We present new and revised data for the phocine distemper virus (PDV) epidemics that resulted in the deaths of more than 23 000 harbour seals Phoca vitulina in 1988 and 30 000 in 2002. On both occasions the epidemics started at the Danish island of Anholt in central Kattegat, and subsequently spread to adjacent colonies in a stepwise fashion. However, this pattern was not maintained throughout the epidemics and new centres of infection appeared far from infected populations on some occasions: in 1988 early positive cases were observed in the Irish Sea, and in 2002 the epidemic appeared in the Dutch Wadden Sea, 6 wk after the initiation of the outbreak at Anholt Island. Since the harbour seal is a rather sedentary species, such 'jumps' in the spread among colonies suggest that another vector species could have been involved. We discussed the role of sympatric species as disease vectors, and suggested that grey seal populations could act as reservoirs for PDV if infection rates in sympatric species are lower than in harbour seals. Alternatively, grey seals could act as subclinical infected carriers of the virus between Arctic and North Sea seal populations. Mixed colonies of grey and harbour seal colonies are found at all locations where the jumps occurred. It seems likely that grey seals, which show long-distance movements, contributed to the spread among regions. The harbour seal populations along the Norwegian coast and in the Baltic escaped both epidemics, which could be due either to genetic differences among harbour seal populations or to immunity. Catastrophic events such as repeated epidemics should be accounted for in future models and management strategies of wildlife populations. KEY WORDS: Epizootic · Harbour seal · Mass mortality · Phocine distemper virus Resale or republication not permitted without written consent of the publisherDis Aquat Org 68: [115][116][117][118][119][120][121][122][123][124][125][126][127][128][129][130] 2006 natural reductions in food supply driven by El Niño conditions have led to high levels of mortality (Trillmich & Dellinger 1991).There is also increasing evidence for mortality resulting from infectious disease. In 1988, up to 60% of North Sea harbour seals Phoca vitulina died during an outbreak of a then newly discovered distemper virus identified by inclusion bodies (e.g. Dietz et al. 1989a, Bergman et al. 1990. This virus was isolated and described as a morbillivirus, phocine distemper virus (PDV) (Osterhaus & Vedder 1988). Subsequently, related dolphin and porpoise morbilliviruses were isolated from cetaceans (Barrett et al. 1993), and widespread screenings suggest that many populations of pinnipeds, cetaceans and sirenians in the North Atlantic had been exposed to these viruses prior to and after the 1988 PDV outbreak (Dietz et al. 1989b, Duignan et al. 1995a,b,c, 1997a,b, Van Bressem et al. 2001. Clinical signs of disease were not recorded in many of the populations in which morbillivirus antibodies were detected (Duignan et al. 1995b, Nielsen et al. 20...

No abstract

The grey seal (Halichoerus grypus) population in the Baltic Sea is recovering after a century of bounty hunting and 3 decades of low fertility rates caused by environmental pollution. A conservative estimate of the population size in 2003 was 19,400 animals, and available data suggest an annual rate of increase of 7.5% since 1990. The growing population has led to increased interactions with the fishery, and demands are being raised for the re-introduction of the hunt. We provide a demographic analysis and a risk assessment of the population, and make recommendations on how to decrease the risk of overexploitation. Although hunting increases the risk of quasi-extinction, the risk can be significantly reduced by the choice of a cautious hunting regime. The least hazardous regimes allow no hunting below a 'security level' in population size. Obviously, to implement such a hunting regime detailed knowledge of the population size and growth rate is required. It is not possible to estimate "true" risks for quasi-extinction, but we used an approach where the relative difference for different scenarios can be compared. With a security level at 5,000 females, the population quasi-extinction risk increases 50 fold at an annual hunt of 500 females compared with a scenario with no hunting. The risk of quasi-extinction is very sensitive to declines in the mean growth rate and to increased variance in growth rate. The variance in the population estimates over the last 14 years imply that it would take 9 years to detect a decline from 1.075 to 1.027 in the rate of population increase. We also show how the age composition of killed animals influences the impact of the hunt. The overall recommendation is that hunting should be kept to a minimum, carefully documented and accompanied by close population monitoring.Harding, K.C., Härkönen, T., Helander, B. and Karlsson, O. 2007. Status of Baltic grey seals: Population assessment and extinction risk. NAMMCO

No abstract

Summary 1.Modelling of the population dynamics of seals require data on an array of vital parameters (fecundity, mortality, age structure, migrations, population growth rate). The most common way to obtain these data is to estimate the parameters from samples taken from the population. However, the in¯uence from skewed samples can be substantial in populations with age-and sex-speci®c features. By quantifying the behavioural dierences among age and sex classes, data from skewed samples can be compensated retrospectively. Awareness of the existence and the potential magnitude of such biases is highly relevant for the design of surveys, sampling programmes and the implementation of management plans of age structured populations. 2. The age-and sex-speci®c behaviour of harbour seals Phoca vitulina and grey seals Halichoerus grypus can be studied by using freeze-branded animals. Since the brand is permanent and visible up to a distance of 500 m, the harassment is limited to one occasion in the lifetime of the seal (the catching day). 3. A method is also given for analysing data arising from re-sighted branded animals, where re-sightings of individual seals were transformed to estimates of relative haul-out frequencies of seals by age and sex. 4. The composition of harbour seal groups on land exhibit a conspicuous seasonal ux, and the fraction on land was not representative of the entire population at any time during the summer. The results have far-reaching implications since most studies of seals are carried out at haul-out sites, and dierential behaviour between the sexes and among age classes is expected in all populations and species of seals. Skewed samples generate biases in estimates of population growth rate, age-speci®c mortality and fecundity. 5. Age-speci®c haul-out patterns must be taken into account when analysing data from populations with non-stable age structures. As a consequence of changes in age structure after the 1988 seal epizootic, surveys under-estimated the size of the Swedish±Danish harbour seal population by 6% in 1988 and over-estimated the same parameter by up to 16% during the following years. 6. The present paper establishes the complications of sampling natural populations that are structured by age and sex, and presents a method on how to quantify sampling errors.

Summary 1.Winter survival rate in Harbour Seal pups is significantly correlated with the autumn body mass of pups. Multi-type mark-recapture statistics were applied to individual re-sighting histories of branded seals, and survival probability was estimated with weight as a covariate. The probability of surviving to an age of 1 year is only 0·63 for the smallest pups at 17 kg, whereas pups at 32 kg have a survival probability of 0·96.2. An energetic model for juvenile Harbour Seals reveals how metabolic rate is related to body mass, skin surface area, blubber thickness and water temperature . There is an increasing thermal stress with decreasing body size of pups. Low winter water temperatures induce a negative energy balance in small pups, which is a probable cause of the observed mass-dependent survival. 3. This study explicitly links a physical property of the environment, sea-water temperature, to energetics and life history. The approach opens possibilities for studying aspects of life-history evolution, such as optimal weaning weight and pupping time, in marine mammals.

We present the first epidemiological data on the 2002 outbreak of phocine distemper virus (PDV) in European harbour seals (Phoca vitulina). The epizootic curve to date supports a mortality rate and probability of infection identical to that of the 1988 outbreak, which killed 58% of the population. Thus immunity is playing no significant role in the dynamics of the current outbreak. Because the timing of the outbreak is important in determining local mortality rates, we predict higher mortality rates on the European continent than in Great Britain or Ireland. A stochastic model is used to quantify how recurrent epizootics affect the long‐term growth, fluctuation, and persistence of the population. Recurrent PDV epizootics with the observed frequency and severity would reduce the long‐term stochastic growth rate of the harbour seal population by half, and significantly increase the risk of quasi‐extinction.

A long-term study of freeze-branded harbour seals (Phoca vitulina) revealed explicit site fidelity. Individuals were followed up to 14 years of age and none of the 163 branded animals were observed to haul out beyond a 32-km distance from the site where they were branded as pups. Within this range, striking spatial segregation by age and sex prevailed. While females' site fidelity increased with age, males spent less time at their natal site with increasing age. These findings have consequences for understanding the population dynamics of harbour seals, since single "colonies" will act as partly isolated "subpopulations" in some contexts but not in others. The differing migration tendencies of the population segments lead to spatially segregated sex and age ratios of subpopulations and create a complex pattern of connectivity among these subpopulations. Ignoring the spatial scale will lead to severe misinterpretations of analyses of basic population-dynamic processes, especially rates of population increase, rates of gene flow, and the dynamics of the spread of diseases. We suggest that when studies have different aims, these should be addressed by encompassing different numbers of subpopulations.