COPING WITH A WARM ENVIRONMENT - BEHAVIORAL THERMOREGULATION BY LAKE TROUT

Temperature-sensing ultrasonic transmitters were used to monitor the b ehavior of eight individual lake trout Salvelinus namaycush in two lak es: a shallow lake that becomes isothermal (15-20 degrees C) during la te summer and a typical dimictic lake with a cold hypolimnion. Use of a cool, nearshore groundwater discharge site, a behavior never before documented for lake trout, was observed among some fish in the shallow lake when ambient lake temperatures exceeded the species' preferred t hermal range of 6-13 degrees C. We suggest that during very warm years , groundwater refugia may be important habitat for lake trout and that these refugia should be protected.