The RV DR FRIDTJOF NANSEN (Fig. 1) was designed as a combination stern trawler (with chute)/purse seiner, with some facilities for longlining, gillnetting and pot fishing. She is 45 m long, has a main engine of 1500 HP and a controllable pitch propeller. Maximum speed is 13 knots.
All winches are hydraulic. In addition to two (split) trawl winches of ample hauling power she carries a normal, smaller trawl winch normally used for bottom trawling, which proved essential for hauling up catches of more than about 6 t which the net drum (Fig. 3) could not handle. The two trawlnet drums are of quite adequate size and power to handle the rather large trial net with chain front weights. The trial gear was operated from the lower drum, which has larger flanges to accommodate 100 m bridles and the up to 520 kg each chain front weights. The upper drum carried the Harstad midwater trawl which was intermittently used for taking biological samples. The hauling speed is on the low side.
The stern chute is rather steep which, together with the short distance to the net arena, limits the amount of catch that could be hauled on deck to about 13 t. The trawl gallows are in conventional position in a portal gallows aft. Their distance from the stern chute is small so that the SÜBERKRÜB otter boards could not be kept completely clear of the chute. Consequently, care was required to avoid fouling of the net and the netsonde cable during shooting and hauling. There is no set of additional trawl gallows as is common on commercial trawlers to enable the quick switch-over from bottom to midwater trawling with different otter boards. Instead, combination bottom and midwater trawling otter boards are used, which are not adequate for midwater trawling with gear of appropriate size.
The navigational equipment includes gyro compass, Bergen Nautic log and satellite navigation. The engine and propeller controls are direct from the wheelhouse and one ship’s echosounder recorder and the netsonde recorder are side by side, altogether an arrangement quite suitable for aimed midwater trawling, even if it does not compare with the “hunting post” provided for the skipper on modern trawlers. The view aft from the wheelhouse covers the stem chute and most of the fishing deck, but only part of the split trawl winches and not at all the small trawl winch and the netsonde cable winch. There is no central winch control. The vessel is equipped with bow and stem thrusters which are of no interest for midwater trawling.
The acoustic equipment includes SIMRAD scientific echosounders with echointegration, one small and one large SIMRAD sonar and a SIMRAD up-or-down netsonde with EX recorder, altogether quite adequate for the trial purposes.
Because of the significance of minimum towing power requirements for the economy of a future commercial fishery, it should be mentioned that the propulsion of the vessel with 1500 HP and controllable pitch propeller was found to be far in excess of towing the trial trawls at 2.5 knots and less, and more than sufficient for even very drastic lifting of the net. For example, the maximum setting for trawling is 750 engine rpm at 83% pitch, while for towing the krill trawl with the front part of 1600 mm meshsize stretched at 2.5 knots, 650 rpm at 66% pitch, and for 2.0 knots, only 590 rpm at 60% pitch were required; 800 rpm at 73% pitch gave a towing speed of 3.5 knots. For the trial trawls and the low towing speed, about 1000 HP would have been sufficient.
In summing up, it can be stated that, for the trial purposes, vessel and equipment were found to be quite adequate. One major drawback was the comparatively long time required for shooting and hauling. Also the lack of fish hold capacity is a disadvantage for fishing trials of this type: fish actually landed is more convincing than reports or photographs. The same applies to the limitations due to the size of the fishing deck and the winch arrangements to the amount of catch that can be hauled on deck for proper assessment of quantity. This, together with the too light construction of aft net and codend of the krill trawl, made it advisable to limit catches by cutting short towing on comparatively promising traces to about 10 t even after the codend cover had been strengthened subsequent to a burst on the stern chute (see Appendix 4, tows 16, 20, 25, 49, 51, 56, and for net damage, tows 10 and in particular tow 58). These unavoidable shortcomings may be kept in mind when judging the fishing results.
The only midwater trawls available on board were a modified krill trawl (Appendix 5) and the so-called Harstad trawl (Appendix 6). The opening area of these nets was estimated by the Norwegian scientists to be in the order of 250 and 400 m2 respectively.
Because of the considerably smaller meshsize in the aft body, the krill trawl obviously provided the better chance for catching small fish. This net has also a much longer small meshed codend with respectively better filtering ability. It was, therefore, selected as the best choice for the trials, with the Harstad trawl as a possible but less suitable alternative.
The towing resistance, or drag, of any body is directly proportional to the area, but increases with the square of the speed. The desired significant increase of the volume filtered with a given towing power is, therefore, achieved better by towing slowly with a larger net rather than by towing faster with a small net. To tow slow would also reduce the water flow in the necessarily small meshed aft belly and codend and also give the fish a better opportunity to react to large mesh netting so that they can be shepherded and guided into the codend rather than being washed out through the meshes by force or to panic prematurely and escape.
Following this reasoning, an enlarged forward extension in two steps was designed which would fit both the krill and the Harstad trawl. The first step had a maximum mesh-size of 800 mm stretched (Appendix 7) and the second of 1600 mm meshsize stretched (Appendix 8). This is a four and eight times increase of the 200 mm meshsize stretched of the original trawls. This significant increase in meshsize is required to keep the power requirements within reasonable limits.
In the following table, the length and circumference are number of meshes × meshsize stretched. The mouth area and opening height/width are calculated according to half the length of the netting along the framing lines. The netting yarn area (m2) is a value for estimating the drag of a trawl and for comparing trawls of similar design. Its value (m2) is number of meshes × 2 × meshsize stretched (mm) × diameter of netting yarn (mm) × 10 6. The knots are disregarded and also the codend is usually not included. Obviously these calculated values for opening size and shape are not to be taken as strictly realistic, because in actual fishing they depend largely on the rig and the mode of operation. They can, however, serve for giving an idea of what can be expected and for the comparison of nets of similar design.
TRAWL TYPE
|
|
Harstad trawl |
Krill trawl |
|||||
|
orig. |
+800 |
+1600 |
orig. |
+800 |
+1600 |
||
|
Length stretched |
m |
63.6 |
85.1 |
101.1 |
66.4 |
99.1 |
115.1 |
|
% |
100 |
134 |
159 |
100 |
149 |
173 |
|
|
Circumference |
m |
287 |
338 |
403 |
284 |
338 |
403 |
|
% |
100 |
118 |
140 |
100 |
119 |
142 |
|
|
Mouth area |
m2 |
256 |
462 |
783 |
256 |
462 |
783 |
|
% |
100 |
180 |
306 |
100 |
180 |
306 |
|
|
Opening height |
m |
16 |
21 |
27 |
16 |
21 |
27 |
|
% |
100 |
131 |
169 |
100 |
131 |
169 |
|
|
Opening width |
m |
16 |
22 |
29 |
16 |
22 |
29 |
|
% |
100 |
138 |
181 |
100 |
138 |
181 |
|
|
Netting yarn area |
m2 |
172 |
160 |
190 |
276 |
315 |
345 |
|
% |
100 |
93 |
111 |
100 |
114 |
125 |
|
In view of the encouraging catches and the limited time available for collecting reasonably reliable results, the krill trawl in combination with the two trial front parts was used exclusively while the original Harstad trawl served for taking biological samples, yielding catches of a few buckets only.
The trial front parts were delivered fully rigged with floats (24 pieces of 0.28 m diameter = about 240 kgf buoyancy) and footrope ballast (100 kg). When the consultant arrived in Dubai, the krill trawl had already been combined with the 800 mm front part so that initial trials with the original net would have been impractical and time consuming.
About one ton of anchor chain for the front weights could be obtained without charge from Capt. Bob Kluzniak, Fleet Manager of the Oman National Fisheries Co., whom the consultant had met during the mission in April/May 1982 mentioned above.
Since the WACO combination otter boards available were not satisfactory for the enlarged trial trawls, cambered SÜBERKRÜB otter boards, as are standard in most one-boat midwater trawling, had to be procured. They were 4.3 × 1.9 m = 8.2 m2, weighing 1.35 t each. The bridles of 100 m length were made up on board. The lower bridles were actually only 94 m long to compensate for 6 m length of the frontweight chains (Fig. 4).
During the trials, only the amount of front weights was varied. The buoyancy on the headline was gradually reduced involuntarily by the cracking of several plastic floats under the heavy pressure on the net drum when hauling up catches of more than about 5 t. Since they have little influence on the net opening during towing, they were not replaced systematically.
After the codend had burst on the chute (tow 10) the damaged last about 6 m were cut off and a stronger cover codend with three ringstrops was improvised using the codend of a new bottom trawl available on board. With the exception of a few torn meshes caused mainly on the net drum during hauling, the trial front parts remained undamaged while the small meshed aft part and the codend of the krill trawl suffered some damage due to larger catches, in particular when hauling in the large catch of the last tow when, in addition to numerous tears, two holes had to be cut in the front part of the long codend to release the excessive fish which could not be handled. The repairs will require some time and labour but no significant expenses. All the trial gear was left on board.
