One of the objectives of MESOPP is to improve estimates of Southern Ocean micronekton biomass in the mesopelagic zone (200 to 1000 m), using both models and active-acoustics. Acoustic instruments, echosounders, produce sound and record backscatter from organisms in the water-column. But the correlation between mesopelagic backscattering intensity and biomass of fish, squids and crustaceans is more complex than a simple multiplication.
The majority of mesopelagic fish (including myctophids, see image above) and siphonophores (below) have gas bladders i.e. a small gas-filled organ, which lowers their overall body density such that the organism can obtain neutral buoyancy and therefore maintain depth with little to no effort.
Gas bladders are responsible for more than 95% of the backscattering intensity produced by these organisms and are the dominant source of backscatter in the mesopelagic zone. Backscattering intensity from gas-bladdered organisms can be even larger at specific depths and gas bladders sizes when resonant backscatter is produced. Therefore, acoustic observations of the mesopelagic zone can be used to approximate biomass of gas-bladdered organisms – since the contribution to mesopelagic backscattering intensity of other mesopelagic organisms e.g. zooplankton and squid, is comparatively very small.
However, since gas bladder volume is not correlated with fish/siphonophore size or weight, then the relationship between mesopelagic backscattering intensity and fish/siphonophore biomass is not correlated. A study from one of the MESOPP partners, Roland Proud from the University of St Andrews, just published, is dedicated to this question.
Roland Proud, Nils Olav Handegard, Rudy J Kloser, Martin J Cox, Andrew S Brierley, From siphonophores to deep scattering layers: uncertainty ranges for the estimation of global mesopelagic fish biomass, ICES Journal of Marine Science, fsy037, https://doi.org/10.1093/icesjms/fsy037