SPAWN conducts an annual mark recapture study on smolts out-migrating from the San Geronimo Creek Watershed

What does this mean?

Spawn Logo

First, a bit about the history of SPAWN:

Salmon Protection and Watershed Network (SPAWN) is a grassroots non-profit that has been monitoring salmonid summer habitat since 1999. SPAWN began monitoring the salmonid spawning activity on the six tributaries of San Geronimo Creek in 2001 and in 2006 added springtime monitoring of out-migrating smolts. SPAWN’s efforts are meant to support and complement the work of partners in the Lagunitas Creek Watershed including the CA Department of Fish and Wildlife, National Marine Fisheries Service, National Park Service, and the Marin Municipal Water District (MMWD).

Fisheries field scientists at work

The willingness for scientists to collaborate at local, county and state levels is crucial to developing strong, uniform protection for migratory species like salmonids.

A bit more about SPAWN:

In regards to Coho Salmon (Oncorhychus kisutch), Lagunitas Creek Watershed supports one of the largest remaining wild populations. SPAWN’s objective is to measure juvenile abundance spatially and temporally throughout in San Geronimo Creek and establish a baseline for how habitat restoration efforts are protecting and improving the population.

Coho are part of NOAA’s Species in Spotlight Initiative

NOAA’s Salmon Recovery Plan recognizes the importance of the Lagunitas Creek Watershed to the recovery of the entire population.

Coho Smolt. Photo by William Bone,

Coho Smolt. Photo by William Bone,

Mark Recapture Method

The Mark Recapture Method answers a simple question: how many fish are in this population?

Upstream photo of SPAWN's field site.


Fun Fact: The first recorded mark recapture study was conducted on the population of London in 1629.




A mark recapture method operates under four assumptions:

  1.     The population is closed. During the study, there are no deaths, births, immigration or emigration within the population in question. In order to ensure accuracy, it is assumed that during the study nothing is happening to disrupt the proportion of marked to unmarked fish.
  2.     There is constant, equal chance for all individuals, marked or unmarked, to be caught during the study.
  3.     There is time for marked individuals to randomly re-disperse into the greater population following each capture.
  4.     Animals do not lose their marks.

The principle: If a proportion of the population is marked and returns to the original population and then after complete mixing a second sample is taken, the proportion of marked individuals in the second sample will be the same as was marked initially in the total population.

R (marked recaptures) / T (total in second sample)


M (marked initially) / N (total population size)


N = (M *T ) / R


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Biologists checking SPAWN’s live trap for fish.

In these mark recapture studies, salmonid smolts are marked with fin clips. A “clip” means a very small section of a few fin rays are distinctively snipped. This is a common, safe marking method in many fisheries studies. Salmonids normally accrue rips and tears on their fins so fin clip schedules will include a small clip from two different fins to ensure biologists know whether they have captured a fish. Markings are predetermined to correspond to calendar weeks. All participating agencies on the Lagunitas Watershed follow a coordinated fin clip schedule.

Wondering how many coho smolts outmigrate from San Geronimo Creek Watershed each year

Wondering how many coho smolts outmigrate from San Geronimo Creek Watershed each year

At the end of the smolt monitoring season, data about how many fish were marked initially is compared to the total number of marked fish that were recaptured. These numbers are processed using the program R to give a final determination of the population of out-migrating smolts for that year.smolt

Past studies conducted by SPAWN and the Marin Municipal Watershed District (MMWD) have shown that the San Geronimo Creek Watershed, within the greater Lagunitas Watershed, contributes up to 50% of smolt production even though it accounts for only 10% of the watershed size.

Swim on!

spawn logo old school

Neely Evanoff is Salmon Protection and Watershed Network (SPAWN)’s salmonid research coordinator working on a research fellowship to study and document the endangered Coho Salmon and threatened Steelhead populations within Marin’s largest watershed. Neely is available for questions at:







Interested in helping protect Coho Salmon? Become a member, donate, or volunteer. SPAWN fields a variety of programs centered around species monitoring, habitat restoration and community education.

SPAWN's Winter Creekwalk

SPAWN’s Naturalist-led Creek walks

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