213 research outputs found
Fiskeriudsigt for tobis i Nordsøen på bankeniveau:Endelig oversigtsrapport vedr. projektet Fiskeriudsigt for tobis i Nordsøen på bankeniveau
Patchy zooplankton grazing and high energy conversion efficiency: Ecological implications of sandeel behavior and strategy
Sandeel display strong site-fidelity, and spend most of their life buried in the seabed. This strategy carries important ecological implications. Sandeels save energy when they are not foraging but in return are unable to move substantially and therefore possibly are sensitive to local depletion of prey. Here we studied zooplankton consumption and energy conversion efficiency of lesser sandeel (Ammodytes marinus) in the central North Sea, using stomach data, length and weight-at-age data, bioenergetics, and hydrodynamic modeling. The results suggested: (i) lesser sandeel in the Dogger area depend largely on relatively large copepods in early spring. (ii) lesser sandeel is an efficient converter making secondary production into fish tissue available for higher trophic levels. Hence, changes in species composition towards a more herring dominated system, as seen in recent times, may lead to a decrease in system transfer efficiency. (iii) sandeels leave footprints in the standing copepod biomass as far as 100 km from the edge of their habitat, but smaller and more isolated sandeel habitat patches have a much lower impact than larger patches, suggesting that smaller habitats can sustain higher sandeel densities and growth rates per area than larger habitats
Patchy zooplankton grazing and high energy conversion efficiency: ecological implications of sandeel behavior and strategy
Impacts of the local environment on recruitment: a comparative study of North Sea and Baltic Sea fish stocks
Abstract
While the impact of environmental forcing on recruitment variability in marine populations remains largely elusive, studies spanning large spatial areas and many stocks are able to identify patterns common to different regions and species. In this study, we investigate the effects of the environment on the residuals of a Ricker stock–recruitment (SR) model, used as a proxy of prerecruits' survival, of 18 assessed stocks in the Baltic and North Seas. A probabilistic principal components (PCs) analysis permits the identification of groups of stocks with shared variability in the prerecruits' survival, most notably a group of pelagics in the Baltic Sea and a group composed of gadoids and herring in the North Sea. The first two PCs generally grouped the stocks according to their localizations: the North Sea, the Kattegat–Western Baltic, and the Baltic Sea. This suggests the importance of the local environmental variability on the recruitment strength. Hence, the prerecruits' survival variability is studied according to geographically disaggregated and potentially impacting abiotic or biotic variables. Time series (1990–2009) of nine environmental variables consistent with the spawning locations and season for each stock were extracted from a physical–biogeochemical model to evaluate their ability to explain the survival of prerecruits. Environmental variables explained &gt;70% of the survival variability for eight stocks. The variables water current, salinity, temperature, and biomass of other fish stocks are regularly significant in the models. This study shows the importance of the local environment on the dynamics of SR. The results provide evidence of the necessity of including environmental variables in stock assessment for a realistic and efficient management of fisheries.</jats:p
Data-driven rules for multidimensional reflection problems
Over the recent past data-driven algorithms for solving stochastic optimal
control problems in face of model uncertainty have become an increasingly
active area of research. However, for singular controls and underlying
diffusion dynamics the analysis has so far been restricted to the scalar case.
In this paper we fill this gap by studying a multivariate singular control
problem for reversible diffusions with controls of reflection type. Our
contributions are threefold. We first explicitly determine the long-run average
costs as a domain-dependent functional, showing that the control problem can be
equivalently characterized as a shape optimization problem. For given diffusion
dynamics, assuming the optimal domain to be strongly star-shaped, we then
propose a gradient descent algorithm based on polytope approximations to
numerically determine a cost-minimizing domain. Finally, we investigate
data-driven solutions when the diffusion dynamics are unknown to the
controller. Using techniques from nonparametric statistics for stochastic
processes, we construct an optimal domain estimator, whose static regret is
bounded by the minimax optimal estimation rate of the unreflected process'
invariant density. In the most challenging situation, when the dynamics must be
learned simultaneously to controlling the process, we develop an episodic
learning algorithm to overcome the emerging exploration-exploitation dilemma
and show that given the static regret as a baseline, the loss in its sublinear
regret per time unit is of natural order compared to the one-dimensional case.Comment: 29 pages, 3 figure
Lasing on a narrow transition in a cold thermal strontium ensemble
Highly stable laser sources based on narrow atomic transitions provide a
promising platform for direct generation of stable and accurate optical
frequencies. Here we investigate a simple system operating in the
high-temperature regime of cold atoms. The interaction between a thermal
ensemble of Sr at mK temperatures and a medium-finesse cavity produces
strong collective coupling and facilitates high atomic coherence which causes
lasing on the dipole forbidden SP transition. We
experimentally and theoretically characterize the lasing threshold and
evolution of such a system, and investigate decoherence effects in an
unconfined ensemble. We model the system using a Tavis-Cummings model, and
characterize velocity-dependent dynamics of the atoms as well as the dependency
on the cavity-detuning.Comment: 9 pages, 7 figure
- …