Seasonal variation of liver and muscle mercury across two populations of a freshwater sport fish
Sofia C Pereira, Timothy J Fernandes, Garth A Covernton, Igor Lehnherr, Brian Branfireun, Stephanie Varty, Taylor Luu, Faduma Jama, Bailey C McMeansAbstract
Objective
North-temperate fishes fluctuate in their energy status, metabolic rate, and behavior in response to seasonal changes in temperature and light, which could influence tissue concentrations of biomagnifying contaminants like mercury (Hg). Yet, repeated sampling within a single year is rarely conducted to quantify if contaminant concentrations vary seasonally across different fish populations.
Methods
We quantified liver total Hg (THg), muscle THg, and liver methylmercury of Lake Trout Salvelinus namaycush, a predatory fish that is harvested for sport and subsistence, from two different lakes in Ontario (Lake of Two Rivers and Lake Opeongo) across all four seasons over two consecutive years. Seasonal increases in Hg were expected to occur during fall, due to decreased Lake Trout energy status following fall spawning, and winter, due to increased winter trophic positions or temperature-driven decreases in elimination rates.
Results
Contrary to our expectations, liver Hg varied seasonally in only one of our study lakes (Lake Opeongo). In this lake, liver THg increased from summer to fall, but only in year 1, and liver methylmercury decreased from fall to winter. Also contrary to our expectations, muscle THg displayed more consistent seasonal patterns and achieved the lowest concentrations in summer in both lakes and years, which could relate to growth dilution. Muscle THg increased by 32% from summer to fall in Lake Opeongo (from 2.078 ± 0.094 to 2.75 ± 0.145 μg/g) and by 55% from summer to winter in Lake of Two Rivers (from 2.054 ± 0.09 to 3.198 ± 0.216 μg/g), which could relate to lower body condition after fall spawning (Lake Opeongo) and higher winter trophic positions (Lake of Two Rivers).
Conclusions
Inconsistent seasonal variation in liver Hg could arise because of dynamic processes of Hg accumulation and redistribution occurring in the liver. However, muscle tissue, which can be sampled nonlethally, could be useful for monitoring seasonal Hg trends based on our findings. Because muscle THg was lower in summer compared with fall and winter, fish Hg samples collected during summer alone may not reflect Hg concentrations during other seasons.