Annex I Equipment and methods

I. a Vessel and equipment

The R/V "Dr. Fridtjof Nansen" is a 150 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 bottom trawl was a 134 foot headline shrimp trawl adapted for demersal fish trawling. The foot rope was equipped with 0.5 m rubber bobbins. Bridles of 40 m gave it a horizontal distance between the wings of about 25 m. The effective vertical opening of the net was about 6 m. The pelagic trawl was of about 120 m circumference, and the vertical opening was normally 13 m. The pelagic trawl had an inner-net of mesh size 1 cm in the cod end. Pelagic trawl operations were usually monitored by aid of a 50 kHz acoustic net sonde.

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.

Two Simrad EK Sounders, 120 kHz and 38 kHz, connected to QM integrators, were run continuously. Settings and performance of the two acoustic systems were:

	120 kHz	38 kHz
Basic range	0 - 100 m	0 - 100 m or 0 - 250 m
Transmitter	1/1 (330 W)	7.5° × 8°
Transducer (ceramic)	10% (circular)
SL + VR	116.7 dB	139 dB (13.9.81)
Bandwith and pulse length	3 kHz, 0.6 ms	3 kHz, 0.6 ms
TVG and gain	20 logR, - 0 dB	20 logR, -20 dB
Recorder gain	4	7
Integrator threshold		0.5
Integrator gain		20 dB (× 10)
Depth intervals		According to recordings

The 38 kHz system coupled to the integrator was used for abundance estimation of fish, while the 120 kHz system was used as an additional aid during the daily analysis of the echo recordings.

I. b Sampling and processing of catch data

For each trawl catch the weight and number of each species were estimated by sampling. Species determination was mainly based on SHER et al. (1981) and partly on BLANCHE et al. (1970). Length measurements were frequently taken, mainly on the commercially important species. The catches and their main composition are listed in Annex III and main results from the length measurements are given in Annex IV.

I. c The echo recordings and their interpretation

Assessment of the abundance of fish resources based on acoustic observations combined with experimental fishing is a method which especially lends itself to fish found in schools or other aggregations in midwater. This is, moreover, a type of behaviour which characterizes some of the fish species found in West-African waters. But there are also notable exceptions, e.g. surface schooling tunas and tuna-like species and strictly bottom dwelling fish as rays and flounders. Any fish found very close to the bottom (½ - 1 m) or in the very surface layer will escape echo sounder detection. For navigational reasons the work with the R/V DR. FRIDTJOF NANSEN is limited to waters deeper than 10 m. The extreme inshore waters could thus not be covered.

Because of differences in behaviour and size, different species or groups of fish species may give rise to various types of echo-recordings. Small-sized pelagic fish are, for instance, often found in well-defined schools, the recordings of which can be distinguished from those of the often looser aggregation in which semi-demersal larger fish are often found. Such classification of the echo recordings is of considerable assistance in interpreting the acoustic observations, but a positive identification by fishing operations is still indispensable and also provides the only means of sampling fish in this type of combined survey.

Based on previous experience and on identification by fishing, the echo recordings in the surveyed waters were classified as follows:

(i) Recordings of true larger schools or dense layer mostly in upper water. These will most often derive from pelagic schooling fish usually of smaller size, e.g. clupeids, scads.

(ii) All other fish recordings which especially comprised looser aggregations of smaller and larger fish near bottom. These are ascribed to demersal or semi-demersal fish such as grunts, seabreams, groupers, croakers, bigeyes etc. This type of recording was also common.

(iii) Recordings of plankton and juvenile fish mostly distributed in scattered layers in upper water.

Average integrator deflection per nautical mile was calculated each five nautical mile steamed. All echo traces were evaluated daily and together with the information from the trawl catches the readings from the integrator were split in three categories:

a) Plankton
b) Small pelagic fish
c) Demersal and semi-demersal fish

a) very scattered (1-9 mm),
b) scattered (10-19 mm) and
c) dense (> 20 mm).

This corresponds to an average target strength of -34.3 dB, derived from the equation TS = -10 log 1 - 22/dB/kg. The method of aquiring the given C₁₇ value is explained further in Annex II.