By Sierra LeTourneau and John Weber
Coho salmon inherently prefer to travel all the way up the watershed into tributaries, small head water streams to spawn. This can be difficult for the species as tributaries are farther upstream than other salmonid spawning sites. The Coho native to Lagunitas watershed, travel through Lagunitas creek up the inkwells that stem into San Geronimo Creek and into the various tributaries that we monitor, most of which require passing through culverts under the roadways. These culverts were created decades ago with little regard for spawning salmon, mostly having some form of a lip creating an obstacle both to spawning adults and emigrating smolts. These creeks are pictured in our sketched map of San Geronimo creek and its tributaries.
Sketched Map of San Geronimo Creek and its Tributaries
The broken portion of the maps indicate seasonality of the tributaries.
Coho, though a more charismatic species, are on the endangered species list. Most concern for their endangered status originates from the degradation of their freshwater habitat. Marin is home to the largest population of historic Central California Coho, SPAWN is one of the organizations that monitors this region’s spawning activity, in collaboration with Marin Water and the National Park Service.
From its inception, SPAWN has worked to improve habitat through various restoration projects to encourage the increase in spawning activity and subsequently the Coho salmon population. Originally neglected, the San Geronimo tributaries have only been surveyed since 1997, since SPAWN has started monitoring them. Below is a graph that shows the amount of redds in San Geronimo creek and the tributaries, additionally included is a graph representing the percentage of redds surveyed in the tributaries specifically. This historically sits around 40% of the total redds counted. From our current findings this season we have identified 28 Coho redds throughout our tributary system. The numbers from the San Geronimo Creek have not yet been independently reported.
Historical Coho Spawning Data
The Coho life cycle in total is about 3 years; they spend around 9 to 12 months where they are spawned in the freshwater ecosystem, then go out to the ocean for 18 months to utilize the food and nutrients of the ocean and grow into mature adults, before they return to their nesting grounds to spawn and shortly after die. The life cycle of anadromous fish allows for nutrients that are carried out of the freshwater ecosystem and into the ocean to be replenished once the previously ocean bound fish return and die. Which is vital for the entirety of the ecosystem supported by the creek.
Aside from the expected adult female and adult male Coho, there is a third category of fish that returns for the spawning season, called a “jack” A jack is a juvenile male, every spawning season some of the salmon come back prematurely, mostly males, though jills have been sighted before. Their reason for their premature return is not always clear and can include factors such as genetic predisposition or faster growth due to a successful season at sea. Below we have charts indicating our Coho sightings broken down into individual creeks. You’ll notice from our numbers we did see a couple jacks this season!
As the season matures and spawning activity reduces, we plan to further analyze our results and historical data with hopes of learning more about spawning trends and factors that influence a successful spawning season in our tributaries. We are interested in further investigating the impact culverts place on spawning and successful smolt emigration and looking into rain events, and the impact of their presence or absence. Ideally we can use our findings to improve our restoration efforts moving forward.
Pictures of Salmonid Monitoring featuring volunteer, John Weber (left) and (right) Montezuma Creek; Jack pictured above the female. Decay of back fin from digging identifies the larger Coho as female.
