Abstract
Persistent organic pollutants are a concern for species occupying high trophic levels since they can cause immunosuppression and impair reproduction. Mass mortalities due to canine distemper virus (CDV) occurred in Caspian seals (Pusa caspica), in spring of 1997, 2000 and 2001, but the potential role of organochlorine exposure in these epizootics remains undetermined. Here we integrate Caspian seal mortality data spanning 1971–2008, with data on age, body condition, pathology and blubber organochlorine concentration for carcases stranded between 1997 and 2002. We test the hypothesis that summed PCB and DDT concentrations contributed to CDV associated mortality during epizootics. We show that age is the primary factor explaining variation in blubber organochlorine concentrations, and that organochlorine burden, age, sex, and body condition do not account for CDV infection status (positive/negative) of animals dying in epizootics. Most animals (57%, n = 67) had PCB concentrations below proposed thresholds for toxic effects in marine mammals (17 µg/g lipid weight), and only 3 of 67 animals had predicted TEQ values exceeding levels seen to be associated with immune suppression in harbour seals (200 pg/g lipid weight). Mean organonchlorine levels were higher in CDV-negative animals indicating that organochlorines did not contribute significantly to CDV mortality in epizootics. Mortality monitoring in Azerbaijan 1971–2008 revealed bi-annual stranding peaks in late spring, following the annual moult and during autumn migrations northwards. Mortality peaks comparable to epizootic years were also recorded in the 1970s–1980s, consistent with previous undocumented CDV outbreaks. Gompertz growth curves show that Caspian seals achieve an asymptotic standard body length of 126–129 cm (n = 111). Males may continue to grow slowly throughout life. Mortality during epizootics may exceed the potential biological removal level (PBR) for the population, but the low frequency of epizootics suggest they are of secondary importance compared to anthropogenic sources of mortality such as fishing by-catch.
Highlights
Marine mammals are frequently cited as sentinels for marine ecosystem health and function
Investigation of the mass mortalities determined Canine Distemper Virus (CDV) as the cause [11,12,13], but concern remains over the potential role in the epizootics of pollution and other environmental factors [14,15]
Since the identification of CDV in Caspian seals, a key question has been whether the virus was endemic and caused mortalities prior to 1997
Summary
Marine mammals are frequently cited as sentinels for marine ecosystem health and function. Between 1997 and 2001, Caspian seals (Pusa caspica) suffered a series of mass mortalities, affecting 1000 s of animals, which raised international concern both about the status of the Caspian seal as a species and the broader Caspian Sea ecosystem [10,11]. Since the early 20th Century the Caspian Sea has been subject to impacts from invasive species, industrial development, pollution, habitat loss, and unsustainable extraction of natural resources [10]. Against this background of broader ecosystem change, the Caspian seal population has declined by 90% from more than 1 million to around 100,000 individuals since the end of the 19th Century, primarily due to unsustainable commercial hunting [16,17]. The species is listed as Endangered in the International Union for Conservation of Nature (IUCN) Red List [10]
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