The State of Canada's Environment — 1996 Chapter 8 — Taiga Ecozones Human Activity and the Environment The long-range transport of contaminants Organochlorine contaminants

Organochlorine contaminants

High levels of organochlorine contaminants, such as polychlorinated biphenyls (PCBs), toxaphene, and dichlorodiphenyltrichloroethane (DDT), can cause various disorders in fish and other wildlife, as they have, for example, in the Great Lakes region (see Chapter 6). Although the use of PCBs and the DDT group of pesticides largely stopped in the Western world in the 1970s, their use continues in some developing countries. In addition, these contaminants can take decades, or even centuries, to break down. As organochlorines accumulate in animal fat, people who consume high levels of fatty tissue of species at the top of food webs are at greater risk of accumulating organochlorines. Polar Bear tissue and seal blubber contain PCBs and DDT, in particular, and walrus blubber from eastern Hudson Bay also contains PCBs. There has been little use of organochlorines in the Taiga, and thus the presence of these substances in the area's biota is evidence of long-range transport.

Table 8.9 provides a summary of findings on organochlorine contamination of fish within the Taiga. With one exception, in all cases where toxaphene was measured, it was present in higher concentrations than PCBs, which, in turn, were more prevalent than the sum of DDT-related compounds and total chlordane. In the majority of cases, analyses were done on muscle tissue. Considerably higher concentrations were found in liver tissue of Burbot. Livers of few other fish species have been analyzed. On a whole fish basis, burbot have similar contaminant levels to lake trout.

Table 8.9 Mean concentrations of some organochlorines in freshwater fish

Keywords for Table 8.9
Species; Broad Whitefish; Burbot; Lake Trout; Lake Whitefish; Northern Pike; Cisco; Walleye; Campbell Lake; Kugaluk River; L 100; Horseshoe Bend; Travaillant Lake; Alexie Lake; Great Slave Lake; Mackenzie River; Trout Lake; Belot Lake; Colville Lake; Gordon Lake; Lac Bienville; Lac Raraire; Lac Morpain; Lac des Loups Marins; Lac Amichinatwayach; Grand-Baleine lakes; Hay River; muscle; skin; liver; Mackenzie Delta; Yellowknife; Fort Resolution; Fort Good Hope; Fort Simpson; Colville; Grande-Baleine; tissue; DDT; PCBs; toxaphene .

Particularly high levels of toxaphene were detected in livers of Burbot from the Slave River, and Health Canada has recommended limiting consumption of Burbot livers from these waters (DIAND 1996). For any individual species, variations in organochlorine contaminant levels from one location to another are attributable to factors such as dietary differences and a tendency for contaminant concentrations to increase from north to south. Also, there tends to be a positive correlation between fish size and level of contamination.

By way of comparison, whereas PCB levels in Lake Trout at Taiga sites were generally less than 50 ppb, levels in the same species in the Canadian Great Lakes ranged from about 500 ppb in Lake Superior to about 2 000 ppb in Lake Ontario during the early 1990s (see Chapter 6). Similarly, for toxaphene, levels in Lake Trout in the Taiga have generally ranged between 10 and 100 ppb, compared with levels as high as 1 900 ppb in Lake Michigan in 1990 (De Vault et al. 1995).

Among mammals, mink eat from both terrestrial and aquatic food webs (small mammals and fish). This makes them particularly susceptible to contaminants in the food web and therefore a good indicator of such contaminants. A study of Mackenzie Valley mink found a distinct trend of increasing organochlorine contaminant burdens with decreasing latitude: mean total PCB residues ranged from 5.32 ppb wet weight in the livers of Inuvik area mink to 27.67 ppb in mink from the Fort Smith area. Dietary differences were not thought to be a significant contributing factor in the trend (Poole et al. 1995). These levels are low in comparison with those found in more southerly areas. For example, total PCB levels averaging 390 ppb and ranging from 34 to 1 797 ppb have been found in the livers of mink in southern Ontario (Muir et al. 1996).

A study of a range of contaminants in Caribou found that, in general, organochlorine residues, including PCBs, were significantly lower in Caribou from the mainland Bathurst and Qamanirjuaq herds than in Caribou from southern Baffin Island. Higher levels in the east were attributed to differences in atmospheric transport pathways (Elkin and Bethke 1995).

Very little long-term information is available on organochlorine contaminants in the North. Although trend data exist for Arctic marine mammals, there is comparatively little information available for freshwater and terrestrial species (Muir et al. 1996). However, declining concentrations of toxaphene and other organochlorines have been noted in the livers of Burbot taken from the Mackenzie River at Fort Good Hope.