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

[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.