[env-trinity] CBB: California Study Indicates Early Coho Salmon Migrants Missing From Population Estimates

[Sari Sommarstrom]

California Study Indicates Early Coho Salmon Migrants Missing From
Population Estimates

 

Coho salmon early life history is more complex and varied than thought and
sampling methods often used by biologists to estimate the number of smolts
leaving freshwater for the sea have missed the mark by almost 10 percent to
31 percent, according to a recent study in northern California.

 

Juvenile coho salmon in California coastal streams generally remain in a
watershed for one year prior to migrating to sea as smolts. However,
researchers have discovered that many of the migrants actually leave the
watershed earlier, so, when scientists measure the size of the outmigration,
they will often miss these fish, which can be a considerable proportion of
the juveniles.

 

The undercounting could affect management of salmon, resulting in estimates
for juvenile production and overwinter survival that are biased low and
estimates of marine survival that are biased high, according to the study. 

 

In addition, it could cause salmon managers to unknowingly ignore important
habitat in restoration efforts, generally in a stream's estuary where many
of the early migrants overwinter.

 

"We knew that some fish were outmigrating from our study watershed before
smolt trapping started in the spring, so they were not included in the smolt
population estimate," said Darren Ward, professor of fisheries biology at
Humboldt State University in California. "We found that early emigration
varied substantially across locations and years, but early emigrants were
common, often the same proportion or higher than regular smolts."

 

The study, "Early emigration of Juvenile Coho Salmon: Implications for
Population Monitoring,"
http://www.tandfonline.com/doi/abs/10.1080/00028487.2014.982258?journalCode=
utaf20#.VOdnXvnF9qU was published online Jan. 8 in the Transactions of the
American Fisheries Society. 

 

In addition to Ward, researchers were Seth Ricker, Colin Anderson, and Mike
Wallace, fisheries biologists with the California Department of Fish and
Wildlife, and Jennifer Rebenak, at the time of the study a Humboldt State
University graduate student in fisheries biology, and now an environmental
scientist with the National Park Service in Colorado.

 

Without a correct accounting of coho juveniles that includes early migrants
"it is impossible to evaluate the population consequences of habitat loss or
habitat restoration in the lower-basin and estuarine overwintering areas
required by the early emigrant life history," the report says.

 

Traditional sampling designs - in this case sampling traps placed in the
stream downstream of rearing areas during two to four months in the spring -
can have unexpected biases, Ward said. Some emigrants, according to other
studies, "move into marine habitats in the fall and winter," the report
says. That is months before scientists employ sampling traps the following
spring.

 

"In addition to messing up our estimates of vital rates, this is important,
because it may keep us from identifying important habitats that the fish we
miss in our traps are using, like winter habitat in the estuary," he said.

 

High rates of early emigration by coho salmon have also been documented in
Oregon and Washington in studies by Miller and Sadro (2003), Roni et al
(2012), and Jones et al (2014). However, until now, no studies had
documented the phenomenon in California streams. 

 

Ward said they conducted the study to find out what proportion of fish
rearing in the Freshwater Creek watershed (Humboldt County) in the fall
ended up being early emigrants. He was surprised by some of the study's
results.

 

"In addition to the high proportion of early emigrants, I found the
variation surprising," he said. "Within the same watershed and year, there
were sites where 30 percent left early and sites where less than 10 percent
left early."

 

Whether adding the early migrants to the actual number of smolts emigrating
to the sea will change the smolt to adult return ratio is unknown, but there
should be a corresponding change, according to Ward. In fact, there have
been studies documenting early emigrants that survive to return as adults,
he said.

 

"We don't know how many of the fish that emigrate early survive to become
smolts. That's a tough number to get, because the main-stem habitat in the
estuary isn't amenable to smolt traps or PIT tag antennas," Ward said. "So,
it's hard to count the early emigrants that make it through the rest of the
winter and complete their journey to the ocean."

 

The information to establish whether there is a change to SAR is
forthcoming. The researchers are continuing the marking study and now have
the first couple of years with marked fish that have returned as adults.

 

"Now, we're starting an analysis to evaluate differences in return rate for
early emigrants and regular smolts," Ward said.

 

THE COLUMBIA BASIN BULLETIN:

Weekly Fish and Wildlife News

www.cbbulletin.com

February 20, 2015

Issue No. 739

 

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