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, hydrographic and plankton observations, and has a satellite navigator for precise positioning.
The bottom trawl was a 134 m 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 about 120 m in circumference, and the vertical opening was normally 13 m. The pelagic trawl had an inner-net of 1 cm mesh-size in the cod end. Pelagic trawl operations were usually monitored with 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:
I b
| 120 kHz* | 38 kHz | |
|---|---|---|
| Basic range | 0 – 100 m | 0 – 100 m or |
| 0 – 250 m** | ||
| Transmitter | 1/1 | Ext. (2500 W NOM) |
| Transducer (ceramic) | 10% (circular) | 8° × 8° |
| SL + VR | 139 dB (13.9.81) | |
| Bandwith and pulse length | 3 kHz, 0.6 m/sec | 3 kHz, 0.6 m/sec |
| TVG and gain | 20 logR, -0dB | 20 logR, -20dB |
| Recorder gain | 3 – 5 | 7 |
| Integrator threshold | 0.5 – 0.8 | 0.5 – 0.8 |
| Integrator gain | 10 dB (×10) | 20 dB (×10) |
| Depth intervals | According to recordings | According to recordings |
* The integrator was not coupled to the 120 kHz system duringthe May - June surveys.
** The depth monitored could be doubled with the aid of a slaverecorder.
The 38 kHz system coupled to the integrator was used for abundance estimation of fish, while the 120 kHz system was used as an aid during the daily analysis of the echo recordings.
All data concerning survey routes, station work, catch, and the judged acoustic data are logged onto an onboard computer terminal (type Texas Instruments TI 771) and stored on floppy disks. The data are later transferred to a main computer for sorting and further processing.
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 Fischer et al. (1981) and partly on Blache et al. (1970) Length measurements were frequently taken of the commercially important species. During the June survey a special study was carried out on the trigger fish (Balistes capriscus) where length/ weight relations and degree of maturation were recorded.
The catches and their main composition are listed in Annex II and main results from the length measurements are given in Annex III.
In connection with the data logging system mentioned above an 7-character alphanumeric species code was developed. This code contains information on the family and genus to which a species belongs and thus faciliates processing of catch data on different levels.
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 behaviour typical for 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 such as rays and flounders. Any fish within 1/2 –1 m of the bottom 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 about 10 m. The extreme inshore waters could thus not be covered.
Because of dissimilarities in behaviour and size, different species or groups of fish species may give rise to different 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 frequently 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 grouped in the following five categories:
| Category | Type | Common feature in traces |
|---|---|---|
| I | Plankton and juvenile fish | Mostly distributed in scattered layer in upper water. Dusty, fine-grained appearance |
| II III | Clupeids and anchovies (Pelagic 1) Horse-mackerels and scads (Pelagic 2) | Recordings of true larger schools or dense layer mostly in upper water. Distinct single fish traces when scattered |
| IV | Trigger fish | Dense to scattered layers mostly in upper water. Distinct single fish traces |
| V | Other fish (Croakers, grunts, sparids, snappers, sharks etc. | Looser aggregations of smaller and larger fish near bottom |
Acoustic abundance estimation
Average integrator deflection per nautical mile was calculated every fifth nautical mile steamed. All echo traces were evaluated daily and combined with the information from the trawl catches. The readings from the integrator were split in the five categories given above.
The integrator deflection was classified into three levels: a) scattered (1–9 mm), b) slightly gathered (10–19 mm) and c) dense (>20 mm). Contour lines were drawn to distinguish between areas of different fish density. This forms the basis for the preparation of charts of fish distributions presented in this report. For each of the areas the mean integrator value and the areas size were calculated and their product gives an index of abundance for that area.
The conversion factor C, used to transform the index of abundance to absolute abundance, was chosen on the basis of information on target strength of fish in the NE Atlantic. C is linearily dependent upon the length of the fish, and we can correct for this by multiplying the index of abundance with a lengthcorrection f, where (L = fish length). After summing up all indices of abundance within a region the length-corrected indices are converted into fish biomass by multiplying with the C value for the fish of standard length 17 cm.
The reason for this procedure is to make it easier to recalculate the biomass figures when better data on target strength is available.
For the 38 kHz system with its standard settings, C17 = 13.6 tonnes/nm2/mm/nm. This corresponds to an average target strength of -34.3 dB/kg, derived from the equation TS = -10 logL -22/dB/kg (Aglenet al., 1982).
