Alix Rommel

I completed a combined honours BSc degree in Oceanography and Biology at the University of British Columbia in Vancouver, Canada. For my thesis, I worked with phytoplankton and trace metals in Dr. Maldonado’s lab. After moving to Germany, I completed a MSc in Marine Biology at the University of Bremen in Germany. I wrote my master thesis in the fisheries working group at the Leibniz Centre for Tropical Marine Research (ZMT). This is where I first encountered hydroacoustic data, and my interest for the deep scattering layer was peaked. I am now working towards a PhD in Dr. Brierley’s pelagic ecosystem working group at the University of St Andrews in Scotland.

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

PhD aim:

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.

PhD objectives:

  • Using a combination of underway fishery-acoustic sampling at sea, trawl sampling, in situ acoustic sampling stereo photographic and holographic sampling, and Target Strength (TS) modeling, we will endeavour to determine how much of the acoustic signal used for fish biomass estimations is due to echoes from siphonophores.

Contact details:
Email: [email protected]