Hydrodynamic Synchronisation of Model Microswimmers

We define a model microswimmer with a variable cycle time, thus allowing the possibility of phase locking driven by hydrodynamic interactions between swimmers. We find that, for extensile or contractile swimmers, phase locking does occur, with the relative phase of the two swimmers being, in general, close to 0 or pi, depending on their relative position and orientation. We show that, as expected on grounds of symmetry, self T-dual swimmers, which are time-reversal covariant, do not phase-lock. We also discuss the phase behaviour of a line of tethered swimmers, or pumps. These show oscillations in their relative phases reminiscent of the metachronal waves of cilia.Comment: 17 pages, 8 figure

Energetic particle influence on the Earth's atmosphere

This manuscript gives an up-to-date and comprehensive overview of the effects of energetic particle precipitation (EPP) onto the whole atmosphere, from the lower thermosphere/mesosphere through the stratosphere and troposphere, to the surface. The paper summarizes the different sources and energies of particles, principally galactic cosmic rays (GCRs), solar energetic particles (SEPs) and energetic electron precipitation (EEP). All the proposed mechanisms by which EPP can affect the atmosphere are discussed, including chemical changes in the upper atmosphere and lower thermosphere, chemistry-dynamics feedbacks, the global electric circuit and cloud formation. The role of energetic particles in Earth’s atmosphere is a multi-disciplinary problem that requires expertise from a range of scientific backgrounds. To assist with this synergy, summary tables are provided, which are intended to evaluate the level of current knowledge of the effects of energetic particles on processes in the entire atmosphere

Stock assessment and the provision of management advice for the short fin squid fishery in Falkland Islands waters

The scientific basis for managing the short fin squid, Illex argentinus, Stock around the Falkland Islands is presented. A target proportional escapement policy is used which permits a level of fishing effort compatible with conservation targets to be set each year. This policy is intimately related to the method of assessing stock size and the rate of fishing mortality presented in a related paper. The results of applying this management procedure for the 1987 and 1988 fishing seasons, the first 2 years of regulated fishing in Falkland Islands waters, are described. The policy has the considerable benefit that the data requirements and monitoring procedures are straightforward, and can be implemented by the limited manpower resources of the islands

Towards the development of a management plan for the mackerel icefish (Champsocephalus gunnari) in Subarea 48.3

Management of the mackerel icefish (Champsocephalus gunnari) at South Georgia is complicated by the likelihood of substantial periodic variations in natural mortality rates. These may be associated with increased consumption of C. gunnari by Antarctic fur seals in years of poor krill availability. Thus natural ortality of C. gunnari may, in some years, increase by a large factor (assumed here to be 4), declining to normal levels again when krill return. This paper outlines a scheme which would use information from studies on krill and predators undertaken as part of the CCAMLR Ecosystem Monitoring Program (CEMP) to interpret or modify information from commercial fisheries and research surveys leading to estimates of stock biomass. An extension of this scheme would use predictions of coming periods of krill scarcity as early warnings of increased natural mortality of C. gunnari. Full implementation of such a scheme would require greater knowledge of quantitative aspects of food web dynamics within the South Georgia ecosystem than we possess at present. There is therefore a need for an interim approach to the setting of precautionary catch limits for this fishery. An approach based on the CCAMLR generalised yield model (GYM), with periodically varying natural mortality, provides a realistic description of perceived icefish dynamics. However, the model generates a significant probability that C. gunnari populations will be depleted even in the absence of fishing, which, given the existing CCAMLR decision rules, would preclude the fishery's ever opening. Several possible modifications to the decision rules are discussed. It is concluded that while the GYM can be used to estimate a temporary, conservative long-term yield, a new approach and set of decision rules will ultimately be required for C. gunnari

The assessment of stocks of annual squid species

Methodology for assessing squid fisheries is presented focusing on the estimation of exploitation rate, stock size and the fishing power of the vessels. The assessment is designed to provide input into management decisions for effort limited fisheries. The fishery for Illex argentinus around the Falkland Islands is used as an example for the methods. The development of a management policy for this fishery is described more fully in a related paper. The methods are based on modifications to standard Leslie-Delury analysis. Various regression methods are utilized depending on the number of fleets for which detailed data are available. The statistical procedures also yield estimates of the standard errors of the estimates for evaluating the assessment advice

An evaluation of the implicit management procedure used for some ICES roundfish stocks

This paper describes a simulation study that evaluated the performance of the scientific advisory process used by ICES to recommend total allowable catches (TACs) for roundfish stocks. A "management strategy evaluation" approach is used, involving development of an operating model to represent the underlying reality, and an observation model to generate pseudo data that are then used within a management procedure. The management procedure comprises an assessment that uses data to estimate parameters of interest and a decision rule to derive TAC recommendations for the following year. There are two important results: including realistic sources and levels of uncertainty can result in far from optimal management outcomes based on the current procedures; and current ICES biomass and fishing mortality reference points are not always consistent, and several are clearly inappropriate. This is because the types of projection used by ICES do not incorporate important lags between assessing stock status and implementing management measures, and they also ignore important sources of uncertainty about the actual dynamics, as well as our ability to collect data and implement management regulations (i.e. model, measurement, and implementation error, respectively). The simulation approach also showed that better management is not necessarily going to be achieved by improving the assessment, because even with a perfect assessment (where the simulated working group knew stock status perfectly), stocks may crash at fishing levels that standard stochastic projections would suggest were safe. It is proposed that, in future, operating models that represent the best available understanding of the actual system dynamics be used to evaluate models and rules considered for application. These operating models should capture the plausible range of characteristics of the underlying dynamics, but not necessarily model their full complexity. In general, they will be more complex than those used by assessment working groups, so developing management procedures that are robust to a broad range of uncertainty. However, the models and rules used as part of the management procedure should be simpler than those used at presen