塞拉利昂海域雨季深海声学散射层生物垂直迁移现象分析

Diel vertical migrations of organisms in deep scattering layers in rainy season off Sierra Leone

应用2021年5月西北非塞拉利昂海域中上层渔业资源与环境调查的船载Simrad EK60科学渔探仪(38 kHz)数据,对深海声学散射层生物的垂直迁移过程和迁移量进行分析研究。结果表明:EK60探测范围内,277~563 m水层存在深海声学散射层;该层部分生物日落时分上升、日出时分下降,上升和下降活动持续时间均约为3.10 h;迁移活动以变速运动形式分2~5层进行,且迁移中会发生层间的分离与聚合现象;净垂直下降速率均值为4.30 cm·s^(-1),大于净垂直上升速率均值3.41 cm·s^(-1);上升后,深海声学散射层内NASC值减少62.63%~92.70%,绝大部分进入96 m以浅的浅水层中;下降后,深海声学散射层内NASC值增加45.59%~79.79%,占浅水层内减少NASC值的32.71%~98.67%。

Sierra Leone has an exclusive economic zone of 104850 km2 and has one of the most productive marine ecosystems in the world.It is not adequately studied in scientific research related to fisheries,especially in diel vertical migration of pelagic organism which is the key to food web between zooplankton and top predators,although the fishery resource of Sierra Leone is rich.The pelagic fishery resource and environmental survey(PFRES)was implemented in the exclusive economic zone of Sierra Leone from 1st to 8th May 2021.Based on the shipborne Simrad EK60 scientific echo sounder(38 kHz)data from PFRES,the net vertical velocity and migration of pelagic organisms in the deep scattering layers were analyzed.The research of the net vertical velocity was examined through visual inspection and acoustic data 1.5 h before and after migration.The selected raw acoustic data could be used for subsequent analysis after preprocessing such as removing background noise and reverberation from the sea surface and the bottom.Three steps would be used in acquire the net vertical velocity.First,the original depth-time with of the migrating biolayer were drawn by applying the maximum Sv line pick and the smoothing filter function module in the preprocessed acoustic data with Echoview software.Second,empirical mode decomposition was used to the original depth-time curves in MATLAB software to remove fluctuations in data and the last intrinsic mode function was chosen to calculate the net vertical velocity.Third,derivative of the last intrinsic mode function was used to get the net vertical velocity.The NASC(nautical area scattering coefficient)value obtained by echo integration on the preprocessed acoustic data 0.5 h to 1.5 h before and after migration was used as an indicator of the migration.The results showed that:Within the detection range of EK60,there was a deep scattering layer at the depth of 277-563 m.Part of organisms in this layer ascended at sunset and descended at sunrise,and ascending and descending activities lasted for 3.10 h.The migration activity was divided into 2-5 stratifications in the form of variable speed movement,and the separation and aggregation between stratifications would occur in the process.The average net vertical descending rate was 4.30 cm·s^(-1),which was greater than the average net vertical ascending rate of 3.41 cm·s^(-1).Before and after ascending,NASC values in the deep scattering layer decreased by 62.63%-92.70%,and most of which entered the shallow layer up to 96 m.Before and after descending,NASC values in the deep scattering layer increased by 45.59%-79.79%,accounting for 32.71%-98.67%of the reduced NASC value in the shallow layer.