Rectangular Midwater Trawls (RMT) for krill during BROKE-West

Regular TrawlAt each regular trawl station a quantitative standard double oblique tow was conducted from the surface down to 200 m (or to within 10 m of the bottom at stations shallower than 200 m). Such a depth range is considered to be the best compromise between the time available for sampling and the likely vertical depth range of krill. During the hauls, ship speed was maintained at a constant 2.5 plus or minus 0.5 knots. Wire speed of 0.7 to 0.8 m/s during paying out and of 0.3 m/sec during hauling (approx. 0.5 m/s and 0.2 m/s respectively at vertical depth change rate). The net mouth angle is remarkably constant during hauling within the speed ranges given above. When the net reaches maximum depth, the winch was stopped for about 30 seconds to allow the net to stabilise before starting retrieval. When hauling, propeller thrust was turned off when the net reached a depth of 15 to 20 m; this was to minimise the effects of the propeller action on the net operation and avoids damage of the samples.Target TrawlWhenever interesting targets were seen on the echo-sounder, or large amounts of krill were required for any purpose, target trawls were performed. Once the position of the target was marked, the ship was turned and navigated to run over the target from direction required within navigation capacity. The ship speed was lowered down to below 2.0 knots before hitting the target, so that the net could be lowered down to the desired depth whenever the net reached the target. Fine adjustments were made throughout the trawl by monitoring the echo-sounder in the aft control room. For live krill target trawl, ship speed was kept as slow as possible to avoid any damage to krill.Sample processing for all regular trawl stations:RMT-81.Measure the total sample volume (Drain water, then measure using water replacement; mandatory only for the regular hauls)2.Sort out all Antarctic krill and count their number. If the sample mainly consists of krill and the volume is more than ~1L, a known portion of the whole sample was sub-sampled for the further processing.3.Stage (TL, Carapace Length, Maturity) of all krill (or subsample), up to 50 to 150 individuals, and digestive gland size (the longest axis) of up to 50 individuals were measured using digital calipers.4.Other zooplankton groups were immediately sorted out from the catch and their numbers were recorded. Preservation of RMT-8 samplesKrill (including those used for onboard demography measurements) were fixed in 10% formalin for their further analysis. Whenever excess amount of krill catch were made, they were sampled and frozen for POP (persistent organic pollutant) measurements, preserved in 80% ethanol for genetic analysis, and frozen under -80C/ liquid nitrogen for chemical analysis. Fish were preserved in formalin, EtOH, or frozen. Squids were preserved in ethanol.RMT-11.The whole sample was fixed with 10 % formalin.2.If the sample volume was too large, then a known proportion of catch was randomly sub-sampled and fixed.This work was completed as part of ASAC projects 2655 and 2679 (ASAC_2655, ASAC_2679).

常规拖网作业：在每个常规拖网站位，均开展定量标准双斜拖网作业，作业水层从海面下探至200米；若站位水深不足200米，则下探至距海底10米范围内。该水深范围被认为是兼顾采样可用时长与磷虾潜在垂直分布深度的最优折中方案。拖网作业过程中，船舶航速保持为2.5±0.5节。放缆时钢缆运行速度为0.7~0.8 m/s，起网时为0.3 m/s（对应垂直深度变化率分别约为0.5 m/s与0.2 m/s）。在上述速度范围内作业时，网口角度在起网过程中保持高度稳定。当网具抵达最大水深后，绞车需停机约30秒以使网具稳定，随后再开始起网回收。起网阶段，当网具降至15~20米水深时，需关闭螺旋桨推力，以尽可能降低螺旋桨水流对网具作业的影响并避免样品受损。

目标拖网作业：当回声测深仪探测到感兴趣的目标，或因科研需求需要获取大量磷虾时，即开展目标拖网作业。标记目标位置后，船舶转向并按照航行能力允许的方向驶向目标正上方。抵达目标区域前，需将航速降至2.0节以下，确保网具抵达目标位置时可下放至预设作业水深。拖网作业全程需通过船尾控制室的回声测深仪进行精细调整。针对活体磷虾的目标拖网作业，需尽可能降低航速，以避免对磷虾造成损伤。

所有常规拖网站位的样品处理流程（RMT-8）：
1. 测定样品总体积：先排尽网具内积水，随后采用水置换法测定体积（仅常规拖网作业强制要求该步骤）
2. 分拣全部南极磷虾并计数。若样品主体为磷虾且体积超过约1升，则从全样中抽取已知比例的子样开展后续处理
3. 使用数显游标卡尺测定全部（或子样）磷虾的体长（TL）、甲壳长度、成熟度，以及最多50尾个体的消化腺长轴尺寸
4. 立即从渔获中分拣其余浮游动物类群并记录其数量

RMT-8型采样网样品的保存方案：
磷虾（包括用于船载种群测量的个体）需置于10%福尔马林中固定，用于后续分析。若磷虾渔获量过大，则需分别取样：一部分冷冻后用于持久性有机污染物（persistent organic pollutant, POP）检测，一部分置于80%乙醇中保存用于遗传分析，剩余部分置于-80℃/液氮中冷冻用于化学分析。鱼类样品可采用福尔马林、乙醇固定或冷冻保存；鱿鱼类样品需以乙醇保存。

RMT-11型采样网样品处理：
1. 全样采用10%福尔马林固定
2. 若样品体积过大，则随机抽取已知比例的渔获子样进行固定

本研究工作作为ASAC项目2655与2679（ASAC_2655、ASAC_2679）的组成部分完成。