ALBERT

Description: We explore a parameterization for mesoscale turbulence, closely related to that of Gent and McWilliams, in which forcing terms proportional to the isopycnal flux of potential vorticity appear in the averaged momentum equations. We show that in the presence of variable bottom topography, the parameterization predicts alongslope mean flow and a corresponding upslope bolus (eddy) flux of tracer that is associated with an alongslope-directed bottom eddy stress. The upslope bolus flux is in qualitative agreement with observations of a cold dome over seamounts. The predicted alongslope flow corresponds to flow fields found in geostrophic turbulence experiments and has some similarity (although conceptually very different, as discussed in the text) to Holloway's prediction based on statistical mechanics. By considering continuous stratification as a limiting case of a multilayer model, we show how to treat the surface and bottom boundaries. Practical application of the parameterization is illustrated using a three-dimensional -coordinate ocean circulation model that is very similar to the Bryan–Cox–Semtner model. The model-computed flow is consistent with observations of anticyclonic flow around a seamount. We show that the bottom eddy stress associated with the parameterization can be large, even compared to the annual mean surface wind stress, and hence could have important implications for the biology and water mass distribution of the coastal ocean as well as for the large-scale ocean circulation. From the climate modelling perspective, the approach adopted here provides a single formalism that combines the advantages of the Gent and McWilliams parameterization with alongslope mean flow similar to that suggested by Holloway.

Description: Three methods for calculating the parameters of the Weibull wind speed distribution for wind energy analysis are presented: the maximum likelihood method, the proposed modified maximum likelihood method, and the commonly used graphical method. The application of each method is demonstrated using a sample wind speed data set, and a comparison of the accuracy of each method is also performed. The maximum likelihood method is recommended for use with time series wind data, and the modified maximum likelihood method is recommended for use with wind data in frequency distribution format.

Description: Many studies on the deep-sea benthic biota have shown that the most species-rich areas lie on the continental margins between 500 and 2500 m, which coincides with the present oxygen-minimum in the world's oceans. Some species have adapted to hypoxic conditions in oxygen-minimum zones, and some can even fulfil all their energy requirements through anaerobic metabolism for at least short periods of time. It is, however, apparent that the geographic and vertical distribution of many species is restricted by the presence of oxygen-minimum zones. Historically, cycles of global warming and cooling have led to periods of expansion and contraction of oxygen-minimum layers throughout the world's oceans. Such shifts in the global distribution of oxygen-minimum zones have presented many opportunities for allopatric speciation in organisms inhabiting slope habitats associated with continental margins, oceanic islands and seamounts. On a smaller scale, oxygen-minimum zones can be seen today as providing a barrier to gene-flow between allopatric populations. Recent studies of the Arabian Sea and in other regions of upwelling also have shown that the presence of an oxygen-minimum layer creates a strong vertical gradient in physical and biological parameters. The reduced utilisation of the downward flux of organic material in the oxygen-minimum zone results in an abundant supply of food for organisms immediately below it. The occupation of this area by species exploiting abundant food supplies may lead to strong vertical gradients in selective pressures for optimal rates of growth, modes of reproduction and development and in other aspects of species biology. The presence of such strong selective gradients may have led to an increase in habitat specialisation in the lower reaches of oxygen-minimum zones and an increased rate of speciation.

Description: We present the results of a series of experiments that examined the effect of feeding history on the growth and tissue composition of juveniles of two tropical cephalopods; the squid Sepioteuthis lessoniana and the cuttlefish Sepia elliptica. Juveniles were reared in individual containers for between 35 and 42 days at different ration levels, three ration levels for the squid and two levels for the cuttlefish. Although differences in ration were sufficient to cause different growth rates, both in body length and mass, the effects on tissue composition were less definitive. Sepioteuthis juveniles on the highest rations had higher concentrations of water, but no difference in lipid, carbohydrate or protein when compared with their lower ration siblings. In the case of juvenile cuttlefish no difference in tissue composition was detected between the two ration levels. RNA:protein ratios were also determined for the juveniles to provide an estimate of instantaneous growth. A significant correlation was found between body size and RNA:protein ratio in the squid; those juveniles that ate more had higher RNA:protein ratios than lower ration individuals. Significantly, the juvenile cuttlefish showed no relationship between growth rate and RNA:protein ratios, which means that we are unable to use this measure to estimate the growth rates of wild individuals. In conclusion, ration level did affect growth rates and food availability is an important factor in modifying growth rates of wild individuals. However, we could not find, at the individual level, an index or measure that could be used to explain the variability of observed differences in growth rates as a function of nutritional history.

Description: An assessment of the South African chokka squid Loligo vulgaris reynaudii fishery is undertaken. Two fisheries catch chokka: the directed jig and the trawl fishery. Chokka is a by-catch in the latter fishery, which targets primarily Cape hake (Merluccius spp.) and Agulhas sole (Austroglossus pectoralis). A population dynamics model is fitted to the jig and trawl CPUE indices, and two biomass indices from scientific surveys, by means of an observation-error estimator. The dynamics are modelled for two periods: January–March and April–December. Parameters estimated are the annual recruitment R and the catchability coefficients corresponding to each biomass index; a composite somatic growth and mortality parameter, g, is fixed externally. Within this approach, two models for annual recruitment are considered: (A) recruitment is constant above a biomass threshold, below which it starts to decline; (B) recruitment depends on jig-induced fishing mortality, larger values of which have an increasing negative impact on reproductive success. The parameters estimated for model A have wide confidence intervals and the model is unable to fit the decline in the early years of the trawl CPUE time-series. A better fit is achieved for the approach taken in model B. The linkage in this model of declines in recruitment to disturbance caused by the jig fishery is, however, somewhat ad hoc, and a basis for an independent test of this link is suggested. Model B estimates the current biomass to be heavily depleted. Stochastic projections under the assumption that the current effort level is maintained, show that the risk of the spawning biomass falling below 20% of its pristine level over the next 10 years is close to 90%. Effort needs to be cut to 2/3 of that at present to achieve a substantial reduction in this level of risk. Some alternative scenarios that are investigated, such as that of a non-linear relationship between jig CPUE and biomass, give more pessimistic results.

Description: A mollusc shell, the xiphoid from cuttlefish was found to be a substrate favouring the deposition of aragonite crystals from stable supersaturated solutions at pH 8.50 and 25°C. The crystallization was studied at constant solution composition, thus making it possible for a relatively large amount of the overgrowths to be formed and to be identified exclusively as aragonite crystals. The apparent order found from kinetics data was n=4.1±0.4, thus suggesting a polynuclear mechanism. A surface energy of 24±3 mJ m−2 was calculated for the growing phase and a four-ion cluster forming the critical nucleus, according to the classical nucleation theory.

Description: A shared characteristic among animals is their propensity to form stable, beneficial relationships with prokaryotes. Usually these associations occur in the form of consortia, i.e. a diverse assemblage of bacteria interacting with a single animal host. These complex communities, while common, have been difficult to characterize. The two-partner symbiosis between the squid Euprymna scolopes and the marine luminous bacterium Vibrio fischeri offers the opportunity to study the interaction between animal and bacterial cells, because both partners can be cultured in the laboratory and the symbiosis can be manipulated experimentally. This system is being used to characterize the mechanisms by which animals establish, develop and maintain stable alliances with bacteria. This review summarizes the progress to date on the development of this model.

Description: The thermal infrared data from the satellites of the European Meteosat series are used to characterize the dynamics of the West African coastal upwelling for the period 1984–1998. Sea surface temperature (SST) images are produced from satellite data with a spatial resolution of 5 km and a time-step of 5 days. These are used to characterize the spatial structure of the upwelling by automatic localisation of the SST minima at each coast location and to derive a normalised upwelling intensity index, based on SST differences. A new index for retention potential is also proposed. The strong interannual variability observed in these indices is quantified, mainly in terms of intensity and seasonal lag. An objective method of space partitioning, based on the variability of the proposed indices is introduced. Objective analysis applied to three characteristic regions shows that the spawning areas of the coastal pelagic species may be clearly delineated in terms of spatial and temporal optimisation of enrichment and retention processes. The importance of the retention processes is shown for the recruitment of Sardinella aurita in Senegal and for Octopus vulgaris in Mauritania. According to the concept of Bakun's triad for upwelling areas, a precise determination of the upwellings intensity and associated spatial processes is essential for evaluating the balance between enrichment and its optimal use in an ecological sense. A Java-based software package is produced as part of this study to visualise long series of pre-processed images and to extract the above indices.

Description: Loligo gahi were collected from bottom trawl samples taken around the Falkland Islands on both commercial and research vessels from January 1994 to November 1995. Commercial samples were taken from both seasons of the fishery (February–May, August–October), research samples were taken in January, June and November. A sub-sample of squid was taken from each sample to provide statoliths for age determination by statolith increment analysis. Sea surface temperature data were derived for an area encompassing the southerly portion of the fishing grounds for L. gahi to provide an indicator of seasonal temperature variations for that region. Age data were derived for 486 females and 445 males. Exponential and power growth models explained the fit to the size-at-age data to a similar extent. Age data were grouped by month and by season of hatch to test the hypothesis that seasonal temperature variability influences the size-at-age relationship in L. gahi. Analyses of covariance showed that the relationships of both predicted mean mantle length and predicted mean body mass per month of hatch, and per season of hatch resembled closely the trends in mean monthly sea surface temperature, with the squid hatched in the summer, i.e., at higher temperatures, being significantly larger than squid of the same age but hatched in the winter months. The growth rates for adult squid in this study were similar over the age range analysed. This study provides evidence that increased temperature during a squid’s early growth period may accelerate growth markedly giving rise to significant differences in size-at-age for adult squid hatched at different temperatures.

Description: Activities of the antioxidative enzymes superoxide dismutase (SOD), catalase, glutathione peroxidase (GPX) and glutathione reductase (GR) were measured in the cephalopods Sepia officinalis and Lolliguncula brevis. Maximal enzyme activities were higher in gill tissue than in the mantle musculature of both species. Activities were generally lower in tissues of L. brevis than in S. officinalis. Comparison with other ectothermic animals showed both cephalopod species to have a low enzymatic antioxidative status despite their high metabolic rate. Furthermore, changes in antioxidative enzyme activities were measured in the cuttlefish S. officinalis with increasing age. The concentrations of malondialdehyde (MDA) and lipofuscin were determined as indicators of lipid peroxidation. Investigated animals were between 1.5 months and over 12 months old. Changes of antioxidative enzyme activities with age were not uniform. SOD and GPX activities increased with age, while catalase activity declined. In contrast, GR activity remained almost unchanged in all age groups. The low level of antioxidative defense might allow for the significant age-induced rise in MDA levels in gills and mantle musculature and for the increase in lipofuscin levels in mantle and brain tissue. It might thereby contribute to increased oxidative damage and a short life span in these cephalopods.