Research Document - 2013/119
Causes of mortality in St. Lawrence Estuary beluga (Delphinapterus leuca) from 1983 to 2012
By Stéphane Lair, D. Martineau, Lena N. Measures
Abstract
Between 1983 and 2012 a total of 469 beluga carcasses from the St. Lawrence Estuary (SLE) population were reported. Standard necropsies were conducted on a total of 222 of these carcasses (117 females, 103 males and two intersexes) using an evolving but relatively uniform protocol. Primary causes of death were determined for each beluga. Age was estimated by counting growth layer groups (GLGs) of the dentine, using a section of tooth. A total of 178 of these carcasses were adults (≥8 GLGs), 25 were classified as juveniles, and 19 were newborn calves. Causes of death were assigned to the infectious disease category for 32% of the beluga examined. These included bacterial infections (11%), verminous pneumonia (11%), verminous gastro-enteritis / peritonitis (4%), toxoplasmosis (2%), protozoal pneumonia (2%) and herpesviral infections (1%). Bacterial diseases, verminous gastro-enteritis / peritonitis and protozoal pneumonia were mainly seen in adult beluga, with an overrepresentation in males, while verminous pneumonia was the most common cause of mortality of juvenile beluga. A total of 39 cancers were diagnosed in 35 adult beluga examined. These malignant neoplasms were identified as the cause of death in 14% of the beluga examined (or 20% of the adult beluga). Cancer was not observed in beluga with an estimated year of birth after 1971. Adenocarcinomas of the gastro-intestinal mucosa were the most commonly observed neoplasms (7% of mature adults) followed by mammary carcinomas (10% of the mature adult females). Dystocia and post-partum complications were identified as the cause of death in 15% of examined females. The occurrence of dystocias / post-partum complications, as well as reports of mortality of dependent calves, have increased recently in stranded beluga. Other causes of mortality include ship/boat strike (4%), primary starvation (2%), fishing gear entanglement (1%) and saxitoxin intoxication (1 case). Other pathological changes observed include benign tumours such as fibroleiomyomas, hyperplasic and degenerative changes of the adrenal and thyroid glands, non-fatal viral gastric papilloma and ulcero-proliferative genital lesions, and incidental infections by different species of helminths. Comparison with other populations of marine mammals and inferences from controlled laboratory studies as well as robust epidemiological studies in public and veterinary health, led to hypotheses regarding predisposing factors that could account for disease prevalence in SLE beluga. One of these hypotheses seeks to explain some of the pathological conditions observed in stranded beluga due to its chronic exposure to industrial contaminants. Although a cause and effect relationship for an observed association between disease in SLE beluga and chemical contamination may never be conclusively demonstrated, a precautionary approach is advisable. Even if the effect of these diseases on the growth of the SLE beluga population remains difficult to determine, the apparent suboptimal growth of the SLE beluga population indicates the presence of some limiting factors in its environment. Understanding causes of mortality and their etiology can provide some insight on these limiting factors.
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