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The impact of health and environmental factors on a population of Mekong River Irrawaddy dolphins (Orcaella brevirostris) in Cambodia

Dove, Verné (2022) The impact of health and environmental factors on a population of Mekong River Irrawaddy dolphins (Orcaella brevirostris) in Cambodia. PhD thesis, Murdoch University.

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The critically endangered Mekong River Irrawaddy dolphin (MRID) (Orcaella brevirostris) population, estimated to comprise 85 individuals in 2010, is at risk of extirpation due to a high level of mortality, particularly in calves, and a very low recruitment level (<1%) into the population; however existing studies have failed to identify the causes of mortality in this population. In this study, a retrospective study was conducted to better understand the causes of mortality in different age classes, so that potentially mortality levels can be mitigated and recruitment rates improved.

Mortality data from 2001-2010 were analysed to identify mortality trends in 102 MRIDs. Samples were also collected from 41 necropsied dolphins and subjected to microbiological, histopathological, genetic and toxicological analyses to identify and quantify the threats to this population. Additionally, a study of skin lesions using photo-identification data of live MRIDs from 2007 to 2010 was conducted.

In this study, significantly high levels of immuno-toxic pollutants, particularly DDT and mercury; genetic factors, specifically genetic drift and outbreeding depression; and disease and immunosuppression; were all contributing towards the population’s decline.

Dead calves were 15 times (95%CI 2.4, 88.1, p=0.001) more likely to have a localised gangrenous, blue/black neck lesion than dead adult dolphins. These lesions were clinically, pathologically and microbiologically similar to necrotising fasciitis of humans. In contrast, adults were 17 times (95%CI of 1.8, 166.1, p=0.006) more likely to have evidence of interaction with fishery activities than juveniles and calves.

The incidence of mortality in adults over a three year period when gillnets were used was 18.3% (95%CI 11.6, 26.9), and this reduced to 6.7% (95%CI 2.5, 14.1) when restrictions on the use of gillnets were introduced. The risk of dying during the three years with gillnets was 2.7 times (95%CI 1.1, 6.5) higher than the period when gillnet use was restricted (p=0.016). In contrast, there was no significant difference in mortality for calves for years with and without gillnet restrictions (p=0.39).

Skin lesions were common in live MRIDs being detected in 35 of 84 individuals observed (41.6%; 95%CI 31.1, 52.2) over a three year period. However, the mean prevalence of MRIDs photographed with skin lesions during 11 surveys was 7% (95%CI 1.9, 16.7), with a significant difference found in the prevalence of skin lesions in MRIDs from pools 1 and 9 (Fisher’s exact test p=0.002).

In necropsied MRIDs (n=8), significantly more calves 100% (95%CI 47.8, 100) had neck lesions and associated neck swelling with wet gangrenous musculature than adults (0%; 95%CI 0.0, 70.8) (p=0.018). The three most frequent bacteria cultured in 13 dolphins examined with neck lesions were Aeromonas hydrophila (54%, 95%CI 25.1, 80.8), Plesiomonas shigelloides (46%, 95%CI 19.2, 74.9), and ß-haemolytic Group C Streptococcus spp. (23%, 95%CI 5, 53.8). Toxicological analysis in a larger sample set (n=20), revealed that the levels of mercury (Hg) and methylmercury (MeHg) were significantly higher in the adult dolphins sampled than in calves (11.21μg/g vs. 1.51μg/g, p=0.019; 7.23μg/g vs. 0.72, p=0.033, respectively). In contrast, calves had significantly higher levels of liver copper (Cu) than adults (40.72μg/g vs. 3.49μg/g, p=0.0005). Adults also had a significantly higher level of Hg (11.21 μg/g vs. 0.91 μg/g, p=0.026) and MeHg (7.23 μg/g vs. 0.17 μg/g, p=0.006) than juveniles. The mean molar selenium:mercury ratio was significantly higher in calves (2.27, p=0.039) and juveniles (2.94, p=0.007) than in adults (0.95). The level of zinc (Zn) in the liver of adults with neck lesions (28.7μg/g) was significantly lower (p=0.049) than that of adults without evidence of neck lesions (65.17μg/g). The concentrations of organochlorines in the blubber of MRID decreased in the order of DDTs > PCBs > CHLs > HCB > PBDEs > Dioxins > OC. The level of ΣDioxins was significantly higher in adults (2.27TEQ) than in calves (1.15TEQ, p=0.004), and also significantly higher in juveniles (3.57TEQ) than in calves (p=0.0094). The levels of PCBs in adults (346.9ng/g) and in juveniles (705.4ng/g) were significantly higher than in calves (142.75ng/g) (p=0.019, p=0.012, respectively). Adult females had higher ΣDioxin levels (61.3pg/g; 2.27TEQ) than female (3.7pg/g, p=0.0017; 1.15 TEQ p=0.02) and male calves (2.8pg/g, p=0.003; 0.99 TEQ, p=0.01). Juvenile females had significantly higher ΣDioxins (3.57TEQ) and ΣPCBs (705.4ng/g) than female (1.15TEQ, p=0.02; 163.2ng/g, p=0.016) and male calves (0.99TEQ, p=0.01; 142.8ng/g, p=0.03) respectively. Adult males had significantly higher PCB levels (606.8ng/g) than female (163.2ng/g, p=0.01) and male calves (142.8 ng/g, p=0.03). The levels of ΣPBDE, ΣCHLDs, ΣDDT and ΣHCB were all significantly higher in MRIDs than in the Chilika Lake Irrawaddy dolphins from the study of Kannan et al. (2005) (p=0.0007).

Phylogenetically the MRIDs appear to be an evolutionary significant unit (ESU), with six new haplotypes identified in this study. The average nucleotide diversity (π) was 0.001 ( SD 0.0001) and the average haplotype diversity (h) was 0.812 ( SD 0.333). 34% (n=11) of the necropsied MRIDs had Infrequent/Rare (I/R) alleles, with an excess found in the adults compared to the calves, indicating a strong likelihood of genetic drift occurring. Paternity tests suggested reproductive failure, as only a few dolphins were breeding in the population and breeding females were genetically related.

It is concluded that two principal factors are driving the population decline. Firstly, the interaction of adults with commercial fishery activities and secondly, necrotising fasciitis associated with neck lesions in the calves. Furthermore, reproductive failure is a major contributing factor limiting recruitment into the population and ultimately affecting the population stability. This study highlights the need for urgent attention to save this population from certain extirpation.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Veterinary Medicine
United Nations SDGs: Goal 14: Life Below Water
Supervisor(s): Robertson, Ian, Warren, Kristin and Holyoake, Carly
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