Health, disease, mortality and survival in wild and rehabilitated harbor seals (Phoca vitulina) in San Francisco Bay and along the central California coast

Abstract

Conventional methods for health assessment of wild-caught and stranded seals were used to describe the disease status of harbor seals in California. Clinical chemistry, infectious disease prevalence, immune function, and contaminant data were collected to evaluate harbor seal health with data collected from three groups of seals. Wild-caught seals of all ages were sampled at two locations: San Francisco Bay (a heavily urbanized estuary) and Tomales Bay (a less developed control site). Stranded seals entered rehabilitation from a more extensive portion of the California coast which included the locations where seals were caught. Hematology reference intervals were generated to provide a baseline for health assessment among the seals. Individual variability in blood variables among seals was affected by age, sex, location, and girth. Disease surveillance focused on pathogens known to cause lesions in harbor seals, zoonoses, and those with terrestrial sources. Specific pathogens of interest were E coli, Clostridium perfringens, Vibrio spp, Campylobacter spp, Salmonella, Giardia, Cryptosporidium, avian influenza virus, Brucella, Leptospira spp., Toxoplasma gondii, Sarcocystis neurona, and Neospora caninum, Leptospira spp, and phocine and canine distemper virus. There was evidence of exposure to all pathogens except for phocine distemper virus. Simple measures of immune response were used to evaluate the immune function of harbor seal pups in rehabilitation that had evidence of previous bacterial infection. The swelling response to a subcutaneous injection of phytohemagglutinin (PHA) was positively associated with growth rate, possibly illustrating the energetic trade-offs between growth and immunity. Blubber contaminant concentrations (PCBs, DDTs, PBDEs, CHLDs, and HCHs) in harbor seal pups were grouped by extent of suckling and strand location. The ratio of PCB:DDT was increased in San Francisco Bay and decreased in Monterey Bay compared with other locations along the coast. Pups that weaned in the wild, lost weight and then stranded had the highest contaminant levels, equivalent to the concentrations detected in stranded adult harbor seals. Dispersal and survival were monitored by satellite telemetry in harbor seal pups released from rehabilitation and recently weaned wild-caught pups to assess the effect of condition, health, and contaminant levels on survival probability. Increased contaminant levels and decreased thyroxine (T4) were associated with decreased survival probability. Increased mass, particularly among the rehabilitated pups, was associated with increased survival probability. This study demonstrates that health and survival of harbor seals pups along the central California coast are impacted by human activities such as contaminant disposal, pathogen pollution and boat traffic, although the variability in individual health measures requires carefully designed studies to detect these effects.