Stockton Fish and Wildlife Office
Pacific Southwest Region

Trawling

A midwater and Kodiak trawl survey is conducted in Sacramento (River mile 55) to estimate the relative abundance and timing of fry and smolts entering the delta. Knowing when the various races and at what size they enter the Delta is helpful in managing water project protective criteria.

Midwater trawling at Chipps Island (River mile 18), to estimate the number of unmarked fish emigrating from the Delta and to recover marked smolts released in mark and recapture survival experiments.

At the end of calendar year 2008, we replaced our Leslie E. Whitesel research vessel with the R/V Confluence for use of Midwater trawling at Chipps Island and scientific special study use.

A Kodiak trawl is also conducted at Mossdale on the San Joaquin River (River mile 54).

Mid-water Trawl

Photo: USFWS

Sampling Map
Midwater Trawl
Crew - one boat operator and two deckhands
Net - 4.6m by 9.1m with a cod end mesh size of 0.64cm
Sampling effort - ten 20 minute trawls per day, three to seven days per week (seven days per week during coded wire tagged fish recovery periods)

The Trawling operation used to sample juvenile salmon are basically smaller versions of commercial trawlers. Trawling has been conducted at Pittsburg in Suisun Bay since 1976, in most years from April through June during peak fall run outmigration, but is now conducted year round to monitor all races of chinook and to collect information on other anadromous species such as American Shad.
The midwater trawl survey at Sacramento uses a smaller net (1.8 X 4.6 meters) and is used to estimate the relative abundance and timing of fry and smolts entering the delta. Knowing when and at what size the various races enter the Delta is helpful in implementing water project protective criteria. This trawl has been deployed at Sacramento since 1988 (in 1990 the site was near Hood) and near Clarksburg on the Sacramento River from 1976 to 1981. Again, early years emphasized the April to June months when fall run outmigration occurs, but since 1992, sampling has been expanded to encompass the outmigration periods of spring, late-fall, and winter run chinook. In December, 1994, this gear was replaced by the Kodiak trawl during the winter months.

Kodiak Trawl
Crew - two boat operators and three deckhands depending on flow conditions
Net specifications - 1.8m by 7.6m with a cod end mesh size of 0.32cm
Sampling effort - ten 20 minute trawls three to seven days per week.

The Kodiak or paired trawl requiring two boats to pull has a larger mouth opening than the midwater trawl nets used historically, thus has a better efficiency for catching larger juvenile chinook salmon such as winter and late-fall run.