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，作业深度范围为从表层至200米；若站位水深不足200米，则将网具下放至距海底10米以内的位置。该深度范围被认为是兼顾采样可用时长与南极磷虾（krill）可能栖息的垂直深度范围的最优折中方案。拖网作业全程，船舶航速维持在2.5±0.5节的恒定值。放缆阶段钢缆运行速度为0.7~0.8 m/s，起拖阶段为0.3 m/s（对应垂直变深速率分别约为0.5 m/s与0.2 m/s）。在上述航速范围内，拖网作业过程中网口角度保持显著恒定。当拖网抵达最大深度时，绞车需停机约30秒以稳定网具，随后再启动起网回收流程。起网阶段，当拖网降至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干扰并避免样品受损。

目标拖网作业
当回声测深仪上观测到感兴趣的目标，或因实验需求需要采集大量南极磷虾样品时，将开展目标拖网作业。标记目标位置后，船舶转向并根据航行能力，沿指定方向航行至目标上方区域。抵达目标区域前，需将航速降至2.0节以下，以便在拖网到达目标深度时可将其下放至预设深度。整个拖网过程中，需通过船尾控制室的回声测深仪进行实时精细调整。若为活体南极磷虾目标拖网，需尽可能降低航速以避免损伤磷虾。

所有常规拖网站位的样品处理流程
1. 测量样品总体积：先排干样品内积水，再采用水置换法测量体积；该步骤仅针对常规拖网作业强制要求执行。
2. 分拣所有南极磷虾并计数。若样品以磷虾为主且体积超过约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（digital calipers）测量全部（或子样）磷虾的总长度(TL)、甲壳长度、成熟度，以及最多50尾个体的消化腺最长轴尺寸，测量样本量为50~150尾。
4. 立即从渔获物中分拣出其他浮游动物类群并记录其数量。

RMT-8型网具采集样品的保存方式
1. 南极磷虾（包括用于船载种群统计分析的个体）需用10%福尔马林固定，以备后续分析。若磷虾渔获量过多，则抽取部分样品分别进行以下处理：冷冻后用于持久性有机污染物（persistent organic pollutant, POP）检测、用80%乙醇保存以用于遗传分析、置于-80℃/液氮中冷冻以用于化学分析。
2. 鱼类样品可采用福尔马林、乙醇固定或冷冻保存。
3. 枪乌贼（squids）样品采用乙醇保存。

RMT-11型网具采集样品的保存流程
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。

本工作作为ASAC项目2655与2679（ASAC_2655、ASAC_2679）的一部分完成。