20-mm Survey

Project Overview

The 20-mm Survey began in 1995 and monitors postlarval-juvenile Delta Smelt (Hypomesus transpacificus) and Longfin Smelt (Spirinchus thaleichthys) distribution throughout the Sacramento-San Joaquin Delta and the upper San Francisco Estuary. Both smelt species are listed under the state Endangered Species Act. Delta Smelt are also listed under the federal Endangered Species Act. Two large pumping facilities located in the south Delta export water fom the Delta into the Central Valley and State Water Projects. The risk of entrainment to these species into the facilities is of particular concern. The 20-mm Survey provides near-real-time distribution data, which can be used by management to enact protections for these species if the risk of entrainment is high. The project also collects data on environmental conditions, other pelagic fishes, and zooplankton.

Methods

The 20-mm Survey typically runs from mid-March through early July. Every other week, three fish samples and one zooplankton sample are collected at each of the 47 fixed sites. Fish samples are collected using an egg and larval, rigid-mouthed net constructed of 1,600 µm mesh. Zooplankton samples are collected using a 160 µm mesh Clark-Bumpus net that is attached directly above the fish net. Three 10-minute stepped-oblique tows are made at each station. Samples are preserved in neutral buffered formalin then fish and zooplankton are sorted and identified in a lab. The vast numbers of species and amounts of detritus in the samples makes this a labor-intensive process.

Relevant Publications

• 20-mm Survey Bibliography
• Mahardja, B., Young, M.J., Schreier, B., and Sommer, T., 2017. Understanding imperfect detection in a San Francisco Estuary long-term larval and juvenile fish monitoring programme. Fisheries Management and Ecology, 24(6), pp 488-503

Related Information

• CDFW 20-mm Survey page
• CDFW Region 3
• See PEN#033 20-mm Survey Data Management Plan
• See PEN#033 20-mm Survey Factsheet