Studies carried out in Scandinavia have demonstrated that intense fishing could significantly
reduce the levels of mercury in fish. However, the mechanisms responsible for this reduction
remained unclear. Over a 6 years period (1997-2002), our study was designed to increase our
understanding of the mechanisms responsible for the reduction of the fish mercury concentrations
following intensive fishing and to eventually determine the feasibility of applying intensive
fishing to young hydroelectric reservoirs and/or to larger natural lakes as a mitigating measure
aimed at reducing potential health risks associated with the consumption of fish. Initially, we
proposed three different mechanisms that could in part explain the hypothesized reduction of
mercury concentrations in fish: (1) a removal of fish biomass could modify the quantity of
mercury in the lake ecosystem; (2) a shift in fish diet could occur, possibly changing mercury
uptake by fish; (3) fish growth rates could increase, resulting in mercury biodilution in fish. Five
oligotrophic experimental lakes (later referred to as 1, 19, 19A, 20 and Des Champs) were fished
at different intensities in the Nemiscau region of the James Bay territory in northern Québec. In a
first approximation, we used an arbitrary ratio of the fish biomass removed divided by the volume
of the lake to classify the intensity of the fish removal Overall, lake 19A had at least four times
more fish biomass removed than the other lakes in relation with the volume of the lakes (table 1).
Lake 19 also exhibits a ratio of the drainage area vs lake area of at least 3,5 time lower than the
others lakes, and is characterized by a lower primary production. The intensive fishing was
carried out from 1997 to 2000, and the lakes were visited again in 2001 to monitor the effect of
intensive fishing on fish communities.
Table 1
Lakes
Total biomass removed/volume of the lake (kg/km3)
Drainage area/lake area (km2/km2)
Primary production (µgChl a/L)
1
Des Champs
19
19A
20
4,97E-05
2,01E-05
4,87E-05
1,62E-04
4,74E-05
12,95
98,70
3,77
132,63
32,89
1,4 ±1,1
1,0 ±0,1
0,6 ±0,1
0,8 ±0,6
0,9 ±0,3
Significant decreases in the Hg levels in walleyes at a standardized length of 400 mm were
observed in lake 19 (from 553±36 ppb to 378±26 ppb) and 19 A (from 544±33 ppb to 449±33
ppb) (ANCOVA, p< 0.05). For lake whitefish at a standardized length of 400 mm, Hg levels
decreased from 81±8 ppb to 64±8 ppb in lake 19 (ANCOVA, p< 0.05). No statistically significant
differences in Hg levels were observed for the other main fish species (northern pike and white
sucker) before and after fishing, although northern pike tend to have lower mercury levels after
fishing in the lakes 19, 19A and 20 and slightly higher Hg levels in lakes 1 and Des Champs.
We tested the three hypotheses that could explain reduction in Hg concentration in walleye and
whitefish. 1) Mass balance calculations in the lakes before and after fishing show that most of
the Hg and MeHg (over 90%) is present in the first layer of sediments and that no significant
amounts of Hg or MeHg is removed from either lakes. Our results show no major change in the
abiotic compartments (sediments, SPM, dissolved fraction) before and after intensive fishing,
indicating very little effect on the inputs of Hg and MeHg at the base of the food web of the lakes.
2) Change of diet: Fish stomach content analyses show no change in the diet of non-piscivorous
species but suggest some changes in the diet of piscivorous species. However, stable isotope
ratios in predatory fish flesh showed no significant change after intensive fishing indicating no
change in trophic levels or food web structure. 3) Biodilution: The increase in growth rate of
walleye and lake whitefish in lake 19 and 19A seem to be related to the decrease in fish Hg levels
observed in both lakes. These results might be explained by the large fish biomass removed from
lake 19A, as compared to biomasses removed from the other lakes (table 1). The low productivity
in lake 19 (caused by the lack of essential nutrient coming from the very small drainage area)
reduced the fish supporting capability of the lake and therefore makes it more vulnerable to
overexploitation (table 1). The massive fish biomass removal in both lakes has created a
significant increase in growth rates (about 20%) for these two species (compensatory response
due to a decrease in competition in the two populations from lake 19 and 19A).
Our study showed that intensive fishing could significantly reduce mercury levels in fish, nor
through a reduction of the lake mercury pool, neither through a shift in fish diet, but through an
increase of fish growth rates. These results will shift our traditional way of studying
bioaccumulation of mercury in fish. From now on, fish growth rates must be included in all
bioenergetics models describing the presence of mercury in fish. On a larger scale, this finding
could also affect the way fisheries managers deal with stock abundance and mercury
contamination of the halieutic resources. Although results of such an experimental project cannot
easily be transferred to larger lakes and reservoirs, the applicability of intensive fishing to young
reservoirs seems less feasible if the main operating mechanism is biodilution. Indeed, prior
measurements of Hg levels in fish from reservoirs often indicate a 3 to 8 fold increases of
mercury levels, in spite of concomitant important increases in growth rates. However, intensive
fishing could be effective in larger lake ecosystems if either: a) the intensity of fishing is
performed at the same order of magnitude as the one realized in lake 19A, or b) the lake in which
fishing is performed is as sensitive as lake 19, in terms of oligotrophic status. Under these
condition we hypothesize that sustained fishing for many decades in lakes of the Canadian Shield
can effectively lead to reduced mercury concentrations in large predatory fish.
The results of this project have been popularized to the Nemaska community in order for its
members to take enlightened decisions about their fishing practices and fish consumption.
Finally, this research project sets the basis for understanding and evaluating the impact of fishing
pressure on lakes and provides a tool for the maintenance of relatively low Hg levels in fish (for
example in Lake St. Pierre), therefore fulfilling COMERN’s fifth objective. New research ideas
have emerged from this study, such as the importance of fish growth rate in the process of
mercury accumulation in fish. Therefore, we will for now on try to link mercury levels in fish to
important factors that affect fish growth rate, such as lake trophic level (amount of food available)
and fishing pressure. Thus, we will be working in larger lakes like Lake St. Pierre (project 3.2.4)
and lakes from the Chibougamau and Abitibi-Témiscamingue regions (project 3.2.2). So far, we
found that Hg levels in predatory fish from lake St. Pierre (Project 3.2.4) were about three times
lower than equivalent size fish from many lakes of the Boreal Forest (Project 3.2.2). In order to
explain this astonishing discovery, we are presently testing the hypothesis that fish in lake StPierre have higher growth rate than the ones from lakes of the Boreal Forest, as Lake St. Pierre
has been intensively harvested for decades by both commercial and sport fishers.
For year 3, a substantial part of our study is symbiotically fed by existing fish and lake
characteristics data set from Lac St-Pierre (project 3.2.4), from six lakes from the AbitibiTémiscamingue regions (project 3.2.2), from three lakes in the Chibougamau region (project
3.2.2) and from five lakes in the Nemiscau region (Y2 project 3.1.3.5). The fish data set already
includes information on species, abundance, length, weight, mercury level and for some of them
fish age and diet. The lake characteristics data set includes data on bathymetry, pH, water color,
conductivity, dissolved oxygen, nutrients, DOC, suspended particulate matters, dissolved Hg and
Chlorophyll a. We will need to get some complementary data on mercury levels in organisms at
the bottom of the food web (plankton, benthos and small fish) for the Chibougamau and AbitibiTémiscamingue lakes and on lakes characteristics for the Chibougamau and Nemiscau lakes.
Also, in 2003, eleven new lakes will be sampled in Abitibi-Témiscamingue with the collaboration
of the Zec Kippawa and the FAPAQ.
We will compare previous data on mercury levels in fish and in their prey from the different
lakes. We will do some age determination on fish structures (otolith, cleithrum, operculum) that
had been sampled from previous year, for the lakes where the fish aging as not been done (lakes
of Abitibi-Témiscamingue and Chibougamau regions). We will estimate growth rate on fish by
species, for every lake, with the Von Bertalanffy growth rate model. To understand what is
affecting the growth rate we will study: 1) the trophic level of the lake considering nutrients
levels (P, N, Ca, S), chlorophyll a, dissolved organic carbon and suspended particulate matter
(Wetzel, 1983). We will use the morphoedaphic index to predict the potential fish yields for a
lake from a minimum of information, namely the total dissolved solids concentration and the
mean depth. 2) the food web structure for the different species captured in the different lakes
using stable isotopes and stomach contents. 3) the fishing pressure will be estimated from data
available in communities and government agencies, and also from data gathered through
questionnaires prepared and use by the health and education teams in lakes of AbitibiTémiscamingue and Lac St-Pierre (project 3.2.1.1 and 3.2.1.2). We will also study fish
population dynamics using some of these parameters: length and age structure, yield index,
abundance, mortality rate, year class strength, age at maturity.
Knowledge that will come out of this critical gap study will help fishing communities to manage
their fish resources and watershed, by applying their traditional knowledge, in order to minimize
the exposure to mercury.