California Department of Fish and Wildlife

20mm Survey

Project Overview

This study monitors postlarval-juvenile delta smelt distribution and relative abundance throughout their historical spring range in the Sacramento-San Joaquin Delta and San Francisco Estuary. Delta smelt in the Central and South Delta areas may be entrained into the pumps of the State Water Project (SWP) and the Central Valley Project (CVP). This survey gets its name from the size (20-mm) at which delta smelt are retained and readily identifiable at the fish facilities associated with the state and federal pumps. The 2004 Biological Opinion requires this survey to provide "recent time" (within 72 hours) information on the distribution and relative abundance of delta smelt throughout the Delta and the upper estuary. These data can be found under Delta Smelt Distribution and Delta Smelt Length Frequency . Based upon the data, recommendations are made to protect delta smelt.

Methods Sampling surveys range from 8-10 annually and run on a fortnightly basis covering stations throughout the delta and downstream to the eastern portion of San Pablo bay and Napa River. Samples are collected using an egg and larval, rigid opening net constructed of 1,600 Ám mesh. Three 10-minute stepped-oblique (bottom to top) tows are made at each station. Samples are preserved in neutral buffered formalin and then all fish larvae are sorted out and identified in a lab. The vast numbers of species (ca. 50) and amounts of detritus in the samples makes this a labor-intensive process.
20mm Survey photo

Sampling Sites


Length Frequency

Fish Distribution Map

Fish Distribution Map + Length Freq.

Zooplankton Distribution Map



FTP Site Smelt/