Patterns of stranding and mortality in common seals (Phoca vitulina) and grey seals (Halichoerus grypus) in The Netherlands between 1979 and 2008.

Pathological examination of stranded marine mammals provides information on the causes of mortality in their populations. Patterns of stranding and causes of death of dead-stranded seals on the Dutch coast were analyzed over a 30-year period (1979-2008). Stranding data (n=1,286) and post-mortem data (n=379) from common seals (Phoca vitulina) and grey seals (Halichoerus grypus) found dead, or that died before admission to rehabilitation, were obtained from the Seal Rehabilitation and Research Centre database. Data for the years 1988 and 2002, when mass mortality occurred due to phocine distemper virus epidemics, were excluded. Common seal stranding increased from one to nearly 100 per year over this period. This coincides with the increase in the number of common seals in Dutch waters over recent decades. Grey seal stranding increased gradually from one to about 40 per year over the period, reflecting recolonization of Dutch waters by this species. For both species, the trend in stranding of dead seals was found to be in line with that of seals observed in Dutch waters during aerial surveys and did not provide any indications of a relative change in the stranding rate of dead seals. The total monthly stranding rates peaked at more than 120 in June and July for common seals and at nearly 60 in January for grey seals. This coincides with the pupping periods of the two species. Besides phocine distemper, the most common causes of death in investigated common seals (n=286) were by-catch (confirmed and inferred) (19%), pup starvation (7%), intestinal volvulus (7%) and parasitic bronchopneumonia (6%). The most common causes of death in investigated grey seals (n=93) were by-catch (confirmed and inferred) (15%), pup starvation (11%) and trauma (5%). The relative occurrence of by-catch significantly decreased over time for grey seals, but not for common seals. Common seals were affected by infectious disease significantly more often than grey seals, mainly because of a higher occurrence of parasitic pneumonia. Phocine distemper caused mass mortalities among common seals, but not among grey seals. These findings in dead-stranded seals differ in part from those reported elsewhere in live-stranded seals, for which pup starvation and parasitic bronchopneumonia were the main causes of stranding. A substantial proportion of seals in Dutch waters die from causes related to human activity. Continued monitoring of stranding patterns and causes of death is warranted for early detection of changes and the possibility of taking timely management actions.

Monk seal mortality: virus or toxin?

During the past few months, more than half of the total population of about 300 highly endangered Mediterranean monk seals (Monachus monachus) on the western Saharan coast of Africa, died in a mysterious disease outbreak. Epizootiological and postmortem findings were reminiscent of similar outbreaks amongst pinniped and cetacean species in recent years, which were caused by an infection with newly discovered morbilliviruses (for review see osterhaus et al.). Virological, as well as toxicological, analysis performed on tissue samples collected from relatively fresh carcasses during the outbreak indicate that infection with a virus closely related to dolphin morbillivirus (DMV), possibly originating from affected dolphins in the same area, was the primary cause of the outbreak. Therefore it is concluded that vaccination with a safe and effective non-replicating vaccine should be considered as a management tool in the conservation of Mediterranean monk seals.

Relative immunocompetence of the newborn harbour seal, Phoca vitulina.

The immune system of many mammalian species is not fully developed at birth, with newborns obtaining temporary immunological protection from maternal antibodies. Little is known of the immune system of the harbour seal, and developmental aspects of its immune system have not been systematically studied. We collected blood and milk samples from nine free-ranging mother-pup pairs throughout the lactation period on Sable Island, Canada, in an effort to characterise developmental aspects of the immune system of this newborn pinniped. Pup lymphocytes responded stronger to the mitogens concanavalin A, phytohaemagglutinin, and pokeweed mitogen than the lymphocytes of their mothers. In contrast to newborn cats and dogs, newborn seal pups developed high specific antibody responses after immunisation with an inactivated rabies vaccine. Circulating levels of total IgG in newborn pups were low (3% of maternal levels), but increased rapidly after colostrum intake (to 65% of maternal levels after 15 days). A similar pattern of increase in pup serum was observed for phocine distemper virus specific antibodies which had been detected in the serum and milk of mothers, suggesting that the transfer of colostral antibodies is an important feature of temporary protection for the pup. We speculate that the relative immunocompetence of the harbour seal at birth reflects an adaptation to its relatively short nursing period and limited maternal care.

Mitogen and antigen induced B and T cell responses of peripheral blood mononuclear cells from the harbour seal (Phoca vitulina).

In vitro assays were developed for studies concerning the functioning of the immune system of the harbour seal (Phoca vitulina). Proliferative responses of peripheral blood mononuclear cells (PBMC) were measured after stimulation with different concentrations of the mitogens concanavalin A (Con A), pokeweed mitogen (PWM), phytohaemagglutinin (PHA) or lipopolysaccharide from Salmonella typhimurium (LPS). Con A and PWM induced strong proliferative responses, while PHA and LPS induced comparatively low proliferative responses. Responses of mitogen stimulated PBMC to recombinant human interleukin-2 (rhIL-2) and in vitro immunoglobulin production by mitogen stimulated PBMC were measured to discriminate between stimulation of T cells and B cells. It was found that Con A and PHA stimulate phocine T cells, PWM stimulates both T cells and B cells and LPS predominantly stimulates phocine B cells. Antigen-specific immune responses were measured after immunization of seals with an inactivated rabies vaccine and/or with tetanus toxoid. Antigen-specific proliferation of PBMC and the presence of antigen-specific antibody forming cells were demonstrated for both antigens in the PBMC of immunized animals. The responses measured in vitro correlated well with the development of specific serum antibody titers to these antigens.