John Harwood

Organochlorine levels in common seals (Phoca vitulina) which were victims and survivors of the 1988 phocine distemper epizootic

We compared concentrations of organochlorines in the blubber of common seals (Phoca vitulina) found dead during the 1988 phocine distemper epizootic with levels in animals which survived it. There were highly significant differences between the live and dead animals, and between sample sites. These were not fully accounted for by seasonal and condition-related changes in blubber thickness.

MARINE MAMMALS AND THEIR ENVIRONMENT IN THE TWENTY-FIRST CENTURY

Abstract Critical habitat for marine mammals is defined in terms of the functioning ecological units required for successful breeding and foraging. This definition is used to consider the potential effects of environmental change in the 21st century on abundance and distribution of marine mammals. Critical habitat for breeding can be identified relatively easily for pinnipeds and some coastal and freshwater cetaceans. Critical habitat for foraging is more difficult to define, particularly for pelagic species. However, telemetry-based studies have indicated that relatively localized areas may be particularly important for some species. Habitat degradation, as a result of reduction in prey density and increased risks of mortality due to human activity, is likely to be a problem for most species. Ice-breeding seals, particularly those that are endemic to inland seas and large lakes, are most likely to be affected by climate change. Climate change will also affect distribution and availability of prey in the short and long term. Although highly mobile species, such as marine mammals, can respond more rapidly to effects of climate change than their terrestrial counterparts, central-place foragers, such as many otariid seals, may still be seriously affected.

Time budgets and topography: how energy reserves and terrain determine the breeding behaviour of grey seals

Abstract Grey seals (Halichoerus grypus) fast during the pupping and mating season; they are polygynous and throughout their breeding range they occupy a variety of habitats. Time allocation patterns are consistent with the fact that grey seal energy reserves are limited during the breeding season. Variations in the activity budgets of seals in different types of habitat appear to be determined by the nature of the terrain occupied. This behavioural flexibility may well have been an important feature in the evolution of the species.

Mass mortality in marine mammals: Its implications for population dynamics and genetics

The death of tens of thousands of common seals (Phoca vitulina) around the coast of Europe in 1988 provoked wide interest in the popular press, but it also raised questions about the importance of mass mortality in the dynamics of marine mammal populations. Here, we summarize published information on the occurrence of mass mortalities among marine mammals and review the mathematical models that have been developed to investigate the role of such disasters in population dynamics. We conclude that mass mortalities may play a more important role than density-dependent factors in the dynamics of some marine mammal populations. This, in combination with recent improvements in our understanding of the structure of these populations, has important implications for their genetics and evolution.

Concentrations of organochlorine compounds in the blubber of seals from Eastern and North-eastern England, 1988

Abstract Samples of blubber from common seals (Phoca vitulina) found dead in eastern England and grey seals (Halichoerus grypus) found dead on the Farne Islands have been analysed for a range of organochlorine pesticides, individual chlorobiphenyl congeners and total polychlorinated biphenyls (PCB). No significant differences were seen between results for the two seal species. HCB and HCH concentrations were low, and total DDT and PCB concentrations (ranging from 0.99–8.0 and 5.7–33 μg g−1 wet wt respectively) were at the lower end of the reported range for seals from the North and Baltic Seas. PCB profiles were dominated by congeners 138 and 153, and amongst the DDT group of compounds the pattern of abundance was DDE > DDT ⪢ TDE.

The descriptive epizootiology of phocine distemper in the UK during 1988/1989

The time, place, individual approach, widely used in characterising human epidemics, was applied to the 1988 phocine distemper virus (PDV) epizootic affecting North Sea seals. Estimates of time of death from 157 (69%) of the 228 dead seals necropsied in 1988 indicated that the number of carcasses which were found more than 14 days post mortem increased as the epizootic progressed. Although information provided by epizootic curves based on when and where carcasses were reported are affected by the accuracy of such data, the PDV epizootic curves were characteristic of a propagative epidemic. The individual characteristics of the carcasses provided more useful information. 1. Common seals were more susceptible than grey seals. 2. Males and older seals had a greater exposure to the virus than females and younger animals. 3. The likelihood of viral transmission was greater on land. Seasonal, sex and age related variation in haul-out behaviour affected transmission probabilities and rates. 4. The risk of infection for a susceptible individual during August was higher for seals in England and Northern Ireland than for those in Scotland. These findings illustrate the importance of population characteristics including behaviour and social organisation in determining the pattern and spread of wildlife epizootics.

Long term risks of recurrent seal plagues

Abstract There has been widespread speculation about the factors which contributed to the mass mortality of North Sea harbour seals during 1988, and the threat that these factors pose to this and other marine mammal populations. We identify the questions which need to be answered before these speculations can be transformed into testable hypotheses and outline an epidemiological context for the preliminary evaluation of such hypotheses.

Plasma thymulin concentrations, the thymus and organochlorine contaminant levels in seals infected with phocine distemper virus

Blood samples collected from live common seals (Phoca vitulina) and grey seals (Halichoerus grypus) around the coast of Scotland and Northern Ireland during, and immediately after, an epizootic caused by phocid distemper virus (PDV) were analysed for thymulin content. Thymulin levels were compared with neutralization titres and concentrations of organochlorine contaminants (DDT and its metabolites, seven chlorinated biphenyl congeners) derived from blood and blubber samples collected from the same animals and analysed independently. Thymulin levels in grey seals (mean 2827 +/- 1355 fg/ml) were negatively correlated with the logarithm of virus neutralization titre. In common seals they varied significantly between age classes. There was no direct relationship between thymulin levels and contaminant levels in either species. However, when an estimate of time since exposure was included in the regression analysis for common seals, there was a highly significant relationship between thymulin and the two chlorinated biphenyl congeners with the highest concentrations in blubber.