1. The glacial relict, Mysis relicta, which occurs mainly in unproductive cold water habitats, shows increased mortality rates in temperatures above 16-18 degrees C. Using a 12-year data set the growth, mortality and fat stores of Mysis in Lough Neagh, an increasingly hypertrophic lake which lacks a thermal refuge and which is subject to warming, were investigated. 2. Mysis showed a significant reduction in fat stores on reaching sexual maturity, contrary to studies elsewhere. 3. The contributions of temperature and eutrophication (total phosphorus, chlorophyll-a concentrations) to growth, mortality and the timing and duration of developmental events were investigated by regression. 4. Water temperatures rose between 1994 and 2005 and the time when temperatures exceeded 16 degrees C doubled over this period. Juvenile and adult growth rates declined and the duration of the juvenile stage increased in warmer years. Eutrophication also affected these variables but, in general, to a lesser extent than temperature. 5. Year class strength (YCS) declined by a factor of 10 over the study period. YCS was independent of temperature up to 2000 degrees C degree days (> 4 degrees C) but declined steeply above that point. Temperature was more important than eutrophication in predicting mortality and YCS. YCS was positively correlated with the length of time water temperature was below 7 degrees C, the upper temperature for Mysis breeding. 6. An established bioenergetic model predicted that water temperature and the timing of embryo release would have appreciable effects on Mysis viability. It also indicated that Mysis would lose mass at summer temperatures, consistent with the observed fat decline.