2.1 Vessel and equipment
2.2 Operation of the acoustic instruments
2.3 Sampling and processing of acoustic data
2.4 Sampling and processing of biological data
“Dr Fridtjof Nansen” is a 150 ft stern trawler with a main engine of 1500 hp. The cruising speed is 10-11 knots. The vessel is equipped for acoustic surveying, bottom and pelagic trawling, hydrography and plankton observations.
The acoustic equipment comprise two echo sounders, 38 and 120 kHz, two echo integrators, a 50 kHz net sonde, and a 24 kHz searchlight sonar. All instruments are manufactured by Simrad.
Three trawls, one bottom trawl and two pelagic trawls are ready for use at any time. The bottom trawl is placed on deck and the pelagic trawls are on drums above deck. The same set of otter boards is used with all trawls.
The bottom trawl is a modified high opening shrimp trawl 1800 meshes in circumference, 40 mm mesh size, with a group rope of 63 ft and headrope of 96 ft length. The ground rope is equipped with 0.5 m rubber bobbins. The bridles are 40 m long, the distance between the wings is 20 m and the vertical opening about 6 m.
The pelagic trawls are one capelin (“Harstad”) trawl, 30x30 m, and one “Krill” trawl, 25.5x25.5 m opening.
The cod ends of all trawls are lined with fine-meshed net of 1 cm mesh size.
The hydrographic instruments used during the cruise were a Brown bathythermograph, depth range 0-140 m, and a Negretti & Zambra continuous temperature recorder at 5 m depth. The vessel also carries Nansen bottles and instruments for salinity and oxygen determinations.
The echo integrators were connected, one to each echo sounder. The instruments had the following settings and performance:
|
Frequency |
38 kHz |
120 kHz |
|
|
Depth range (m) |
0-100 + 100-200 |
0-100 |
|
|
Transducer (ceramic) |
8° x 8° |
10° circular |
|
|
Transmitter |
External |
1/1 |
|
|
|
output |
2500 W |
355 W |
|
SL + VR |
138 dB |
102 dB |
|
|
Bandwidth and pulse length |
3 kHz, 0.6 m sec |
3 kHz, 0.6 m sec |
|
|
TVG and gain |
20 log R+2aR -20 dB |
20 log R+2aR 0 dB |
|
|
Recorder gain |
7 |
4 |
|
|
Integrator threshold |
1 |
1 |
|
|
Integrator gain |
20 x 10 |
20 x 10 |
|
|
Depth intervals (m) |
4-50/50-200 |
4-50/50-100 |
|
The acoustic instruments were watched continuously. Integrator outputs were read per nautical mile and averaged over five miles intervals. Trawl hauls were made to identify the various types of echo traces.
The 38 kHz sounder was chosen as the main instrument. By comparing the values from the two sounders one had a means of separating fish and plankton echoes.
The integrated values were divided into three categories: pelagic fish, demersal fish, and plankton/mesopelagic fish/swimming crabs according to the echo traces and trawl catch composition.
The integrated values were plotted along the course-line, and charts of density distribution were drawn.
Trawling time was recorded from the moment the trawl was at the selected fishing depth until hauling in started. Normal towing time was 30 minutes.
Catches were sorted into species, weighed and counted. Some species were measured to length (LT).
Samples from the catches were frozen or preserved in formalin and taken to Djibouti and Bergen, Norway for identification and further examination.
We are indebted to Dr Jean Michel Rose, Aquarium Tropical de Djibouti, and Dr Jakob Gjøsæter, University of Bergen for their help in identification of several species.
Fish names are, as far as possible in accordance with FAO Species Identification Sheets for Fishery Purposes (FISCHER and WHITEHEAD 1974).
