In this article the observation and spread of brook lamprey (Lampetra planeri) will be discussed as a bio-indicator and a ‘subtle sign’ in the boreal. Brook lamprey is a small non-parasitic freshwater lamprey species indicating good ecological health of aquatic habitats. This article presents knowledge co-produced through a 7-year monitoring programme in the south boreal catchment area of the Jukajoki River, North Karelia, Finland. Over the past one hundred years, this basin has been negatively affected by human land use. Monitoring methods employed as part of this programme have included both rigorous scientific sampling and large-scale traditional and local knowledge (TEK) monitoring. International long-running community monitoring efforts are assessed to position these Finnish traditional knowledge flows. Examples provide for the discussion of new monitoring and restoration methods of boreal aquatic habitats and contribute to the new realisation of these landscapes that were once hidden and now positioned to emerge, providing the suitable social-geographical space is present and accessible to allow for that.