2. MATERIALS AND METHODS

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

2.1 Vessel and equipment

The R/V "Dr. Fridtjof Nansen" is a 1,50-foot, stern trawler with a main engine of 1500 horsepower. The vessel is equipped for acoustic surveying, bottom and midwater trawling, hydrography and plankton observations.

The acoustic equipment consisted of two scientific sounders (120 and 38 kHz), two echo integrators, a searchlight sonar (18 kHz) and a net sonde (50 kHz). The bottom trawl was a high opening shrimp trawl, 1800 meshes (40 mm mesh size) in circumference, the lengths of head line and ground rope being 96 feet and 63 feet respectively. The pelagic trawl was 1600 meshes (200 mm mesh size) in circumference, with four equal panels (sides), particularly designed for catching small pelagic fish. Both trawls had a small meshed inner-net of meshsize 1 cm in the cod ends.

Hydrographic observations were carried out with Nansen bottles with which temperature readings and samples for salinity and oxygen determinations were collected at standard depths. The salinity was determined with an inductive salinometer and dissolved oxygen by the Winkler method.

2.2 Operation of the acoustic instruments

Each of the two echo sounders was operated in conjunction with one echo integrator and the two acoustic systems had the following settings:

	38 kHz	120 kHz
Basic range	0-100 or 0-250	0-100
Transmitter	Ext. (10/1)	1/1
Band width and pulse length	Narrow, 0.6 m sec	Narrow, 0.6 m sec
TVG and gain	20 log R - 20 dB	20 log R - 0 dB
Recorder gain	4	3
Integrator threshold	8	8
Integrator gain	20 (x 10)	20 (x 10)
Depth intervals	4-25 and 25-bottom	4-25 and 25-bottom
(according to	4-50 and 50-bottom	4-50 and 50-bottom
bottom depths)	4-100 and 100-bottom

The sonar was running continuously at basic ranges 2000 or 1250 m, sweeping 60 degrees to each side. The gain settings of the sonar were adjusted according to the reverberation conditions in the water masses.

2.3 Sampling and processing of acoustic data

Continuous watch was kept on the acoustic systems and whenever the echo recordings changed their characteristics trawling was carried out for identification and biological sampling purposes. The outputs of the echo integrators were grouped in four types of recordings - pelagic fish - demersal fish, mesopelagic fish and plankton - according to the trace patterns on the echogram and the composition of the trawl catches. Average values over 5 nautical mile distances were calculated for each type of recording and plotted in maps, and arithmetic mean values were made within each 30 x 30 nautical mile square. An echo abundance index for each square was calculated by multiplying the mean integrator output with the area of the square. Echo abundance indices for any area can now easily be arrived at by summation.

The number of sonar contacts for each 5 nautical mile sailing distance were noted in the acoustic log and plotted in maps together with the visual surface spottings.

During the first days of operation, a checking procedure was carried out in order to secure reliable and adequate sampling of acoustic data. This work resulted in the system settings given above, and in addition it was decided that the echo integrator outputs from the 120 kHz system should be used to obtain indices of echo abundance of fish. The reason for this is explained by the observations given in the following table which shows corresponding echo integrator outputs at 38 and 120 kHz for different types of scattering layers:

Frequency	Scattering layer of small pelagic fish
38	29	37	45	69	140	34	150	157	48	108
120	27	26	30	50	114	41	98	119	17	106

Frequency	Scattering layer of planktonic organisms
38	8	9	12	33	31	28	33
120	1	2	2	4	3	3	4

These observations show that the two systems gave almost equal outputs (M₁₂₀ » 0.8 M₃₈) as long as the recordings consisted of small pelagic fish, while plankton layers contributed much more to the 38 kHz system than to the 120 kHz system. Consequently, even moderate concentrations of plankton would mask weak fish echoes in the 38 kHz system; a feature which was frequently observed throughout the survey. The echo abundance indices of fish were therefore calculated from the 120 kHz data which were much less influenced and biased by planktonic scattering. It is believed that the observed frequency differences were caused by resonance scattering phenomena, but neither the identity nor the size distribution of the planktonic scatterers is known.

2.4 Sampling and processing of biological data

All catches were sorted into species to the extent which time permitted, and measurements of total length (cm below) were collected for the predominant species. Weight and maturity observations were also made for selected species. Many samples were preserved in formaldehyde or frozen for later analyses at PPFC, as well as for scientific and processing/ marketing purposes.