The warnings come a day after the first commercial opening for gillnetters to catch the province's prized fish, with the next opportunity not expected until next Monday.
Fisheries representatives estimated on Tuesday that 1.8 million salmon will return to the Fraser River during the early summer sockeye run.
A meeting of the Fraser River Panel Tuesday estimated near 50 per cent of the run has already migrated from the ocean, essentially meeting pre-season forecasts.
The panel says test catches show that approximately 90 per cent of the summer run of salmon are currently passing through Johnstone Strait, along the northeast coast of Vancouver Island.
The panel cautions that water temperatures in the Fraser River are expected to climb 1.5 degrees to 19.5 degrees Celsius, while water levels at Hope, B.C., have been measured at 22 per cent lower than average. The panel says sustained water temperatures in that range can cause severe stress to migrating sockeye and may lead to "significant" deaths while en route.
Estimates for the late run salmon expected later this summer has been pegged at a return of 23 million fish.