Seaquest DSV: a compact Deep-water Sonar and Visual sampler for exploring the marine twilight zone

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Animals in the deep sea, including a diverse array of fish, squid and zooplankton, are hard to sample, but play important roles in ocean ecosystem function (e.g. they are food for species such as tuna and some cetaceans), biogeochemical cycling (e.g. helping transport atmospheric carbon to the deep sea, buffering climate change), and may be targeted directly by fishers. We need fundamentally to gain a good understanding of which species are where, and in what abundance. We propose an acoustic and optical sampling device that will help with this, opening a new window on the mesopelagic zone (200 to 1,000 meter depth range) by overcoming some present day sampling difficulties.Traditional net surveys suggest that there are about 1,000 million metric tonnes (MT) of fish in the mesopelagic zone. In 2014, an international team suggested, controversially, that these old estimates were an order of magnitude too low, and that there may in fact be more than 10,000 MT of mesopelagic fish [1]. Their estimate was made using single-frequency (38 kHz) scientific echosounder data collected on a single circumnavigation of the globe. They assumed that all of the echo energy from the mesopelagic came from fish but did not have any net samples to confirm this. Different sized fish return different intensities of echo energy, and some zooplankton thought to be abundant in the mesopelagic (siphonophores) have gas-bearing pneumatophores that can return stronger echoes than some fish. In the absence of species or size information, therefore, there is scope for considerable uncertainty in any 'fish' biomass estimate arising from a blanket scaling of echo intensity to fish biomass. Due to this headline figure, there is now growing commercial interest in mesopelagic biomass as a potential major source of protein. We need as a scientific community to better understand mesopelagic community composition so we can better inform society of the ecosystem services of the organisms that live there and their potential for harvest.Basic acoustic theory (e.g. [2]), our own work [3] and that of colleagues [4] focusing on the mesopelagic, has shown that fish and siphonophores cannot be differentiated by single frequency sampling. Multiple frequency data can however give information on size and, in some circumstances, can enable separation of species [5]. Typical ranges of frequencies used for fish/zooplankton identification/sizing range from tens to several hundred kHz. The physics of sound propagation limits the effective range of the high end of this spectrum to a few tens of m in seawater, so in order to acoustically sample the mesopelagic we need to lower the echosounder into deep water. The instrument we propose will enable this. Furthermore, we will use stereo video to capture images of some of the organisms we detect acoustically. This will enable us to determine the acoustic target strength (TS, a ratio measure of the proportion of sound energy backscattered from a target) of species of known size (size influences TS) across a spectrum of frequencies and so enable quantitative evaluation of acoustic survey data and progress towards better understanding of global biomass distribution. Combining acoustic and stereo optics provides an innovative and world-leading new way to sample the mesopelagic.1. Irigoien, X. et al. 2014. Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nat Comm, 5: 3271. 2. Simmonds, E., and MacLennan, D. 2005. Fisheries acoustics. Blackwell Science Ltd. 3. Proud, R., et al. 2018. From siphonophores to deep scattering layers: uncertainty ranges for the estimation of global mesopelagic fish biomass. ICES JMS.4. Kloser, R. J. et al. 2016. Deep-scattering layer, gas-bladder density, and size estimates using a two-frequency acoustic and optical probe. ICES JMS. 73: 2037-2048. 5. Brierley, A. S. et al. 1998. Acoustic discrimination of Southern Ocean zooplankton. DSR Part II:TSIO. 45: 1155-1173.