This month, May 2020, the St Andrews team was supposed to be in the final stages of preparation for going to sea on the SUMMER voyage on the Spanish Research Vessel Sarmiento de Gamboa (http://www.utm.csic.es/en/instalaciones/sdg) from Vigo to Barcelona led by colleagues from CSIC (Spanish National Research Council). We were to deploy our new ‘Seaquest DSV’, a compact Deep-water Sonar and Visual sampler for exploring the twilight zone (low-light level sub-surface depth zone). The Covid-19 pandemic has however thwarted travel, delayed assembly of the DSV, and led to postponement of the voyage. Instead of reporting on the voyage, we will take the opportunity to provide some background information on the DSV, that we hope we will have the opportunity to use later in SUMMER.
One of the objectives of SUMMER is to estimate the biomass of mesopelagic (200 to 1000 m depth zone) fish in selected study areas in the North East Atlantic and the Mediterranean. Scientific echosounders, which produce soundwaves and record echoes from organisms in the water-column, provide valuable data on the depth- and spatial distribution of biota in the twilight zone, but in order to scale echo intensity data to biomass by species or taxonomic group we need information on the species composition and size of organisms that reflect sound. During echosounder surveys of conventional fish stocks (for example North Sea herring), trawl samples are taken regularly to obtain data on fish size and maturity, and to confirm that echoes believed to originate from herring are in fact from herring. Whereas trawl sampling works well for single-species aggregations of fish in relatively shallow water, obtaining biological samples from the mesopelagic is problematic. The mesopelagic community is taxonomically and anatomically diverse, and contains organisms that vary substantially in size and swim speed. Net samples are therefore biased: they under-sample fast-swimming animals such as squid that can avoid a net, and gelatinous organisms such as siphonophores (pictured left), which produce very strong echoes, that are broken by it. The challenge is to obtain unbiased ‘ground truth’ samples with which to inform our acoustic analysis.
The DSV is intended to provide data on the identity, size and acoustic reflectivity (i.e. target strength) of species in the mesopelagic zone. It comprises a self-contained Simrad WBAT broadband echosounder and a programmable stereo pair of housed 4K low-light sensitive video cameras with synchronised white and red LED lights. The instruments are packaged together on a small frame that is lowered on a non-conducting wire to the depth of choice. Several other institutes have lowered broadband or multifrequency echosounders (e.g. CSIRO’s Profiling Lagrangian Acoustic Optical System), but they are either deployed on the CTD frame (which is large, and causes avoidance reactions by some organisms) or require a conducting cable, which means they cannot be deployed from some vessels. The DSV is small and self-contained: it is our hope that it will illicit minimal avoidance. We hope that during the course of SUMMER the DSV will be used on several voyages and will lead to the accumulation of data on species composition, size, and target strength. With these data we hope to be able to partition echo energy data from acoustic surveys and determine biomass estimates for fish, siphonophores and other groups. Together this will deliver, we hope, a more precise estimate of biomass in the twilight zone.
Written by: Andrew Brierley, Roland Proud and Yang Yang, University of St Andrews