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

Lead Research Organisation: University of St Andrews
Department Name: Biology


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.

Planned Impact

Impact will be reduction in uncertainty of mesopelagic fish biomass and better characterisation of the mesopelagic community. This will lead to better-constrained estimates of the active component of the biological carbon pump and aid future fisheries management. We expect that it will be in high demand. Impact will be tracked by N days at sea per year, N deployments, N papers, N grants enabled by possession in the UK of this instrument.

The largest source of uncertainty in mesopelagic fish biomass estimates is the unknown contribution of gas-bearing siphonophores to observed mesopelagic echo intensity [1,2]. The Seaquest DSV will enable the quantification of siphonophore density and therefore significantly reduce uncertainty in mesopelagic fish biomass estimates. This information will be used by commercial fishers to evaluate the economic potential of the resource and, coupled with other information, will help facilitate sustainable ecosystem-based fisheries management of open-ocean fisheries.

Mesopelagic organisms that vertically migrate in the water-column on a daily basis (to feed at night under cover of darkness at the surface to avoid visual predators) actively transport carbon, energy and nutrients between the surface and the deep ocean (diel vertical migration) [3]. This process forms the active (as opposed to passive) component of the so-called biological carbon pump (BCP), that contributes towards the global carbon cycle. Reducing uncertainty in mesopelagic biomass estimates will reduce uncertainty in the BCP and allow us to better understand the contribution of this ecosystem service in alleviating climate change.

The Seaquest DSV will generate a lot of data (echosounder observations and video/images of mesopelagic organisms) through deployment on both research vessels and ships of opportunity. These deployments will be made by our own group at the University of St Andrews and by a number of research groups within the UK science community (including BAS, NOC, SAMS and University of Aberdeen). We have the capacity to store these data and through collaboration we could potentially form a working group (research network) to facilitate standard data collection and processing standards. Such working groups have been previously setup through the ICES Working Group on Fisheries Acoustics, Science and Technology, of which, all named academic beneficiaries are members.

These data will also be useful for the modelling community. Ecosystem models are often driven by satellite-inferred primary production data (bottom-up control) or top predator observations (top-down control) but the mid-trophic level (composed of mainly mesopelagic organisms) is often not constrained or assessed due to a lack of data. The Seaquest DSV will provide such information.

1. Proud, R., Handegard, N. O., Kloser, R. J., Cox, M. J., and Brierley, A. S. 2018. From siphonophores to deep scattering layers: uncertainty ranges for the estimation of global mesopelagic fish biomass. ICES Journal of Marine Science.
2. Proud, R., Cox, M. J., and Brierley, A. S. 2017. Biogeography of the Global Ocean's Mesopelagic Zone. Current Biology, 27: 113-119. Elsevier Ltd.
3. Brierley, A. S. 2014. Diel vertical migration. Current Biology, 24: R1074-R1076. Elsevier.


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