D e e p - S e a R e s e a r c h . 19,56. Vol . 3, pp. 287 t o 241 . Pergamon Press L t d . , L o n d o n .

Echo-sounder observations of midwater nets and their towing cables

RICHARD H. BACKUS and J. B. HERSEY

(Received 21 March, 1956)

Abstract--By echo-sounding from a following ship it has been possible to determine the shape of towing cables as well as the depth to a net with varying amounts of wire out. It was found that at 2.5 knots the depth to the Isaacs-Kidd midwater trawl can be closely approximated by computing it from the wire angle measured at the surface.

INTRODUCTION

ECHo-sounders have been put to many uses since their development for measuring water depth. A recent application was that of WOOD and PARRISH (1951) who observed ground and midwater trawls in action. They examined the dimensions of the mouths of nets being towed and the contact of ground trawls with the bottom.

A programme of fishing with two-metre stramin closing nets and the ten-foot Isaacs-Kidd midwater trawl ( IKMT) was carried out by our institution during the summer of 1953. In the absence of a reliable depth meter it was suggested that the echo sounder be used to determine the relationship between length-of-wire and depth- of-net. At the same time a telemetering depth meter was being developed for use with nets (Dow, 1954) and it was proposed that echo soundings provide one of the checks on the performance of this meter. On the echo-sounder record of an explora- tory run, portions of the towing cable were clearly detectable as well as the net itself. It was then decided that with suitable range information, the shape of the towing cable could be determined.

PROCEDURE

The I K M T is described by the Scripps Institution of Oceanography 0953). The two-metre net used is described by LEAVITT (1935 and 1938). These nets were towed by the research vessel Blue Dolphin on ~ , 7 by 7, galvanized aircraft cord at approxi- mately 2.5 knots. The length of the towing cable was measured by a standard type

5 n - meter wheel (similar to the G. M. Mfg. Co.'s # 400) made for ~ wire. A correction of 6~o has been added to indications of wire out to allow for the use of ]" wire on this meter. The research vessel Bear made the echo-sounder observations with an Edo Corp. AN/UQN-1B 12-kc echo sounder.

Since the fishing gear did not tow directly astern of Blue Dolphin but slightly off the starboard quarter it was necessary to corm Bear by sighting along the towing cable and relaying instructions by radio telephone. Range between Bear and Blue Dolphin was determined by stadimeter. On Bear's first echo-sounder contact with the fishing gear, range observations were begun. A series of " marks " from the stadimeter operator on Blue Dolphin were relayed to the echo-sounder operator on Bear by radio telephone. These marks were entered on the echo-sounder record and the corresponding ranges were logged on Blue Dolphin.

237

238 RICHARD H. BACKU$ and J. B. HERSEY

On 23 August, a total of seven echo-sounding runs was made. In these runs the IKMT depressor (without net) was fastened to the end of the towing cable. The telemetering depth meter was placed about five metres above the depressor and the two-metre stramin net was fastened to the wire about 5 metres above the depth meter. This gear was first set with 636 metres of wire out and Runs 1-3 were made. The gear was then retrieved and it was discovered that the net had not opened. The gear was then set as before save that 535 m of wire were paid out. Echo-sounding Runs 4-7 were then made. When the gear was retrieved it was found that the net had again failed to open and also that the frame that supported the depth meter was badly bent. This indicated that at some time during the haul the various parts of the gear did not tow in their proper relationship.

On August 24 a total of 21 echo-sounding runs was made on the IKMT (complete) alone. These runs were made with varyiog lengths of towing cable as follows : 320 m e t r e s - 2 runs, 5 3 0 - 2 , 7 4 2 - 2 , 903 -1 , 1007-8, 1113-2 and 1221-4.

Fig. I. Edo echo-sounder record of Run # 6, Aug. 23, 1953 on two-metre stramin, telemetering depth meter and 10-foot IKMT depressor.

ANALYSIS OF RECORDS

The following method has been used to interpret the echo-sounder records (Fig. l) which resulted from these runs :

Echo-sounder observations of midwater nets and their towing cables 239

I. A time scale in arbitrary units was laid off on the echo-sounder record. The number of time units from some zero point (usually the trace of the net or " M a r k " 1) to the successive range marks was plotted on graph paper against the corresponding range as determined by stadimeter. This relationship should be linear on the assump- tions that Bear's rate of closure with Blue Dolphin was constant and that the rate of paper advance of the echo sounder was constant. A straight line was fitted to these points and this graph was used to determine corrected ranges at any point on the record during an echo-sounder run.

2. At a number of points where the echo-sounder record could be clearly read the indicated depth to the cable or net was recorded. Zero correction, sound-velocity correction and transducer-depth correction were added in that order. Because the towing cable was in a sloping plane " u p " which the Bear echo sounded, the depth values at a given range are too small because the shortest effective path between the transducer and the wire is not normal to the sea surface but normal to the wire. The amount of this correction is determined by the slope of the wire ; the greater the slope, the greater the correction. For this purpose the echogram of the cable was divided into a few straight segments and the slope of these approximations calcu- lated from the change in indicated depth and the change in corrected range. Once these slopes were known the suitable depth-correction factor was computed. This correction varies from about ~ of 1% to about 6%.

An error for which one cannot correct is that which enters from the Bear's echo sounder not being directly over the towing cable or net. Assuming an effective sound transmission and reception cone of 30 ° at the apex, the maximum error resulting from transmission along an hypotenuse would amount to about 4%. We have attempted to minimize the results of this error by showing some of the variation encountered in the more successful runs (in which the error would likely be small).

Furthermore, we have previously noted errors in traveltime measurement by the Edo echo sounder as high as 6%. This error is due to friction in the belt drive of the recording styli and therefore the error is a negative one (depth indications too low). In the present case this error, if existent, was unknown and we have assumed it to be zero.

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AUGUST 24,1953 180 + RUN 3

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30 60 90 120 i50 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 D I S T A N C E A S T E R N , M E T E R S

Fig. 2. Shape of the cable towing a 10-foot Isaacs-Kidd midwater trawl with 530 m of wire out. Speed about 2.5 knots.

RESULTS

Fig. 2 shows the shape of the lower two-thirds of the cable when the complete I K M T was being towed alone (Runs 3 and 4, 24 August). The values obtained for both runs are plotted but the minimum soundings are used to draw the line represent-

240 RICHARD H. BACKUS and J. B. HERSEY

ing the cable. The part of the cable for which we have soundings is slightly concave downward. I f the entire cable were of this shape the net should reach a greater depth than that computed from the wire angle measured at the surface. We find that the depth to the net, as determined by echo sounder, is always slightly less than that computed from the wire angle. Therefore we conclude that the upper one-third of the cable (for which we have no soundings) must be slightly concave upward and we have shown it as such in the figure. However, we might assume a negative error of 10% in depth measurement due to friction in the stylus belt of the echo sounder, and increase all soundings on the cable by this amount. Then, assuming the cable to be flat in the upper portion, the wire angle at the surface would necessarily be about 69 °. Since this is within the limits of our observation (67 ° ± 2) a flat shape in the upper portion is a possibility.

We estimate from the data in SIO (1953) that the tension on the towing cable during these runs was about 1500 pounds.

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120: Z I.- 150 AUGUST Z3,1953 o 0.. L,d o RUN ! o 0 180 + RUN 6

• RUN 7

,oll I o Z 4 0 0 30 6 0 90 IZO 150 leO 2:10 ; ' 40 2 7 0 3 0 0 3 3 0 3 6 0 3 9 0 4 2 0 4 5 0 4 8 0 510 5 4 0 570

D I S T A N C E A S T E R N , M E T E R S

Fig. 3. Shape of the cable towing a closed two-metre stramin net, telemetering depth meter and 10-foot IKMT depressor. Gear is presumed to have been fouled during Runs 6 and 7 and clear

during Run 1.

Fig. 3 shows the shape of the cable during Runs 6 and 7 on the 2-metre stramin net, depth meter and IKMT depressor on 23 August. On these runs the gear is presumed to have been fouled. Two soundings are shown at the very end of the cable for each run. These represent echoes from two parts of the gear, probably the rim of the net and the depressor. In this case the cable shape is slightly concave upward. The gear is probably behaving as any towed object in which the weight-drag ratio is less than that of the towing cable. Also shown are a few soundings on the cable during Run 1, 23 August, when the gear is presumed to have been towing properly. During this run the cable shape was probably the same as that when the complete IKMT was being towed.

It has been suggested to us by Mr. JOHN ISAACS, one of the designers of the IKMT, that the IKMT depressor should be very unstable when used without its net. This probably explains the observations in Runs 6 and 7 but the results of Run i show that this is not always so.

DEPTH-WIRE OUT RELATIONSHIP

The relationship between the length of the towing cable and the depth to the net from soundings on the I K M T itself is shown in Fig. 4. All soundings obtained are

Echo-sounder observations of midwater nets and their towing cables 241

plotted but the minimum soundings are used to draw the line. The relationship is also shown that would obtain if the depth were computed from the wire angle read at the surface during the runs (67 ° ± 2 from the vertical). The latter values are about 6% higher than the former. These curves would lie even closer together if an

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Fig. 4. A. Relationship between depth of 10-foot IKMT and length of towing cable at about 2.5 knots as observed by echo sounder. Figures in parentheses indicate number of observations at a particular point where more than one. B. Same relationship as in A. Computed from wire angle

at surface.

error in the echo sounder, o f the mechanical sort earlier described, of up to 6% existed. I f the error exceeded 6% (a maximum of 10% seems reasonable) the actual depth ~ou ld exceed the computed depth by some small amount (up to about 4 %).

Woods Hole Oceanographic Institution, Woods Hole, Mass., ~S.A.

Contribution No. 834 of the Woods Hole Oceanographic Institution. The work was done under Contract NObsr-43273 with Bureau of Ships, U.S. Navy Department.

REFERENCES

Dow, W. (1954), Underwater telemetering. A telemetering depth meter. Deep-Sea Res., 2, 145-151, 9 figs.

LEAVITT, B. B. (1935), A quantitative study of the vertical distribution of the larger zo6plankton in deep water. Biol. Bull., 68 (l) 115-130, 3 figs.

LEAVITT, B. B. (1938), The quantitative vertical distribution of macrozo6plankton in the Atlantic Ocean basin. Biol. Bull. 74 (3) 376-394, 5 figs.

SCRIPPS INSTITUTION OF OCEANOGRAPHY (1953), Isaacs-Kidd midwater trawl. SIO Ref 53-3 18 pp., 11 figs., 3 pls.

Wooo, H. and PARaISH, B. B. (1951), Echo sounding experiments on fishing gear in action. J. Cons. Explor. Mer. 17 (1) 25-36, 7 figs.