Jellies in the deep: quantifying siphonophore abundance and biodiversity in the mesopelagic (200-1,000 m depth) and how siphonophores bias acoustic

[标题续)中层鱼类生物量估算：中层带(200-1000米深)的一个显著特征是存在深层散射层，所谓深层散射层，是因为聚集了大约500米的生物体，并散射声纳装置发出的声音。在DSL及其周围发现的中上层鱼类，全球总生物量可能达到10亿吨，因此可能是地球上最后也是最大的未开发野生蛋白质来源。在它们成为商业目标之前，我们必须了解这些鱼类提供的生态系统服务，因此了解捕捞可能面临的风险。这种理解的一个关键部分是对鱼类生物量的可靠估计，这对于未来的任何可持续开发也是必要的。然而，目前，我们的知识被估计鱼类生物量的常见方法中的偏差所笼罩。渔网回避使人对拖网调查的估计产生怀疑，而声学调查受到这些鱼类所居住的深层散射层回声来源的不确定性的阻碍：许多明显的“鱼”回声能量可能来自管吸水团中的气泡。因此，本论文的目的是提高对中上层海洋物种多样性的理解，从而能够更好地解释声学调查数据，从而改进生物量估计，最终改善渔业管理，并更好地理解DSL在碳减少中的作用。

[title cont.] estimates of mesopelagic fish biomass.A distinguishing feature of the mesopelagic zone (200 - 1000m depth) is the presence of the Deep Scattering Layer (DSL), so-called because of the mass of organisms that aggregate around 500m and scatter sound from SONAR devices. Mesopelagic fish, found in and around the DSL, may have a total global biomass of 1 billion tonnes, and could therefore represent the last and largest untapped source of wild protein on the planet. Before they are targeted commercially, it is crucial that we understand the ecosystem services that these fish provide and hence, what might be at risk from harvesting.A key part of that understanding is a robust estimate of fish biomass, which is also necessary for any future sustainable exploitation. At present however, our knowledge is clouded by biases in the common methods of estimating fish biomass. Net avoidance casts doubt on estimates from trawl surveys, and acoustic surveys are hampered by the uncertainty around the source of echoes in the deep scattering layers, in which these fish reside: much of the apparent "fish" echo energy could arise from gas vesicles in siphonophores.The aim of the thesis is therefore to improve the understanding of the species diversity in the mesopelagic, and hence enable better interpretation of acoustic survey data, leading to improved biomass estimation and - ultimately - improved fisheries management and a better understanding of the DSL's role in carbon drawdowns.