Interagency Ecological Program San Francisco Estuary 20-mm Survey 1995 - 2024

The Interagency Ecological Program San Francisco Estuary 20-mm Survey was initiated by the California Department of Fish and Wildlife (CDFW) in 1995 to monitor the distribution and abundance of post-larval and juvenile Delta Smelt (Hypomesus transpacificus). Delta Smelt are considered to be an annual fish endemic to the upper San Francisco Estuary that spawn in early spring and rear through the summer and fall in fresh to brackish water. Young of the Year (YOY) Delta Smelt are considered juveniles at sizes greater than or equal to 20-mm, and this survey targets those Delta Smelt. The 20-mm Survey currently samples 61 stations at fixed locations from San Pablo Bay through Suisun Bay and into the Sacramento-San Joaquin River Delta every other week from March to July. The survey has historically sampled between 41 and 55 stations. At each station, three 10-minute stepped oblique tows are conducted for fish. Each stepped oblique tow follows a tow schedule to sample the water column up to 32 ft, and the volume of water sampled is estimated using a General Oceanics flowmeter. The net is a cone shaped plankton net composed of 1600μm mesh and measuring 5.5 meters in length with a mouth area of 1.51m2 lashed onto a rigid D-frame with skis. Post-larval and juvenile fish samples are preserved in the field in 10% Formalin dyed with Rose Bengal and brought back to the CDFW Stockton Lab for identification to species and enumeration under a microscope. Fish are measured to fork length or total length when fork is not present. At each station, environmental and water quality data are also collected, and include: top temperature, top and bottom electrical conductivity (normalized at 25 degree C), secchi (cm), surface water clarity (FNU), and tidal condition.

跨部门生态计划旧金山河口20毫米调查（Interagency Ecological Program San Francisco Estuary 20-mm Survey）由加州鱼类与野生动物部（California Department of Fish and Wildlife, CDFW）于1995年启动，旨在监测稚鱼后期及幼体三角洲胡瓜鱼（Delta Smelt, Hypomesus transpacificus）的分布与丰度。
三角洲胡瓜鱼为一年生特有鱼类，仅分布于旧金山河口上游区域，于早春产卵，整个夏季和秋季在淡水至半咸水环境中育幼。当年幼鱼（Young of the Year, YOY）体长≥20毫米时即被视为幼体，本调查即针对此类个体展开。
当前该调查于每年3月至7月间每两周一次对固定站位进行采样，目前共设61个站位，覆盖范围从圣巴勃罗湾、苏森湾直至萨克拉门托-圣华金河三角洲。历史采样站位数量介于41至55个之间。
在每个站位，需开展3次时长10分钟的阶梯式斜拖网采样。每一次阶梯式斜拖网均按既定流程采集深度达32英尺的水柱，采样水体体积通过通用海洋学流量计（General Oceanics flowmeter）估算。所用网具为锥形浮游生物网，网目尺寸1600μm，全长5.5米，网口面积1.51平方米，固定于带有滑雪板式支架的刚性D形框架上。
稚鱼后期及幼体鱼类样本在野外以添加玫瑰红染色剂的10%福尔马林溶液保存，随后送至加州鱼类与野生动物部斯托克顿实验室，在显微镜下进行物种鉴定与个体计数。样本鱼类需测量其叉长，若无叉长数据则测量总长度。
每个站位同时采集环境与水质数据，涵盖表层水温、表层及底层电导率（于25℃下标准化处理）、塞氏盘深度（厘米）、地表水浊度（FNU）以及潮汐状况。