Using fine‐scale catch predictions to examine spatial variation in growth and catchability of Panulirus cygnus along the west coast of Australia

Puerulus settlement has been monitored throughout the western rock lobster Panulirus cygnus fishery for nearly 40 years. These data, in combination with indices of effort and water temperature, were used to produce recruitment‐catch relationships for each 1° transect of latitude in the coastal part of this fishery from Kalbarri to Cape Leeuwin, as well as at the offshore Abrolhos Islands (total of eight transects). The fine spatial scales of these models provided estimates of certain life history traits that are known to affect lobster catches between adjacent fishing ports. This catch modelling showed that the proportions of 3‐ and 4‐year‐old post‐settlement lobsters contributing to the catches varied markedly from the southern to northern transects, suggesting that juvenile lobsters grow substantially faster in the warmer northern and offshore waters of this fishery. These proportions provide accurate estimates of juvenile growth rates, which are vital in the construction of location‐specific growth algorithms required by the age‐structured models used in the management of this fishery. Model estimates of density‐dependent mortality were greater in the more densely populated centre of the fishery and markedly lower at the northern and southern limits of this species distribution. Annual increases in fishing efficiency were also found to be lowest at the northern and southern extremes of the fishery and greatest in the centre of the fishery, where technology advances and increased fleet mobility have enabled the fleet to increase efficiency by 1–3% each year. Catchability (q) was found to be most influenced by water temperatures in the cooler southern transects, whereas at the Abrolhos Islands, changes in water temperature produced almost no discernable change in q. The catch modelling was also used to quantify the impact of management changes introduced in the 1993/94 fishing season. Increased protection of female lobsters and an 18% pot reduction resulted in a 3–4% permanent reduction in the catch rates of lobsters throughout most of the coastal fishery, whereas at the offshore Abrolhos Islands, catch rates increased by c. 20%, presumably owing to a reduction in the level of pot saturation.     

Bounds on the stability of 3D magnetic equilibria in the solar corona

A necessary and a sufficient condition are derived for the ideal magnetohydrodynamic stability of any 3D magnetohydrostatic equilibrium using the energy method and incorporating photospheric line-tying. The theory is demonstrated by application to a simple class of theoretical 3D equilibria. The main thrust of the method is the formulation of the stability conditions as two sets of ordinary differential equations together with appropriate boundary conditions which may be numerically integrated along tied field lines one at a time. In the case of the shearless fields with non-negligible plasma pressure treated here the conditions for stability arenecessary and sufficient. The method employs as a trial function a destabilizing ballooning mode, of large wave number vector perpendicular to the equilibrium field lines. These modes may not be picked up in a solution of the full partial differential equations which arise from a direct treatment of the problem.     

The stability of 2D current sheet models of prominences

A necessary and sufficient condition for the ideal magnetohydrodynamic stability of 2D current sheet models of prominences suspended in a potential coronal field with line-tying is developed using the energy method. This condition takes the form of two simple coupled second-order differential equations which may be integrated along a field line to find marginal stability. The two conditions (85) and (86) of Anzer (1969) are now only sufficient for stability. Two current sheet models are investigated and it is shown that for a potential coronal field allowing perturbed electric currents to flow, line-tying can completely stabilize the equilibria for realistic heights.     

Zostera muelleri as a structuring agent of benthic communities in a large intertidal sandflat in New Zealand

The influence of seagrass beds on intertidal infaunal communities has been widely studied, with vegetated areas typically having higher diversity and abundances than adjacent bare sand patches. Such “seagrass–sand” comparisons, however, do not reflect the gradient of seagrass cover that may exist across large landscapes. We studied the large-scale distribution of intertidal macrozoobenthos over approximately 10,000 ha of sandflat on Farewell Spit, New Zealand. The benthic fauna, sediment composition and surface cover of the seagrass Zostera muelleri were studied at 192 sites evenly spaced along 30 transects covering the length of the 30 km spit. Most sites had Zostera present, generally at low densities (1–25% surface cover). Overall, invertebrate taxon diversity increased with Zostera cover, from a median of 4 taxa at sites with no Zostera to 23 at sites with high Zostera cover. Multivariate analyses of 37 frequently occurring taxa (of the 91 recognised) indicated that there was a site gradient of taxon abundances that reflected seagrass cover, with 23 taxa increasing as Zostera cover increased. Only three taxa tended to be found more where Zostera was scarce. Seventeen taxa were identified as being significant indicators of Zostera cover; in all cases abundances peaked with high Zostera scores. Cluster analysis revealed a number of major groupings. One group was associated with low Zostera; two were strongly associated with high Zostera cover; a fourth was probably distinguished by low tidal elevation and proximity to channels. On the Farewell Spit tidal flats, large-scale patterns of abundance seem to be largely structured by the presence and density of Zostera.     

Magnetic equilibrium in coronal arcades

Solar Physics - An analytical solution to the magnetohydrostatic equations is presented that generalises a solution due to Birn et al. (1978) to include the effect of gravity. There exist two...     

SWAT.nz: New-Zeland-based “Sand Waves and Turbulence” experimental programme

The SWAT.nz (“New-Zealand-based Sand Waves and Turbulence”) research programme was carried out to advance understanding of subaqueous sand waves. The programme was based around detailed measurements at varying scales of bed morphologies and associated flow fields as sand waves formed from plane-bed conditions and grew to equilibrium. This paper outlines the philosophy and details of the SWAT.nz programme, with the aim of providing insight into experiment and analysis design and methodologies for studies of highly-variable bed surfaces and flows. Example challenges addressed in the SWAT.nz programme include the measurement over large spatial domains of developing flow fields and three-dimensional bed morphology, including flow measurements below roughness (sand-wave) crests, and how to interpret the collected measurements. Insights into sand-wave dynamics that have arisen from the programme are presented to illustrate the values of the SWAT.nz programme and the developed methodologies. Results are presented in terms of mobile-bed processes, and flow-bed interaction and flow processes for fixed-bed roughness and erodible beds, respectively.     

Investigation of mangrove macroalgae as bioindicators of estuarine contamination

This study examined the potential use of macroalgae epiphytic on mangrove aerial roots as indicators of estuarine contamination. The distribution and abundance of macroalgae was investigated in four estuaries in the vicinity of Sydney, Australia, and compared to water and sediment metal concentrations, nutrient concentrations and physicochemical parameters over four seasonal surveys. Macroalgal diversity and distribution appeared to be highly influenced by the ambient contaminant concentrations, while biomass appeared to be linked with nutrient concentrations. The distribution of the Rhodophyta species, Catenella nipae Zanardini significantly decreased as metal concentrations increased among the estuaries during all seasonal surveys. This species showed strong potential for use as a bioindicator of estuarine contamination.     

Magnetohydrostatic structures in the solar atmosphere

Separable two-dimensional solutions to the isothermal magnetohydrostatic equations are presented which include the effect of gravity. Examples of three types of linear solution are given in which photospheric magnetic fields are prescribed and the field topologies are discussed. In addition, a new nonlinear solution is discussed. The functional form of the pressure distribution is restricted by the separable assumption. An analysis suggests that these are the only separable analytical solutions.