Time Trends in Fish Tissue Methylmercury in Northern Watersheds: Implications of Phosphorus Loading and Eutrophication on Subsistence Fisheries

Abstract

Subsistence fisheries for Michipicoten First Nation (MFN) in habitats across an area north of Lake Superior in Ontario, Canada were assessed. This assessment used reports by Ontario, private entities (e.g., mines), and MFN to evaluate contaminant concentrations in fishes from the 1980s to 2021; methylmercury was determined to be the contaminant of primary concern in fish tissues. Methylmercury tissue concentrations for varied fish species from four lakes and one river were used to establish contaminant-fish length relationships. Observed methylmercury tissue concentrations for these fishes allowed for the creation of updated consumption recommendations in MFN’s subsistence fisheries. This study recommended updated consumption rates for fish species including Northern Pike (Esox lucius), Walleye (Sander vitreus), Lake Trout (Salvelinus namaycush), Burbot (Lota lota), Lake Whitefish (Coregonus clupeaformis), White Sucker (Catostomus commersoni), Longnose Sucker (Catostomus catostomus), and introduced Smallmouth Bass (Micropterus dolomieu). Elevated methylmercury concentrations followed increased eutrophication in these naturally oligotrophic watersheds from loading of plant nutrients, from both diffuse and defined regional sources. Nutrient mitigation measures to control in situ methylmercury production cannot be implemented as neither the nature or extent of past or current nutrient loading from various sources has been identified or estimated in the region.