Subglacial discharge plumes increase submarine melting of marine-terminating glaciers significantly; however, in-situ data on their properties and behaviour are limited. We present oceanographic data collected by ringed seals (Pusa hispida) instrumented with GPS-equipped conductivity-temperature-depth satellite relay data loggers (GPS-CTD-SRDLs) in Kongsfjorden, Svalbard, during 2012. The seals foraged just outside the plumes and collected hydrographic data from within the plumes' upwelling cores as they returned to the surface. The seals encountered water with fractions of subglacial discharge as high as 27% at 60 m below the ocean surface. The ringed seals responded rapidly to spatial and temporal variations in subglacial discharge at the glacier terminus, suggesting that prey becomes available quickly following the appearance of plumes. The seals' dive locations were used to monitor the presence of plumes over a four-month period. High surface runoff from Kronebreen catchment created strong plumes, but weak plumes were present even during periods of low surface runoff. The continued retreat of Kronebreen, and other tidewater glaciers, will lead to the loss of these marine-termini as the glaciers retreat onto land. The techniques presented here improve our understanding of the drivers of glacial retreat and the implications of future habitat loss for glacier-associated birds and mammals.