Fraser River sockeye salmon: data synthesis and cumulative impacts

Key metrics: The most important metric is temperature. The key metrics examined changes overtime and found clear temporal patterns. The pattern varies among Fraser sockeye stocks and arelikely to be similar for non-Fraser sockeye stocks such as those in the Columbia River.Biological hypothesis: Thermal and related issues play a large role in among- and within-stockvariations in en-route and pre-spawn mortality. Differences in migration distance and entrytiming among stocks directly affects their thermal migratory experience (those that enter earlyand travel long distances accumulate more hot days and hotter days). The Early Summer andSummer stocks encounter the highest Fraser temperatures.Thermal changes: Since the 1950s, there has been an increase of about 2 degrees Centigrade insummer Fraser water temperature, and an increase of around 1 degree C in the last 20 years.Discharge patterns are changing and many recent years have had extreme high summertemperatures. Since 1996, late-run sockeye have been entering the river 4 – 6 weeks earlier thannormal, instead of holding as in the past at the mouth of the river, and thus encountertemperatures up to 5 degrees C warmer than usual. River temperatures of more than 18 degreesC are now routinely experienced during return migrations.High temperature can kill salmon in different ways. Even when temperatures are sub-lethal, theycan still cause mortality before the salmon reach their spawning grounds. It is not known whetherthese effects have intergenerational consequences.Evidence: Lab studies of swimming and cardiac performance under different temperature andflow conditions simulated in swim tunnels show that optimum and lethal high temperaturethresholds vary among stocks. The results show that stocks are thermally adapted to theirmigration timing, in some cases with a small window between the optimal and critical hightemperature thresholds. For example, Early Stuart is adapted to river temperatures between 14and 16 degrees C. The Late runs that are now migrating earlier face much warmer temperaturesthan in the past.When adult late-run sockeye are held in water warmed to 20 degrees C in the lab, they all die,even if the water is subsequently cooled to 18 degrees. Mortality is only slightly abated if thewater is cooled to around 10 degrees (which speaks to the value of thermal refuges in lakes).Potential interactions among the examined factors and other stressors include pathogens anddisease and rate of senescence. Most such interactions are negative or neutral, dependent onmigration timing.For example, Parvicapsula is picked up by all returning Fraser sockeye in the Lower Fraser.Experiments showed that kidney infections in lab-held sockeye increased rapidly whenindividual fish had accumulated around 370 degree days. There is a correlation between the levelof disease and mortality and many diseases are thermally mediated, so the more hot days, themore rapidly the disease agents will act. Field studies show that differences between the Missionand spawning ground estimates (i.e. en route mortality) are related to exposures to high rivertemperatures and flows. Survival estimates from telemetry studies also show stock-specificrelationships between river survival and temperature. For Chilko, higher temperatures did notseem to matter, but other stocks, like Quesnel, seem highly affected. For most stocks, 18 degreesC seems to be a tipping point, with 40% or more of the run lost at 20 degrees C.In 2002, a major research program was launched to investigate the early migration of returningLate-run sockeye. Studies showed much higher mortality in recent years with early migration. It62