fish report header
Rotary Screw Traps
Efficiently Incarcerating Adolescent Fish

After a salmon is hatched from the gravel of the riverbed in the upper reaches of a watershed, it will begin a long and difficult journey out to the ocean where it will mature before returning to the stream to spawn. When juvenile salmon are migrating out to the ocean it is important to monitor both the quantity and quality of the fish that are being produced in the streams so that biologists can better evaluate how various watershed activities and management strategies are influencing the salmon. There are many different methods used to sample outmigration, but one of the preferred methods is the use of rotary screw traps (RSTs). Sampling the small outmigrating juveniles can be difficult, but RSTs can simplify the process of estimating abundance because you are constantly sampling a portion of the run throughout the entire outmigration season.


There are a variety of different calculations you can use to estimate efficiency of the RST, which is then used to estimate the abundance of outmigrating salmonids from the trap catch. The simplest method is based on an estimate of the percent volume of water sampled by the trap. This is calculated by determining the amount of flow that passes through the trap's cone relative to the volume of water flowing in the channel. If the salmonid catch is low over the course of the season and mark/recapture experiments are not possible, then this is one of the few options available, but the problem with this method is that it assumes that fish are evenly distributed throughout the water column, and does not factor in fish behavior.


A more favorable method is to utilize mark and recapture experiments conducted with wild caught fish over the course of a monitoring season. A known number of marked fish are released upstream of the trap, and the proportion of fish recaptured is the efficiency of the trap. There are different ways that data from mark/recapture trap efficiency experiments can be used to estimate abundance. A common method assumes that trap efficiencies are constant throughout the time period between experiments. The resulting trap efficiency of a mark-recapture experiment is then applied to the catch data for the period of time until the next experiment is run. Since trap efficiency will change throughout time, the more often a mark/recapture experiment can be conducted the more accurate the abundance estimates will be. Alternatively, if researchers have developed a large mark/recapture dataset that has shown consistencies over multiple seasons, they can apply a regression model to estimate trap efficiency based on variables such as fish size, turbidity and flow.


Regressions have been valuable in estimating abundance at one of our trapping locations on the Tuolumne River because in some years catch can be too low to properly conduct mark/recapture experiments. When this occurs we are able to use catch, fish size and flow data collected from previous trapping seasons to estimate the daily trap efficiency and fish abundance for the trapping location (Figure 1). For example, in 2009 and 2010 there were not enough fish captured to conduct trap efficiency experiments, but by using a regression based on years of experiments we can confidently estimate abundance for those years.

Follow Us!  Like us on Facebook  View our photos on flickr  View our videos on YouTube

email list


Recent Blog Post
Don't overlook the little guys
Adult striped bass inhabiting the California San Francisco Bay-Delta are known to be significant fish eating predators (piscivorous), Striped bassbut until recently there has been little research on population-level prey demand of sub-adults. Striped bass are anadromous as adults, migrating between the Sacramento and San Joaquin Rivers and the Pacific Ocean, but millions of sub-adults remain in the Bay-Delta estuary to feed and grow. Studying trends in striped bass consumption is not only important to understanding their feeding requirements, but also to assessing the potential impacts on prey species... Read more >

Recent Job Announcements  


Figure 1. Trap efficiency estimates observed at the lower trapping location on the Tuolumne River relative to river flow at Modesto (MOD), 1999-2008 and 2011.
IN THE NEWS: Recent stories you might have missed...
Anglers flock to Monterey Bay in search of salmon

Mercury News

A full moon shone brightly over the Santa Cruz Small Craft Harbor very early Saturday as a steady stream of boats, stocked with fishing lines, headed out to sea. Some returned in a few hours, their ice chests filled with the allowed quota of large shimmering fish - a sign, they say, this salmon season will be prolific. "It was excellent," said Brad Miller of Ultimate Fish Charters. "We got early limits for our passengers. They were nice, decent size keeper fish"... Read more > 

Government funds UC Davis study on Delta smelt
Sacramento Bee

Federal water officials plan to fund a study at UC Davis to learn more about the basic behavior of Delta smelt, the small threatened fish at the vortex of water supply and environmental problems in the Sacramento-San Joaquin Delta. 

The U.S. Bureau of Reclamation announced the study on a government grants website. It plans to pay UC Davis $73,957 to try to answer basic questions about smelt behavior and explain its "fundamental niche" in the environment... Read more > 

DFG Newsletter

Large schools of Pacific herring once again returned to the waters of San Francisco Bay this winter, providing a feast for the bay's many marine mammals and seabirds. The return was also celebrated by the commercial herring fleet. The spawning biomass estimate for the 2011-2012 season was 60,985 tons, an increase over last season's estimate of 57,082 tons and well above the historical average (1978-1979 season to present) of 49,670 tons... Read more > 

Salmon forecast promises boost for Lower Columbia communities

The Daily News  

The improving fortunes of Pacific salmon may help Pacific County's struggling economy reel in some big bucks this summer. Last week, the Pacific Fisheries Management Council gave both sport and commercial fishermen in Washington and Oregon their longest season in memory - one that will stretch salmon fishing off the mouth of the Columbia from June into September. "Any time you can extend the season like that, it brings more money into the community. Read more > 

Three herbicides imperil West Coast salmon, says government report 

Central Valley Business Times  

A new draft biological opinion issued by the National Marine Fisheries Service in Seattle, Wash., Wednesday morning has found that three commonly used herbicides are increasing the chance of extinction for threatened and endangered Pacific salmon and steelhead runs. The report makes specific recommendations that, if implemented, could impact farming practices in the Central Valley, spawning grounds for some of the endangered fish... Read more >