Trawl experiments

In 1979 the Krill trawl was used with different weights on the lower wings. Two hauls with 80 kg weights on the wings gave trawl openings of 10 and 13.5 m. When the weights were increased to 190 kg, the vertical opening varied between 12 and 18.5 m. Higher speed gave smaller vertical opening.

To compare the efficiency of the Krill trawl and the Harstad trawl, 10 parallel trawl hauls were carried out (Table 13). The effective opening of the trawls are estimated to be 250 m² and 400 m² for the Krill and Harstad trawl respectively, and the catches were corrected according to their openings.

If we look at catch per nautical mile, the Krill trawl caught in average 143 kg and the Harstad trawl 82 kg. This difference was analysed using a Wilcoxon paired-sample test, and found to be significant.

The Harstad trawl was towed with higher speed as it was difficult to keep the speed constant during the trials. Average catch per hour trawled for the Krill trawl and the Harstad trawl was 293 and 203 kg respectively. This difference was also analysed and found to be significant.

Table 13. Comparison of Krill trawl and Harstad trawl 1979. (Original data, not corrected for trawl opening).

Tr. st. No.	Type of trawl	Depth	Layer	Speed knots	Trawl opening m²	Catch pr. n. mile	Catch pr. hour
159	Krill	42	N I	2.3	14	47	105
160	Harstad	41	N I	2.7	15	33	80
162	Krill	176	D I	2.0	14	118	248
163	Harstad	183	D I	2.1	14	180	405
166	Harstad	38	N I	3.3	15	28	90
167	Krill	40	N I	2.2	14	44	105
172	Krill	35	N I	1.5	13.5	31	47
174	Harstad	35	N I	2.2	13	55	120
175	Krill	280	D II	2.0	15.5	26	49
176	Harstad	291	D II	2.3	13	13	30
179	Krill	48	N I	2.0	17	145	270
180	Harstad	36	N I	2.3	15	44	110
181	Harstad	33	N I	2.5	15	206	495
182	Krill	33	N I	2.2	14	236	495
190	Krill	204	D I	2.0	16	77	150
191	Harstad	187	D I	2.0	17	131	255
192	Krill	315	D II	2.2	17	38	90
193	Harstad	277	D II	2.8	17	15	41
194	Krill	20	N I	2.2	12	135	270
195	Harstad	20	N I	3.3	12	118	400
199	Harstad	30	N I	2.9	14	21	60
200	Krill	30	N I	3.0	14	27	80

Fig. 16. Size distribution of B. pterotum in the Gulf of Oman, July 1979.

Fig. 17. Size distribution of B. pterotum in the Gulf of Oman, January - February 1981.

Fig. 18. Size distribution of B. pterotum in the Gulf of Aden. February 1981.

Possibly a larger part of the smallest fish are filtered through the meshes in the Harstad trawl than in the Krill trawl. Then fish caught in the Harstad trawl should be expected to be larger than those caught in the Krill trawl. Fig. 19 shows the length distribution for the fish caught in the Krill trawl and Harstad trawl in the 10 parallel trawl hauls. The figure does not give any reason to believe that there is any significant difference in size.

In the same way we have split these length distributions in fish, caught at day and at night with the two trawls, but none of the distributions indicate a difference in size of fish from the two trawls.

Experiments with fine meshed net bags attached to the outside of the panels of the Krill trawl was carried out to catch fishes filtered out through the meshes in the trawl. Three net bags were attached in positions shown on Fig. 20. The results of these experiments are given in Table 14. The catches in the net bags showed great variations and no correlation to the catches in the cod end of the trawl. It is difficult to draw any conclusions about the filtering effect of the trawl from these experiments.

Fig. 19. Length distribution of B. pterotum from the Krill trawl and the Harstad trawl, 1979.

Table 14. Catches in bags on the Krill trawl 1979.

Tr.st. no.	Total catch	Number of fish in each bag			Number of fish per kg catch
		Forward	Central	Backward	Forward	Central	Backward
159	70	8	2	0	0.11	0.03	0
161	120	14	4	0	0.12	0.03	0
162	165	0	5	0	0	0.03	0
164	250	0	3	3	0	0.01	0.01
165	40	0	2	1	0	0.05	0.03
167	70	10	1	17	0.14	0.01	0.24
168	230	18	0	11	0.08	0	0.05
169	195	50	1	51	0.26	0.01	0.26
170	90	19	4	2	0.21	0.04	0.02
172	27.5	0	0	0	0	0	0
175	33	2	25	1	0.06	0.76	0.03
178	90	4	3	3	0.04	0.03	0.03
179	180	115	1	7	0.64	0.01	0.04
182	330	54	4	36	0.16	0.01	0.11
183	900	1	3	3	0.001	0.003	0.003
187	20	1	0	0	0.05	0	0
190	100	4	1	0	0.04	0.01	0
194	135	2	0	0	0.02	0	0
195	200	7	77	2500	0.04	0.39	12.50
196	45	0	55	0	0	1.22	0
201	40	0	0	0	0	0	0
203	40	0	0	0	0	0	0
206	60	6	0	4	0.10	0	0.67
224	10	13	0	0	1.30	0	0
225	17	0	0	5	0	0	0.29
226	20	0	0	0	0	0	0
233	15	4	0	0	0.27	0	0
234	20	0	0	0	0	0	0
235	25	5	0	0	0.20	0	0
236	15	0	0	0	0	0	0
240	14	4	2	0	0.28	0.14	0
242	25	19	0	2	0.76	0	0.08

Fig. 20. Net bags used on the trawls. The figures indicate the mesh size in the trawl.

DISTRIBUTION OF EGGS AND LARVAE OF BENTHOSEMA PTEROTUM IN THE GULF OF OMAN

One of the objectives of this cruise to the Gulf of Oman was to find and identify eggs of Benthosema pterotum and to study the spawning biology of this species. These important biological aspects have not been described previously.

The distribution of myctophid larvae in the Gulf of Oman in January/February 1981 is presented in Figs. 21 A and B. Larvae of Benthosema pterotum predominated in these samples and completely outnumbered larvae of all other fish species.

B. pterotum larvae were found in 69, of 77 Juday net hauls (50-0 m) during the first survey (24 January - 1 February). The highest number of larvae found in one haul were 664. Three different areas with more than 200 larvae/m² were found (Fig. 21 A). One in the western part of the Gulf, close to the shelf region and northwards, one in the middle of the Gulf extending towards the Iran side of the Gulf and one area in the outer part of the Gulf of Oman in the slope region just east of 58°E.

Fig. 21. Distribution of larvae of B. pterotum in the Gulf of Oman. Number of larval per m² surface area. - A: 24 January - 1 February.

Fig. 21. Distribution of larvae of B. pterotum in the Gulf of Oman. Number of larval per m² surface area. - B: 9-13 February.

During the last survey (9-13 February) a reduced amount of B. pterotum larvae were found (47 of 51 Juday net hauls) compared to the first survey. Only one area with more than 200 larvae/m² were found (Fig. 21 B). The relative reduction of larval quantity was estimated by integration of the two distribution maps. During the second survey the quantity of larva was reduced by 63% compared to the quantity found during the first cruise.

This shows that the total concentration of larvae varies and changes quickly during a short period of time. This is most probably due to heavy predation on larvae, but could also be because the B. pterotum population in the area may spawn in cohorts.

The average number observed was 43 larvae/m² (109 samples). This density was equivalent to Fursa's (1973) observations in the Gulf in January-April in 1969. Nellen (1973), however, found the average of 335 larvae/m² in March-April 1965. Nellen (1973) also points out that B. pterotum larvae were the only myctophid species in the samples. In no other area such high concentrations of one single species of Myctophidae larvae have been observed. According to Fursa (1969, 1973) and Nellen (1973) the spawning period of B. pterotum in the Gulf of Oman clearly varies with the season, however most larvae seems to be found during the winter monsoon in January to April.

The eggs and newly hatched larvae of B. pterotum are not previously described. We were, however, successful in fertilizing and incubating B. pterotum eggs on board the R/V "Dr. Fridjof Nansen", and thereby able to identify eggs and the early larval stages. Ripe fish were found in trawl samples from 200-300 meters depth. Eggs were dissected from the ovary and fertilized in sea water (20°C 36.6‰). Ripe eggs from the ovary had an opaque chorion, segmented yolk with one oil droplet. The eggs were slightly eliptical with a diameter of 0.7 - 0.8 mm (long axis). Transferred to sea water and fertilized, the eggs absorbed water and the diameter of the chorion increased to 1.0 -1.1 mm. The chorion became transparent and almost spherical, the shape of the yolk mass was unchanged. The chorion of fertilized eggs was very fragile. (Details will be published elsewhere.) The incubation time was 16 hours and 10 hours at 20.5°C and 25.0°C respectively. The larvae were transparent without pigmentation at hatching, and the yolk sac was fully resorbed within three days at 25°C.

Broken hauls were taken from 100-50 and 50-0 meter on every second station during the last cruise. The results are presented in Table 15 as the distribution of B. pterotum eggs and larvae in double broken hauls through 24 hours.

Table 15. The distribution of B. pterotum eggs and larvae in broken hauls from 100-50 and 50-0 meters depth through 24 hours. (2-11 February, Gulf of Oman, see Fig. 21 B).

Hours	06h - 12h		12h - 18h		18h - 24h		24h - 06h
Depth	100-50	50-0	100-50	50-0	100-50	50-0	100-50	50-0
% Eggs	100	0	0	0	0	0	81	19
% Larvae	30	70	1	99	20	80	27	83
No. of hauls	4	4	6	6	5	5	7	7

Most larvae were found in the upper 50 meters during 24 hours. Eggs were only found at midnight and in the early morning hours, and most eggs were found in the deepest hauls (100-50 m). The eggs found at midnight were all in the early clevage stages, and must have been spawned only a few hours prior to sampling. Eggs were only found in two hauls between 06 hours to 12 hours and these eggs were close to hatching. In one set of triple broken hauls, 300-200, 200-100 and 100-0 meters taken at 23 hour (5 February), eggs were only found in the two deepest hauls, 5 and 61 eggs respectively.

These observations indicate that B. pterotum spawns in the mesopelagic layer at night, and that the eggs slowly ascends towards the surface and hatch before they reach the upper 50 meters of the water column. With a few exceptions yolk sac larvae were found in hauls from 50-0 meters. This means that the most vulnerable larval stages are spent in deeper waters.

The main spawning area was found in the western part of the Gulf of Oman. In this area the highest concentration of both fish and larvae of B. pterotum were found. Judging from the distribution of larvae presented in Fig. 21 A it seems, however, that spawning occurs all over the Gulf.

From the data on the spawning period (Fursa 1969, 1973 and Nellen 1973) and our data on the spawning biology and distributions of eggs and larvae of B. pterotum in time and space, a very high effort, distributed over most of the year, will be necessary to give reliable estimates of stock size of B. pterotum from egg and larval surveys.