It is that time again. Time for the Trout Count. Biologists with the SFPUC have been working to estimate the total number of adult rainbow trout living in Calaveras Reservoir.

Rainbow trout (Oncorhynchus mykiss) are a cold water species native to the rivers and lakes of North America. Like all native fishes, it’s important to preserve them and our ecological systems. Monitoring fish populations is an important part of protecting trout in Calaveras Reservoir.

To estimate the rainbow trout population size, biologists use a technique known as mark and recapture. During spring, when adult rainbow trout migrate from the reservoir upstream to Arroyo Hondo to spawn, the biologists capture and mark as they can. During summer, after the adult fish move back down into the reservoir, the biologists fish for them and check for tags. Knowing how many trout were originally tagged in Arroyo Hondo, they can use the ratio of tagged to untagged fish caught in the reservoir to estimate the total number that are there.

SFPUC’s biologists will continue track the fish through the end of September. After Calaveras Reservoir is refilled, they will check the population size again to see if there is a difference.

Periodically tracking the rainbow trout population size is also important because it helps SFPUC biologists determine whether they are healthy and thriving or being negatively impacted by reservoir operations.