The primary goal of this project is to achieve a better understanding of how the chemistry of Hg
and its distribution in small piscivorous mammals varies with exposure, and how dietary MeHg
exposure impacts individuals and populations of these species. Initially we set out four main
objectives:
1. To determine the nature and magnitude of adverse physiological, immunological, genetic,
and developmental effects of food web MeHg biomagnification on these top trophic level
predators;
2. To compare Hg exposure in mink and otters breeding in environments with differing natural
background levels of Hg, and differing environmental chemistry characteristics (e.g. acidified vs. circumneutral environments);
3. To determine if MeHg exposure at environmentally relevant concentrations adversely affects
the development, genetic integrity, immune and endocrine function, and/or behavior of
captive fish-eating birds in a controlled experiment; and
4. To characterize Hg speciation in otter and mink tissues, including the Hg/Se complex.
To address the first two objectives, we are gathering regional data on concentrations of Hg in
various tissues of mink (Mustela vison) and river otter (Lutra canadensis) from six sites in eastern
Canada. Two sites have been selected in Nova Scotia, one coastal and one interior. In both sites
the exposure is quite high relative to the other provinces (based on known concentrations in fish)
but the coastal region is lower than the interior and it is expected that Se intake is higher in the
coastal region due to marine influences. In Quebec samples are being collected in the Outaouais
(low exposure) and Mistissini (high exposure) regions and in Ontario samples are collected in the
Kinmount (high) and Parry Sound (low) areas. Selenium status of prey fish is unknown at
present in Ontario and Quebec. Animal collections were scheduled over three years to allow us
to collect a meaningful number of individuals from each region. 58 otters and 84 minks were
collected from NS in the first trapping season, completing our requirements for that region. No
collections were made in Year 1 in Quebec, but in Year two, 27 otters and 24 minks were
collected. In Ontario, 72 otter and 26 mink were collected in Years 1 and 2. We are confident
that we will reach our sampling targets in all regions by the end of Year 3. Necropsies and total
Hg analyses have been completed on all animals collected in the 1st trapping season, and most of
those collected in the second season but Se and MeHg analyses are still underway.
Tissues from selected trapped individuals are also used to examine genotoxicity and brain
biochemistry in these animals as a function of exposure. Particularly for genotoxicity tests, the
freshness of the tissues is of paramount importance. For this reason, only selected samples are
suitable.
Although data collection and analyses are currently on-going, some remarkable trends have
already begun to emerge. First, the concentrations of Hg in all sampled tissues are much higher
in NS than in Ontario or Quebec. This is undoubtedly due to differences in fish-Hg
concentrations between these regions; however, the differences among regions are very large.
For example, the average otter fur concentration from our ‘high Hg’ region in NS was 54 ppm
compared to 26 for the ‘low Hg’ region. Both of these values are substantially higher than values
from ON, where fur from ‘high’ and ‘low’ regions averaged 9.5 and 5.7 ppm respectively. What
is extremely interesting is that the brain concentrations do not show the same differences as the
fur concentrations. The slopes of the relationships between fur and brain concentrations for the
various regions are: 12.6, 7.2, 6.7, and 3.5 for the NS high, NS low, ON high and ON low regions
respectively (we have insufficient data from Quebec yet to do a meaningful analysis). Thus, there
appears to be a relationship between the level of exposure and the partitioning of Hg between fur
and brain, with more Hg being diverted to the fur as exposure increases. To our knowledge this is
the first time such a relationship has been demonstrated for any species. It suggests that
elimination of Hg through fur is being used as a detoxification mechanism. Because we are still
waiting for some age analyses, we are not yet able to test the hypothesis that the population
demographics in these trapped animals differ among the regions. Those data should be available
shortly.
As we begin year 3 of this project, our focus is shifting away from use of trapped animals to that
of a captive population. While we had originally intended to use otter trapped in the wild, we
have decided to use mink because we can maintain a larger population in captivity. This work is
intended to address objective three and enhance objective four. From the captive study we will
obtain important information on kinetics of Hg uptake and distribution, while providing high
quality tissues for our biochemistry and genotoxicity projects. We will determine Hg
concentrations in prey-sized fish from lakes within the range of trapped animals. While we have
good data on Hg concentrations in fish from many lakes, we want to have a better measure of the
specific intake of these animals. We will also measure Hg and C and N isotope ratios in gut
contents. These data will be used with the results from both trapping and captive studies to
produce a pharmacokinetic model of Hg distribution and cycling in these animals.
The role of Se in Hg toxicity remains largely unknown. Our fourth objective was designed to add
to our understanding of the role of Se. We have developed a hypothesis to explain the role of Se
in Hg methylation and demethylation in the brain. Initially, we are examining the relationship
between Se and Hg in approximately 300 brains from otter that had been collected by NS Fish
and Wildlife prior to the start of COMERN. Those measurements have been completed and will
provide a starting platform around which experiments will be designed to test the role of
selenocysteine in enzymatically demethylating methylHg in the brain using our captive
population of mink exposed to various levels of meHg and Se in their diets. Following these
experiments, we will test for specific proteins in brains of trapped animals.
Thus, as we start our third year of the project, we are well on our way to completing the four
objectives set out in our proposal. We have added and changed a few aspects of the project, such
as the building of a model for Hg uptake. This model should have applicability beyond the target
species. We are also exploring the possibility of undertaking some capture/recapture experiments
in collaboration with partners.