Mid-December is typically peak spawning time for coho salmon in Lagunitas Creek, but this season has been anything but typical. The season started with unusual sightings of pink and chum salmon, and in the last month it’s turned unusually dry. During that period MMWD and Watershed Stewards Program biologists observed two pink, three chum, and 16 Chinook salmon, bringing the season totals to 13, seven, and 29, respectively.
We saw our first school of coho salmon on November 29 and another school on December 12. To date we’ve observed 46 coho, with only three of those fish seen on redds. The rest of them appear to be waiting for rain before finding a spot to lay and fertilize their eggs. We lowered a GoPro camera into one of these pools and confirmed that the salmon were all coho. That video can be seen below:
So far this season we’ve observed 78 salmon redds (nests), with salmon seen on only 17 of those redds. This means we’ve had to deduce what species of salmon excavated the rest, which can be challenging when an unprecedented four species are in the creek at the same time. Some redds are pretty distinctive, like the shallow, sprawling redds often built by coho. The redds seen back in October were likely built by pink salmon since no other species were seen that month. But most of the redds seen since mid-November (when Chinook, chum, and coho were present) weren’t particularly distinctive. That’s resulted in 36 redds being classified as “Unknown Salmonid” – a frustrating result when we’re trying to use redd counts to estimate how many endangered coho salmon have returned.
In the not-too-distant future we may be able to identify these unknown redds using environmental DNA (eDNA) collected from water in the redds. A recent article by FishBio summarizes the technique. Scientists have already identified the genetic markers for each of these species, and are now working out the protocols and thresholds for using eDNA to classify these redds. That will improve the accuracy of redd counts and hopefully ease the frustration of salmon biologists everywhere.