Abundance mesopelagic fish collected during Maria S. Merian cruise MSM26, spring 2013@en

Mesopelagic fish were collected using a 1 m**2 Double-MOCNESS (Multiple Opening and Closing Net and Environmental Sensing System) and 4.5 m**2 IKMT (Isaacs-Kidd midwater trawl). The main portion of the IKMT was 20 mm knotted nylon, and the tail bag was 3 mm knotless nylon. Oblique IKMT tows were made to a maximum depth of 500 m at a tow speed of 3.5 knots. The original cruise plan intended for nighttime IKMT tows, but tow times varied due to operational constraints. The MOCNESS was equipped with 20 nets of 333 µm mesh size; 10 nets per side. The towing speed was 2 knots. Samples were collected to a maximum depth of 1250 m. The first oblique nets sampled from the surface to the max depth, and the other nets sampled depth stratified bins of the water column. MOCNESS hauls were performed during day and night to investigate diel vertical migrations. Mesoplelagic fish were processed on board. All fish were picked from all IKMT nets, most oblique MOCNESS nets, and the left side nets of the depth stratified MOCNESS samples. The Depth stratified nets from the right side of the MOCNESS frame were preserved in 5 % formalin for future quantitative analyses of the nekton. Fish were identified to the lowest possible taxa using Whitehead et al. (1984) and Fahay (2007). Standard length of each fish was measured to the nearest 0.1 mm using a digital caliper. Measured and identified fish were frozen in an -80 °C freezer, and shipped to the University of Hamburg at the end of the cruise.本研究采用1 m²双开闭式网与环境传感系统（Double-MOCNESS, Multiple Opening and Closing Net and Environmental Sensing System）及4.5 m²艾萨克斯-基德中层拖网（Isaacs-Kidd midwater trawl, IKMT）采集中层鱼类样本。该IKMT的主体网衣采用20 mm有结尼龙网布，尾袋则使用3 mm无结尼龙网布。斜拖IKMT作业的最大作业深度为500 m，拖曳速度为3.5节。原巡航计划仅安排夜间IKMT拖网作业，但受作业约束影响，实际拖网时间存在变动。 本次使用的MOCNESS配备20张网孔尺寸为333 µm的网具，两侧各10张。拖曳速度为2节，最大采样深度达1250 m。其中第一组斜拖网从表层至最大采样深度进行采样，其余网具则对水层开展分层采样。MOCNESS拖网作业于昼夜均进行，以探究鱼类的昼夜垂直迁移规律。 中层鱼类样本均在船上完成预处理。所有IKMT网具采集的样本、大部分斜拖MOCNESS网具样本，以及分层采样MOCNESS样本的左侧网具采集的样本均被分拣收集。而MOCNESS架右侧的分层采样网具样本以5%福尔马林溶液保存，用于后续自游生物(nekton)的定量分析。 鱼类分类鉴定参考Whitehead等人（1984）与Fahay（2007）的研究成果，尽可能鉴定至最低阶分类单元。采用数显卡尺测量每尾鱼的标准体长，测量精度达0.1 mm。经测量与鉴定后的样本置于-80℃冰柜中冷冻保存，并于本次航次结束后运往汉堡大学。