Mesopelagic fish collected during Maria S. Merian cruise MSM26, spring 2013

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.

深海中层鱼类样本采用1 m²双网口开合式环境传感网（Double-MOCNESS，Multiple Opening and Closing Net and Environmental Sensing System）与4.5 m²艾萨克斯-基德中层拖网（IKMT，Isaacs-Kidd midwater trawl）采集。该IKMT的主体网具采用20 mm打结尼龙材质，尾袋则为3 mm无结尼龙材质。IKMT斜向拖网作业的最大作业深度达500 m，拖速为3.5节。原巡航计划仅安排夜间开展IKMT拖网作业，但受作业约束影响，实际拖网时间存在波动。
本次使用的MOCNESS配备20张网孔孔径333 μm的网具，两侧各10张，拖速为2节，采样最大深度达1250 m。首批斜向拖网的采样范围覆盖从表层至最大作业深度，其余网具则按水层分层进行采样。MOCNESS拖网作业覆盖昼夜时段，以探究深海中层鱼类的昼夜垂直迁移现象。
深海中层鱼类样本在船上完成前处理：所有IKMT网具、多数斜向MOCNESS网具以及分层采样MOCNESS样品的左侧网具均完成鱼类分拣。MOCNESS架右侧的分层采样网具经5%甲醛溶液固定，用于未来开展游泳动物的定量分析。
依据Whitehead等（1984）与Fahay（2007）的分类手册，将鱼类鉴定至尽可能低的分类阶元。使用数显卡尺测量每条鱼的标准体长，测量精度至0.1 mm。经鉴定与体长测量的样本置于-80 ℃冰箱冷冻保存，航次结束后运往汉堡大学。