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, one or both connected to 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 ground rope of 63 ft and headrope of 96 ft length.
The pelagic trawls are one Capelin (“Harstad”) trawl, 16 x 16 fathoms, and one Krill trawl of 14 x 14 fathoms opening.
The cod ends of all trawls are lined with fine-meshed net of 1 cm mesh size.
For plankton sampling, a Juday net with 80 cm diametre opening and mesh size of 180 µ was available.
The hydrographic instruments consist of one Brown bathythermograph, depth range 0-140 m, Nansen bottles and continuous temperature recorder at 5 m depth. The laboratory onboard has instruments for salinity and oxygen determinations.
The echo integrators were connected to the 38 kHz echo sounder which was chosen as the main recording instrument. The acoustic instruments had the following settings:
|
Frequency |
38 kHz |
120 kHz |
|
Depth range (m) |
0-100 + 100 or 0-250 + 250 |
0-100 |
|
Transducer |
30 x 30 cm Ceramic 8° x 8° |
10° Circular |
|
Output |
2500 W |
355 W |
|
SL + VR |
138 dB |
102 dB |
|
Bandwidth and pulse length |
3 kHz 0.6 ms |
3 kHz 0.6 ms |
|
TVG and gain |
20 logR -20 dB |
20 logR 0 dB |
|
Recorder gain |
7 |
4 |
|
Integrator threshold |
1 |
|
|
Integrator gain |
20 x 10 |
|
|
Depth intervals (m) |
4-50/50-100/100-150/150-200 |
|
The acoustic instruments were watched continuously. Strong and “interesting” recordings were trawled on for identification and for biological sampling purposes.
Integrator outputs were read per nautical mile and averaged over two mile intervals. The standard procedure onboard “Dr. Fridtjof Nansen” is to average over five mile intervals, but it was the wish of the Tunisian to use two mile intervals for comparison with previous investigations (RIJAVEC & al. 1977).
The integrated values were divided into three groups: pelagic fish, demersal fish, and plankton/fish larvae according to the echogramme pattern and catch composition. The 120 kHz recordings were used, by comparing them to the 38 kHz recordings, as an aid to separate plankton and fish registrations.
The integrated values were plotted along the course-line, and charts of density distribution were drawn for the three groups.
The product of average integrator values
(
) and the area of fish
distribution (A) was calculated within subareas. These abundance indexes were
converted to fish biomass by using the conversion factor C = 0.8 L tonnes/n.
mile2 per echo integrator unit.
is average fish length in
cm. This corresponds to an average target 5 strength of
(ref 1 kg/m2)
which earlier has been used for surveys with “Dr. Fridtjof Nansen”
(AGLEN & al. 1982). Fish biomass (B) was then estimated as
A 0.8 L (tonnes). Due to
the high avoidance observed during pelagic trawling, the larger fishes are
expected to be under-represented in the catches. A rough estimate of 15 cm is
therefore used as the average fish length during the calculations.
Effective trawling time was normally half an hour, i.e. from the moment the trawl was at the selected depth until hauling started. Catches were sorted into species and weighed. From larger catches representative samples were taken, while the rest was measured in baskets. Important species were measured to length. Total (extreme) length was recorded for most species. Sex and maturity, and parasites were studied in selected species.
Samples were preserved for further examination ashore and for the specimen collection at the La Goulette institute. Some mesopelagic species were taken to the Department of Fisheries Biology, University of Bergen.
