Interagency Ecological Program San Francisco Estuary 20mm Survey 1995 - 2021

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 47 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. During high outflow years, 5 additional stations are sampled in San Pablo Bay to provide greater spatial coverage of potential Delta Smelt habitat. 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 entire water column, 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. At each station, environmental and water quality data are also collected, and include: top temperature, top and bottom electrical conductivity (normalized at 25degreeC), water clarity, and tidal condition.

跨部门生态计划旧金山河口20毫米调查由加州鱼类和野生动物部（California Department of Fish and Wildlife, CDFW）于1995年发起，旨在监测三角洲胡瓜鱼（Hypomesus transpacificus）后期幼体与幼体的分布及种群丰度。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，随后在淡水至微咸水环境中完成夏季至秋季的育幼阶段。体长≥20毫米的当年生幼鱼（Young of the Year, YOY）被归类为幼体，本调查正是针对此类个体开展。目前，该调查在圣帕布罗湾、苏森湾及萨克拉门托-圣华金河三角洲范围内布设47个固定采样站位，于每年3月至7月每两周开展一次采样。在高径流量年份，调查会在圣帕布罗湾额外增设5个采样站位，以提升潜在三角洲胡瓜鱼栖息地的空间覆盖度。历史上，该调查的采样站位数量介于41至55个之间。每个站位均开展3次时长10分钟的阶梯式斜拖网采样以采集鱼类样本；每次拖网均遵循标准化程序覆盖全水层，采样水体体积通过General Oceanics流量计进行估算。该采样网为锥形浮游生物网，网目尺寸1600微米，网长5.5米，口部面积1.51平方米，固定安装于带有滑雪板式支撑结构的刚性D型架上。后期幼体与幼体鱼类样本在野外采用添加玫瑰红染剂的10%福尔马林溶液固定保存，随后运送至加州鱼类和野生动物部斯托克顿实验室，在显微镜下完成物种鉴定与个体计数。每个站位同时采集环境与水质数据，具体包括表层水温、表层与底层电导率（25℃下标准化值）、水体透明度及潮汐状况。