Accounting for seasonal and composition‐related variability in acoustic material properties in estimating copepod and krill target strength

Abstract

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.

Essential omega‐3 fatty acids are depleted in sea ice and pelagic algae of the Central Arctic Ocean

“…In the Bering and Chukchi Sea, where summer temperatures in surface waters rise to over 10°C, DHA proportions in pelagic POM actually increased, rather than decreased, with temperature (Figure S5). The low EPA and DHA proportions in microalgae and zooplankton from the coldest CAO indicate that components other than EPA and DHA may contribute to homeoviscous adaptation (DeLong & Yayanos, 1985; Guschina & Harwood, 2009; Sinensky, 1974) and that wax ester phase transition from liquid to gel forms can be tolerated (Sakinan et al., 2019). How nutrients and light may limit EPA and DHA production in the ice‐covered CAO, and possibly in other global regions, is further examined below.…”

“…In the field, lowest EPA proportions in sea ice and pelagic POM occurred at the end of the dark winter season. Yayanos, 1985;Guschina & Harwood, 2009;Sinensky, 1974) and that wax ester phase transition from liquid to gel forms can be tolerated (Sakinan et al, 2019).…”

Two scales of distribution and biomass of Antarctic krill (Euphausia superba) in the eastern sector of the CCAMLR Division 58.4.2 (55°E to 80°E)

“…Seasonal changes in krill morphology will also affect krill target strength (TS; [ 28 , 30 ]). This survey took 26 days to complete and krill growth during this time will potentially increase krill length and girth [ 31 , 32 ].…”

Small-Scale Spatiotemporal Pattern in the Spawning of Pacific Herring (Clupea pallasii) in the Jinhae Bay, Korea, Estimated Using Hydroacoustic Survey

“…The adult prosome length of A. omorii and C. abdominalis was less than 2 mm in Jinhae Bay [25]. The contribution of these copepods to SV was negligible because of the low mean TS, which was reported to be below −100 dB for 2.57-mm-long copepods at 70 kHz [26].…”