ABSTRACT
The St. Lawrence River estuary (SLE), downstream from the Great Lakes, is home to the southernmost population of beluga whales (Delphinapterus leucas). Hunted for food and as a pest until the 1950s, the population has dwindled from an estimated 5000 to approximately 500 animals in the 1970s and has been listed as an endangered population (Reeves and Mitchell, 1984; Michaud, 1993). The population has not recovered over the past 30 years and has been under scrutiny for a decade. Post-mortem examination of carcasses retrieved from the shores of the St. Lawrence estuary since 1982 has shown a high prevalence of infectious, degenerative or necrotic lesions, often associated with mildly pathogenic organisms or neoplasms (Martineau et al., 1988; DeGuise et al., 1994, 1995a). The frequency, diversity and severity of lesions described in this population were considerably higher than those found in marine mammals elsewhere. Worldwide, a total of 75 tumors have been reported in cetaceans, of which 28 (37 per cent) have come from the small SLE population of beluga whales. Consequently, a link was suggested between toxic contaminants in the SLE food web and these observations. Because SLE beluga whales are generally restricted to the St. Lawrence estuary, they are exposed to sediments, and prey on invertebrates and fish, contaminated from industrial and agricultural discharges originating from the Great Lakes, the St. Lawrence River and its tributaries. In addition, the relatively long life span of the beluga whale, its diet and the year-long residency in the St. Lawrence, result in a greater accumulation of contaminants compared to other marine mammals such as seals. Higher concentrations of organohalogens, benzo[a]pyrene (B[a]P), polychlorinated biphenyls (PCBs), dichlorophenyl trichloroethane (DDT), mirex, mercury and lead are found in tissues of SLE belugas compared to Arctic belugas (Martineau et al., 1987; Muir et al., 1990; Wagemann et al., 1990). Among these chemicals, many were demonstrated to have adverse effects on different aspects of the normal physiology of laboratory animals and wildlife, including effects on the reproductive system. The immune system also may be a potential target. There is ample evidence that organohalogens (Loose et al., 1977; Thomas
and Hinsdill, 1978; Imanishi et al., 1980; Exon et al., 1985; Davis and Safe, 1988; Smialovicz et al., 1989; Tryphonas et al., 1989; Kerkvliet et al., 1990), pesticides (see review by Voccia et al., 1999) and metals (see review by Bernier et al. 1995) have detrimental effects on the immune system of humans and animals, decreasing resistance to viral, bacterial and parasitic infections as well as inducing neoplasms. Because little was known about the immunology of marine mammals, research was undertaken to develop assays to evaluate immune functions in belugas, to further evaluate the possible effects of environmental contaminants on their immune systems.
