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Southern blue whiting Micromesistius australis occurs around the southern coasts of South America and has been known to undertake summer feeding migrations to the Antarctic Penin- sula and South Orkney Islands. The largest stock spawns to the southwest of the Falkland Islands and has been the subject of a major fishery on the Patagonian Shelf since 1978. Fish are infected with cysts of the Myxosporean parasite Kudoa alliaria which make the flesh commercially unattractive for fillets. A 10 yr study initiated in 1989 established that prevalence of the parasite is over 80% for fish older than 1 yr, and that average infection intensities are about 14 cysts per fish. We constructed a combined generalised linear/generalised additive model (GLM/GAM) of parasite abundance in M. australis. The combination of up to 30 ages and 8 yr of data spanning 10 yr allowed us to investi- gate rates of parasitism in 33 individual cohorts. Strong age and cohort effects in the model implied that the fish acquire parasites in their first 1 to 1 1 /2 yr of life and that parasite abundance is set at these ages for the lifetime of the cohort. Several cohorts have statistically significantly lower parasite abun- dance than the majority, and these instances are roughly coincident with the 7 yr periodicity of sea- ice and sea surface temperature (SST) fluctuations in the Drake Passage. There are significant corre- lations between SST in the Drake Passage, the duration of sea-ice around Signy Island (South Orkney Islands) with a 1 yr time lag and parasite abundance in a cohort. We hypothesise that these correla- tions represent changes in the distribution and density of adult M. australis in warm and cold years, which thereby influences the density of the parasite field that juvenile fish encounter.

The results of a sampling programme for salmon lice on sea trout caught in the North Sea off the East Anglian coast during 1992 and 1993 are compared with similar data from 1972 and 1973 for fish caught in the North Sea off the coast of Yorkshire. These areas are completely free of any salmonid mariculture, nor has East Anglia any significant sea trout rivers, the trout migrating into the area mostly from the north-east of England. These attributes provide an ideal basis for examining levels of infection in the absence of any potential interaction with salmonid farming activities. Counts of the two principal species of lice that occur on salmonids, Lepeophtheirus salmonis and Culigus elongatus, were made over a number of months in each year. The inter-annual and seasonal patterns of infection by both species showed mean intensity to be both low and fairly stable. The statistical distribution of the parasites amongst the host population generally showed levels of overdispersion (aggregation) typical of parasite populations. These results are discussed in terms of the dynamics of the parasite population, the interaction between parasite and host, and parasite transmission dynamics. 0 British Crown copyright 1997

SUMMARYEnvironmental sex determination in the mermithid nematodeRomanomermisculicivorax is examined in the context of parasite reproductive success and population regulation. Experimental results show that the sex ratio of the nematode within its mosquito host (Culex quinquefasciatus) is dependent on parasite density. Sex ratios are biased to females at low parasite burdens and to males at high parasite burdens. Low temperature further enhances female-biased ratios. The net effect of density-dependent sex determination on parasite and host population growth is shown to be critically dependent on the frequency distribution of parasite numbers/host. Mermithid parasite distributions within natural host populations show low degrees of aggregation relative to other helminth species. The population regulation of the parasite is examined with respect to environmental sex determination and parasite-induced host mortalities by means of simple mathematical models of the dynamics of parasite transmission via its life-cycle. The significance of regulatory constraints on population growth are discussed in relation to the use of mermithids as biological control agents of insect pests or disease vectors.

The DuMont ``Vitascan'' Color TV system produces live color television pictures of high quality without the use of expensive and complicated color television cameras or highly trained and skilled camera and maintenance crews. It is, in fact, a modern all-electronic version of the earliest forms of mechanical live television pickup which employed the flying-spot scanning principles.

This new color-television system, the Vitascan, an all-electronic version of the old-style mechanical type, is the simplest and most reliable live-action color system now in existence. It can be regarded as using the reverse of conventional studio methods, as the flying-spot tube is substituted for the image-orthicon camera tube and clusters of phototubes replace regular studio lighting equipment.

In a study of faecal egg counts of Schistosoma mansoni from 359 people of all ages from a rural Kenyan community, a positive association was demonstrated between infection intensity in individuals before treatment and reinfection intensity in the same individuals 9 months after treatment in certain age groups of the sampled population. Consequences and possible causes of these observations are discussed in terms of the epidemiology and control of schistosomiasis.