Experimental study of differentially rotating supersonic plasma flows produced by aluminium wire array Z-pinches

A novel approach to cylindrical wire array z-pinches has been developed in order to create a rotating plasma flow analogous to astrophysical accretion discs. The method involves subjecting the wire array to a cusp magnetic field (B_r) to create converging off axis ablation streams to form a rotating flow. The rotation is sustained by the ram pressure of the ablation streams in a quasi-equilibrium state for approximately 150 ns. This corresponds to one full rotation of the plasma about the axis. The rotating plasma is supersonic with Mach number ~2 and a radially constant rotation velocity between 60 and 75 km/s; the angular velocity therefore has an r^-1 dependence and the flow is differential. A Thomson scattering diagnostic is used to measure the electron and ion temperatures as Te ~30 eV and Ti >55 eV and the ionisation of the plasma (Z) between 6 and 8. These parameters are used to calculate the Reynolds number (10^5 to 10^6) and magnetic Reynolds numbers (20 to 100) which are large enough for viscous and resistive effects to be negligible on the large scale of the flow. These are of sufficient magnitude for the experiment to be scalable to astrophysical accretion discs. Further more the Reynolds number for the experiment is large enough for shear instabilities to manifest in the plasma. Some evidence for this can be seen in XUV images and Thomson spectra which indicate the development of perturbations and vorticity within the flow. Predictions for the growth rate of the Kelvin Helmholtz instability, 12 to 40 ns, agree reasonably well with the observed perturbation growth of ~30 ns. It is also possible that shear instabilities are driving hydrodynamic turbulence. Turbulent heating of the plasma could explain the approximately 500 eV increase in the ion temperature observed from some Thomson spectra. Further work is required however to prove the existence of shear flows and turbulence within the experiments.Open Acces

Uncertainty estimation and model selection in stock assessment models with non-parametric effects on fishing mortality

Uncertainty coming from assessment models leads to risk in decision making and ignoring or misestimating it can result in an erroneous management action. Some parameters, such as selectivity or survey catchabilities, can present a wide range of shapes and the introduction of smooth functions, which up to now have not been widely used in assessment models, allows for more flexibility to capture underlying nonlinear structures. In this work a simulation study emulating a sardine population is carried out to compare 3 different methods for uncertainty estimation: multivariate normal distribution, bootstrap (without and with bias correction) and Markov chain Monte Carlo (MCMC). In order to study their performance depending on the model complexity, five different scenarios are defined depending on the shape of the smooth function of the fishing mortality. From 100 simulated data sets, performance is measured in terms of point estimation, coefficients of variation, bias, skewness, coverage probabilities and correlation. In all approaches model fitting is carried out using the a4a framework. All three methods result in very similar performance. The main differences are found for observation variance parameters where the bootstrap and the multivariate normal approach result in underestimation of these parameters. In general MCMC is considered to have better performance, being able to detect skewness in posterior distributions, showing small bias and reaching expected coverage probabilities. It is also more efficient in terms of time consumption in comparison with bootstrapping.JRC.D.2-Water and Marine Resource

WORKSHOP ON THE IBERIAN SARDINE MANAGEMENT AND RECOVERY PLAN (WKSARMP)

The Workshop on the Iberian Sardine Management and Recovery Plan, chaired by Manuela Azevedo (Portugal), met in Lisbon, Portugal, 1–5 April 2019 to evaluate if the management and recovery strategies jointly proposed by the Portuguese and Spanish administration meet the client´s objective and are precautionary according to ICES precautionary criterion. The request also asked for the re-examination of the Biological Reference Points (BRPs) for the Iberian sardine stock. The working group decided to keep the current BRPs, Blim of 337.4 thousand tonnes and FMSY of 0.12year-1, corresponding to the scenario of medium stock productivity, because the updated estimates as well as the estimates from the analysis of the effect on the BRPs of the assessment retrospective showed that both were within confidence bounds (95%) of the current adopted reference points. The working group also estimated BRPs considering the recent low stock productivity to be in the period 2006–2017 as specified in the request. Blim_low was estimated to be 196.3 thousand tonnes and FMSY_low was estimated to be 0.032year-1, corresponding to the scenario of low stock productivity. The estimated BRPs were used to set the biomass and fishing mortality reference levels of the catch rules and also, following the ICES guidelines for the evaluation of management plans, the basis to compute performance statistics of the management strategy evaluation under each operating model

Large-scale ocean connectivity and planktonic body size

Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents. However, the role that abundance and body size play in determining spatial patterns of diversity remains unclear. Here we analyse spatial community structure - β-diversity - for several planktonic and nektonic organisms from prokaryotes to small mesopelagic fishes collected during the Malaspina 2010 Expedition. β-diversity was compared to surface ocean transit times derived from a global circulation model, revealing a significant negative relationship that is stronger than environmental differences. Estimated dispersal scales for different groups show a negative correlation with body size, where less abundant large-bodied communities have significantly shorter dispersal scales and larger species spatial turnover rates than more abundant small-bodied plankton. Our results confirm that the dispersal scale of planktonic and micro-nektonic organisms is determined by local abundance, which scales with body size, ultimately setting global spatial patterns of diversit