Northern Water staff work alongside Colorado Parks and Wildlife (CPW), which annually conducts fish population surveys by sampling fish from numerous locations in these lakes and reservoirs. CPW provides a handful of sampled fish to Northern Water staff, who then collect a small amount of tissue from each. The mercury content of the tissues is determined by an independent laboratory, before Northern Water submits the data to the Colorado Department of Public Health and Environment (CDPHE). CDPHE then convenes the Fish Consumption Advisory Technical Advisory Committee, a group of experts, to evaluate the mercury levels and determine if advisories are needed. When they are needed, CDPHE provides graphics that Northern Water uses to print and post signs – usually near boat ramps.
Mercury – much of which is emitted from various industrial processes – over time falls from the atmosphere, some ending up in lakes and reservoirs, where, under the right conditions, is converted by some naturally occurring bacteria into chemical forms that can enter the food web. If mercury reaches concerning levels in fish, usually in larger predator fish, the public is advised to limit their consumption of those particular fish.