The Role of Canine Distemper Virus and Persistent Organic Pollutants in Mortality Patterns of Caspian Seals (Pusa caspica)

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

Boxplot of blubber thickness (cm) for Caspian seal carcases stranded 1997–2009, showing seasonal variation in body condition.

<p>Boxplot of blubber thickness (cm) for Caspian seal carcases stranded 1997–2009, showing seasonal variation in body condition.</p

Relative frequency histograms showing distribution of ∑PCB and ∑DDT levels for necropsied seals (n = 67).

<p>Relative frequency histograms showing distribution of ∑PCB and ∑DDT levels for necropsied seals (n = 67).</p

Boxplot for number of Caspian seal carcases recorded monthly (1971–2008) in the 10 km Apsheron monitoring zone.

<p>Central line of boxes gives median carcases per month. Red line gives mean carcase number.</p

Time series histogram for annual Caspian seal mortality 1971–2008, along the 10 km Apsheron monitoring zone (40.523 N, 50.119 E and 40.501 N, 50.226 E).

<p>Time series histogram for annual Caspian seal mortality 1971–2008, along the 10 km Apsheron monitoring zone (40.523 N, 50.119 E and 40.501 N, 50.226 E).</p

Comparison of PCBs, DDTs and Blubber thickness in CDV+ and CDV− animals (Welch 2 sample <i>t</i>-test).

<p>Comparison of PCBs, DDTs and Blubber thickness in CDV+ and CDV− animals (Welch 2 sample <i>t</i>-test).</p

Comparison of Total TEQs in Caspian seals dying in 2000 with seals in the Netherlands feeding experiment (experimental data from Ross et al., 1995; Caspian data from Kajiwara et al. 2002; Kuiken et al. 2006).

<p>Comparison of Total TEQs in Caspian seals dying in 2000 with seals in the Netherlands feeding experiment (experimental data from Ross et al., 1995; Caspian data from Kajiwara et al. 2002; Kuiken et al. 2006).</p

Histogram of age structure among Caspian seal carcases stranded 1999–2002.

<p>Histogram of age structure among Caspian seal carcases stranded 1999–2002.</p

Boxplots of contaminant burdens by sex-age class (top) and CDV status (bottom).

<p>Red dashed line indicates proposed threshold for potential toxic effects for ∑TEQ levels (196−220 pg/g lipid weight) derived from experimental studies in harbour seals (De Swart et al. 1994, 1995a, 1995b, 1996, Ross et al. 1995). y axis is plotted on log10 scale.</p

Anova comparison for log10(PCBs), log10(DDTs) and log10(Blubber+1) versus, Sex-age class, Country, Year and Season.

<p>Anova comparison for log10(PCBs), log10(DDTs) and log10(Blubber+1) versus, Sex-age class, Country, Year and Season.</p