John Dalen

Acoustic estimation of size distribution and abundance of zooplankton

A series of investigations were undertaken to observe and describe the sound backscattering process from larger zooplankton (euphausiids). The target strength versus frequency, size, and aspect angle of the organism was measured. The target strength is highly dependent on the density and sound speed contrasts between the target and the medium, and both these parameters were measured. From the target strength observations it was concluded that the fluid sphere model was insufficient as a scattering model for krill. Observations of the tilt angle distribution of krill at natural field conditions showed that they were distributed over a large region of tilt angles. The deficiencies of the fluid sphere models required the development of a new scattering model based on experimental data. This model predicts a decreasing target strength versus frequency in the geometric scattering region. The backscattering spectra of euphausiids were better described by our empirical model than by the fluid sphere models. Appl...

Density- and sound speed contrasts in sub-Arctic zooplankton

SummaryThe sound speed was determined for Meganyctiphanes norvegica, for a mixture of Thysanoessa raschii and Thysanoessa inermis and for a mixture of Calanus finmarchicus and Calanus hyperboreus. The sound speed contrasts ranged from 1.014 to 1.044. Seasonal variations in specific density were measured for Thysanoessa inermis, Thysanoessa raschii, Meganyctiphanes norvegica, Calanus finmarchicus and Calanus hyperboreus. The density of 20 mm T. inermis was lowest in November (1.052 g/cm3) and highest in February–March (1.065 g/cm3). For a 20 mm T. raschii the minimal density was determined in December (1.059 g/cm3) and the maximum in February–March (1.074 g/cm3). M. norvegica individuals of 35 mm also had their lowest density in December (1.060 g/cm3), but reached their maximum density in July (1.076 g/cm3).The density of the euphausiids was found to be size dependent. The density increases as the size decreases. C. finmarchicus and C. hyperboreus had densities less than seawater (1.026 g/cm3) during most of the year. Just before spawning the density increased to 1.028 g/cm3 and 1.036 g/cm3 for C. finmarchicus and C. hyperboreus respectively. The seasonal variations of the density were closely related to the lipid content of the animals.

First data from sea trials with the new MS70 multibeam sonar

The new multi‐beam sonar, MS70, is a horizontally observing sonar yielding very high spatial resolution when operating all 500 beams, covering the frequency band 75–112 kHz. The sonar has undergone sea trials from R/V G. O. Sars from December 2005. The presentation includes preliminary results obtained during this period. Topics covered will be sonar performance objectives, calibration methods, and preliminary results from calibration and school data acquisition. Examples of raw and processed 3‐D and 4‐D data from a stationary and surveying vessel on small sprat schools and large herring schools will be shown.