Estimation of abundance or biomass, using acoustic techniques requires knowledge of the frequency dependent acoustic backscatter characteristics, or target strength, of organisms. Target strength of zooplankton is typically estimated from physics-based models that involve multiple parameters, notably including the acoustic material properties (i.e., the contrasts in density and sound speed between the animal and surrounding seawater). In this work, variability in the acoustic material properties of two zooplankton species in the Gulf of Maine, the copepod (Calanus ﬁnmarchicus) and krill (Meganyctiphanes norvegica), was investigated relative to changing season as well as, for the copepod, temperature and depth. Increases in the density and sound speed contrasts of these species from fall to spring were observed. Target strength predictions based on these measurements varied between fall and spring by 2-3 dB in krill. Measurements were also conducted on C. ﬁnmarchicus lipid extract at changing temperature and pressure. The density contrast of the extract varied negatively with temperature, while the sound speed contrast changed by more than 10 % over the temperature and pressure ranges that the organism expected to occupy. C. ﬁnmarchicus target strength predictions showed that the combined effect of temperature and pressure can be signiﬁcant (more than 10 dB) due to the varying response of lipids. The large vertical migration ranges and lipid accumulation characteristics of these species (e.g., the diapause behaviour of Calanus copepods) suggest that it is necessary for seasonal and environmental variability in material properties to be taken into account to achieve reliable measurements.
Sakinan, S., Lawson, G. L., Wiebe, P. H., Chu, D., & Copley, N. J. (2019). Accounting for seasonal and composition‐related variability in acoustic material properties in estimating copepod and krill target strength. Limnology and Oceanography: Methods, 17(11), 607-625. https://doi.org/10.1002/lom3.10336