Results of the acoustic-trawl survey of walleye pollock (Gadus chalcogrammus) on the U.S. Bering Sea Shelf in June-August 2018 (DY1807)

Eastern Bering Sea shelf walleye pollock (Gadus chalcogrammus) midwater abundance and distribution were assessed from Bristol Bay to the U.S.-Russia Convention Line from 6 June to 26 August 2018 using acoustic-trawl (AT) survey methods aboard the NOAA ship Oscar Dyson. In addition to surveying the traditional (core) survey area, we also surveyed a region northeast of St. Matthew (the northern extension). Water column temperatures were cooler in 2018 (mean sea surface temperature (SST) 8.5 °C, bottom temperature 3.7 °C) compared with temperatures during the previous AT survey in 2016 (mean SST 11.4°C, bottom temperature 3.9 °C), but still much warmer than in prior cold years (2006-2014; SST means between 4.9° and 6.8 °C). Walleye pollock biomass was concentrated in the regions north of Pribilof and Zhemchug canyons, and east of Pervenets Canyon, primarily between the 100- and 200-m isobaths. The estimated amount of pollock in midwater (between 16 m from the sea surface and 0.5 m off the sea floor) in the U.S. EEZ core survey area was 5.57 billion fish with a biomass of 2.50 million metric tons (t), less than half of the 2016 estimate of 12.2 billion fish with a biomass of 4.83 million metric tons. This estimate includes the lower water column (3 m to 0.5 m), estimated by combining acoustic and bottom trawl data and previously reported separately. Pollock east of 170° W numbered 1.28 billion fish and weighed 0.74 million t (27% of the total shelf-wide biomass, including the northern extension which represented 9% of shelf-wide biomass). Five- and 6-year-old pollock (43 and 45 cm mean fork lengths (FL), respectively) together composed 77.6% of the biomass east of 170° W. Pollock abundance west of 170° W was 4.29 billion fish weighing 1.75 million t (64% of total shelf-wide biomass). These estimates include an ‘unsampled’ portion of the core area (the three westernmost transects, comprising 6.1% of the core area) which was not surveyed due to ship motor failure. Most pollock west of 170° W were aged 5 and 6 years (42 and 44 cm mean FLs, respectively). The mean biomass-weighted depth of adult pollock (≥ 30 cm FL, age-3+) was 79 m in the region east of 170° W and 97 m west of 170° W. The mean weighted depth of juveniles (< 30 cm FL, ages 1 and 2) west of 170° W (relatively few juveniles were observed east of 170° W) was 106 m, 9 m deeper than adults (97 m). The mean euphausiid abundance estimate for 2018 was similar to that in 2004, a relatively low year in the index. 2020 NMFS (National Marine Fisheries Service) AFSC (Alaska Fisheries Science Center) Submitted https://doi.org/10.25923/7njp-tc70 Public Domain 1861

2018年6月6日至8月26日，研究团队搭载美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）“奥斯卡·戴森号”（Oscar Dyson）科考船，采用声学拖网（acoustic-trawl, AT）调查法，对布里斯托尔湾至美俄公约线范围内的东白令海陆架黄线狭鳕（walleye pollock, Gadus chalcogrammus）的中层水体资源丰度与分布格局开展了评估。本次调查除覆盖传统（核心）调查区域外，还额外对圣马太岛东北部的北部延伸区域进行了采样。2018年的水柱温度较2016年同期声学拖网调查偏低：表层海水温度（sea surface temperature, SST）平均值为8.5℃，底层水温平均值为3.7℃；而2016年的平均SST为11.4℃，底层水温为3.9℃，但相较于2006-2014年的偏冷年份（SST均值介于4.9℃至6.8℃之间），2018年水温仍明显偏高。黄线狭鳕的生物量集中分布于普里比洛夫峡谷与热姆丘格峡谷以北、佩尔韦涅茨峡谷以东区域，主要栖息在100至200米等深线之间的水层。美国专属经济区（Exclusive Economic Zone, EEZ）核心调查区域内，距海面16米至海底以上0.5米的中层水体中，狭鳕资源量估算为55.7亿尾，总生物量达250万公吨（metric ton, t），仅为2016年估算值（122亿尾，生物量483万公吨）的不足一半。本次资源估算涵盖了通过整合声学与底拖网数据得出的下层水柱（3米至海底以上0.5米）资源量，该部分数据此前曾单独报道。170°W以东区域的狭鳕资源量为12.8亿尾，总生物量74万公吨，占全陆架总生物量的27%，其中北部延伸区域的生物量占全陆架总生物量的9%。在170°W以东区域，5龄与6龄狭鳕（平均叉长（fork length, FL）分别为43cm和45cm）的生物量合计占比达77.6%。170°W以西区域的狭鳕资源量为42.9亿尾，总生物量175万公吨，占全陆架总生物量的64%。本次估算包含了核心区域内因船舶电机故障未完成调查的“未采样”部分，即最西侧的3条调查断面，其面积占核心区域总面积的6.1%。170°W以西区域的多数狭鳕为5龄和6龄个体（平均叉长分别为42cm和44cm）。成年狭鳕（叉长≥30cm，3龄及以上）的生物量加权平均水深在170°W以东区域为79米，以西区域为97米。170°W以西区域的幼鱼（叉长<30cm，1龄和2龄）的加权平均水深为106米，较同期成年狭鳕（97米）深9米；而170°W以东区域观测到的幼鱼数量极少。2018年的平均磷虾（euphausiid）资源量估算值与2004年相近，属于该资源指数中的相对偏低年份。本数据由美国国家海洋渔业服务局（National Marine Fisheries Service, NMFS）阿拉斯加渔业科学中心（Alaska Fisheries Science Center, AFSC）于2020年提交，DOI标识：https://doi.org/10.25923/7njp-tc70，采用公有领域授权，编号1861。