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 whether contaminant concentrations vary seasonally across different fish populations.

Methods

We quantified liver total Hg (THg), muscle THg, and liver methylmercury (MeHg) 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, LTR; Lake Opeongo, OPE) 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 (OPE). In this lake, liver THg increased from summer to fall, but only in year one, and liver MeHg 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 from summer to fall in OPE and from summer to winter in LTR, which could relate to lower body condition post-fall spawning (OPE) and higher winter trophic positions (LTR).