante discrete time smooth wealth path with high probability, with shortfalls below the path penalized with a convex function of the shortfall so as to force the investor to remain above the wealth path. This results in a lower investment fraction than the Kelly strategy with less risk, and lower but maximal growth rate under the assumptions. A mixture model with Markov transitions between several normally distributed market regimes is used for the dynamics of asset prices. The investment model allows the determination of the optimal constrained growth wagers at discrete points in time in an attempt to stay above the ex-ante path

Optimal capital growth with convex shortfall penalties

The optimal capital growth strategy or Kelly strategy, has many desirable properties such as maximizing the asympotic long run growth of capital. However, it has considerable short run risk since the utility is logarithmic, with essentially zero Arrow-Pratt risk aversion. It is common to control risk with a Value-at-Risk constraint defined on the end of horizon wealth. A more effective approach is to impose a VaR constraint at each time on the wealth path. In this paper we provide a method to obtain the maximum growth while staying above an ex-ante discrete time wealth path with high probability, where shortfalls below the path are penalized with a convex function of the shortfall. The effect of the path VaR condition and shortfall penalties is less growth than the Kelly strategy, but the downside risk is under control. The asset price dynamics are defined by a model with Markov transitions between several market regimes and geometric Brownian motion for prices within regime. The stochastic investment model is reformulated as a deterministic program which allows the calculation of the optimal constrained growth wagers at discrete points in time

An extreme paucity of second population AGB stars in the normal globular cluster M4

Galactic Globular clusters (GCs) are now known to harbour multiple stellar populations, which are chemically distinct in many light element abundances. It is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs show different abundance distributions in light elements compared to those in the red giant branch (RGB) and other phases, skewing toward more primordial, field-star-like abundances, which we refer to as subpopulation one (SP1). As part of a larger program targeting giants in GCs, we obtained high-resolution spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter we report an extreme paucity of AGB stars with [Na/O] > -0.17 in M4, which contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB abundance distribution is consistent with all AGB stars being from SP1. This result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor) avoid the AGB phase. This is an unexpected result given M4's horizontal branch morphology -- it does not have an extended blue horizontal branch. This is the first abundance study to be performed utilising the HERMES spectrograph.Comment: 5 pages, 2 figures, 4 tables (full Table 1 online). Accepted for publication in MNRAS Letter

Individual differences in rhythmic skills: links with neural consistency and linguistic ability

Durational patterns provide cues to linguistic structure, and so variations in rhythm skills may have consequences for language development. Understanding individual differences in rhythm skills, therefore, could help explain variability in language ability across the population. We investigated the neural foundations of rhythmic proficiency and its relation to language skills in young adults. We hypothesized that rhythmic abilities can be characterized by at least two constructs, which are tied to independent language abilities and neural profiles. Specifically, we hypothesized that rhythm skills that require integration of information across time rely upon the consistency of slow, low-frequency auditory processing, which we measured using the evoked cortical response. On the other hand, we hypothesized that rhythm ic skills that require fine temporal precision rely upon the consistency of fast, higher-frequency auditory processing, which we measured using the frequency following response. Performance on rhythm tests aligned with two constructs: rhythm sequencing and synchronization. Rhythm sequencing and synchronization were linked to the consistency of slow cortical and fast frequency-following responses, respectively. Furthermore, while rhythm sequencing ability was linked to verbal memory, reading, and nonverbal auditory temporal processing, synchronization ability was linked only tononverbal auditory temporal processing. Thus, rhythm perception at different time scales reflects distinct abilities, which rely on distinct auditory neural resources. In young adults slow rhythmic processing makes the more extensive contribution to language skill

AGB subpopulations in the nearby globular cluster NGC 6397

It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anti-correlations of light element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here we investigate the AGB of NGC 6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 RGB and 8 AGB stars, deriving Fe, Na, O, Mg and Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC 6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M 4 (with contrasting results), since the same tools and methods were used.Comment: 10 pages, 7 figures, 8 tables (2 online-only). Accepted for publication in MNRA