Fishing trials have been conducted in the Gulf of Oman with the Norwegian fishery research vessel DR FRIDTJOF NANSEN to assess the economic feasibility of harvesting the very considerable resource (biomass several million tons) of the lanternfish (Benthosema pterotum), which is so far completely unutilized.
The RV DR FRIDTJOF NANSEN of the Norwegian Agency for International Development (NORAD) is operated under the basic agreement TF/INT 41 (NOR)-/FI with the Fisheries Department of the Food and Agriculture Organization of the United Nations, as part of the United Nations Development Programme/FAO project (GLO/82/001) Survey and Identification of World Marine Fish Resources.
As part of the operation of the project, FAO assigned Dr J. Schärfe as Consultant Fishing Technologist to assist in preparation of the trawl gear and to supervise the actual trawling trials in the Gulf of Oman with the RV DR FRIDTJOF NANSEN.
As a result of several acoustic surveys since 1975, this resource in the Gulf of Oman, its annual distribution and diurnal migrations are well known. Previous sampling and limited fishing trials with conventional midwater trawls had given encouraging results, which were to be followed up with trials by more suitable trawlnets enabling the filtering of a considerably larger volume of water per unit of time and effort in order to collect the sparsely (a few grammes per cubic metre) distributed and rather small (Fig. 5) (length around 35 mm) fish in sufficient quantities (about 100 t/day) for the commercial production of fish meal and oil. This requires the largest possible net opening and lowest possible towing resistance, i.e., large meshsize and low towing speed.
To this end the existing small mesh krill trawl (Appendix 5) was extended with a front part of 800 mm meshsize stretched (Appendix 7) and alternately with a front part of 1600 mm meshsize stretched (Appendix 8) providing a two to almost four times larger net opening at only a small increase in towing resistance. Drag was further saved by employing cambered SÜBERKRÜB otter boards (Fig. 2) and by using low towing speeds of mostly less than 2.5 knots. As an example, in this way, with an opening area of 750 m2 and a towing speed of 2.3 knots, about 3.2 million m3 of water can be filtered per hour. At a fish density of 5 g/m3 and a catching efficiency of 50 percent, a catch of 8 t/h could be expected, which would be sufficient for catching about 100 t/d, provided that fishing can be done round the clock. The crucial question was whether this species can be guided and shepherded by large mesh netting, and a low towing speed was supposed to be desirable to give the supposedly poor swimmers sufficient time to react.
The densest aggregation of B. pterotum is in the so called Dl layer which appears in the echogramme as a ragged band of about 20 m width in about 100 to 150 m depth during the day. At sunset the fish rise to the surface and form the so called N1 layer of considerably less density, which reaches from the surface to about 50 m depth. While the D1 layer proved to yield reasonable catches during 12 hours of the day, the other layers, D2, N2 and N1 were found to be completely useless for commercial fishing. For the N1 layer this is in contrast to earlier experience (e.g., 1975/76).
The more intensive trials with artificial light, utilizing all outside illumination available on the vessel, confirmed earlier observations according to which B. pterotum is not attracted (as was hoped), but repelled. The effect is a denser concentration of the N1 layer in depths of about 30 to 60 m, making the fish more vulnerable to one-boat midwater trawling. Limited trials (five tows) indicate that, by proper lighting of the water surface around and behind the trawler, commercial catches can also be obtained during the night, i.e., fishing round the clock is feasible.
According to previous surveys, the time selected for the present trials was favourable. The acoustic survey conducted along with the trials, however showed a smaller total biomass of B. pterotum and in particular a far less wide distribution of the D1 layer and a much lower fish density in this layer than found in the previous surveys. This would indicate that the fishing results obtained (summarized below, see also Appendix 4) are very encouraging indeed, providing excellent prospects of success for the proposed Mesopelagic Fishery Pilot Project in the Western Arabian Sea.
Recommendations on the staffing of vessels and equipment resulting from these trials refer to switching from pair to one-boat trawling, only one trawler with fish meal and oil processing on board, additional acoustic and electronic equipment with related staff and the deletion of purse seining from the trial programme.
SUMMARY OF FISHING TRIALS AND RESULTS
Full days available for fishing trials: 11.5
|
Krill trawl with front part of 800 mm meshsize stretched |
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|
|
Tows |
hours fished |
Catch |
|
|
t |
t/h |
|||
|
On good D1 layers |
10 |
10.9 |
41.2 |
3.8 |
|
On other layers |
10 |
10.7 |
4.5 |
0.4 |
|
Total |
20 |
21.6 |
45.7 |
2.1 |
|
Krill trawl with front part of 1600 mm meshsize stretched |
||||
|
On good D1 layers |
17 |
24.9 |
109.4 |
5.0 |
|
On poor D1 layers |
4 |
6.4 |
8.4 |
1.3 |
|
On N1 layer, with light |
5 |
8.0 |
17.7 |
2.2 |
|
Sub-total |
26 |
39.3 |
135.5 |
3.4 |
|
On very dense D1 layer |
1 |
about 0.5 |
about 50 |
100 |
|
Grand total |
47 |
61.4 |
231.2 |
3.8 |
