The units of acoustic reflection is 0.1 xm2/nm2 reflecting surface. An arbitrary scale has been used to illustrate different levels of concentration. The integrator values were allocated to the following groups on the basis of trawl sampling and characteristic behaviour:
Pelagic fish type 1 : Clupeids and anchovies, i.e mainly sardines.
Figure 1. Course tracks and fishing stations Cap Safi to Cap Yubi .
Figure 2. Course tracks and fishing stations Cap Yubi to Cap Blanc.
Figure 3. Temperature at sea surface Cap Safi to Cap Yubi.
Figure 4. Temperature at sea surface Cap Yubi to Cap Blanc.
Pelagic fish type 2 : Carangids, scombrids, barracudas etc. Demersal fish in mid water. In this survey predominantly horse mackerel and mackerel.
The distribution of the two groups are presented in Figures 5-6 and 7-8 respectively. The figures show the distribution by four levels of echo intensity. The conversion into biomass is both species and size dependent, but roughly speaking a density of 10 units corresponds to slightly less than 30 tonnes/nm2 for a 17 cm fish. The highest densities in the map, representing more than 300 units have thus an average density exceeding 900 tonnes/nm2.
Cape Safi to Cape Ghir
Sardines were found all over the inner part of the shelf. Three patches of very high densities were found close to the shore. The modal lengths in the catches were 10 and 19 cm with the first predominating. The biomass of sardines was estimated to 430 000 tonnes with 265 000 tonnes or 61 % as very dense concentrations.
Horsemackerel and/or mackerel were found at most part of the shelf, but mainly as scattered distributions. The mackerel tended to have a more shallow distribution, often mixing with the sardines, while the horse mackerel (Trachrus trachurus) also was registrated at the outer part of the shelf. The biomass estimate for the two species combined is 74 000 tonnes. Most of the biomass, 72%, was in scattered distributions. The remaining 28% was in a slightly aggregated patch, but then co-occurring with the sardines. The findings do not justify a targeted fishery on mackerel or horse mackerel in the area, at least not at this season.
Cape Ghir to Cape Juby
Sardines were found in the shallow waters in the whole region Some aggregations were found close to Agadir, and the main part of this had a very shallow distribution, less than 15 m bottom depth. The main concentrations were however found between Cape Dra and Cape Juby, where integrator readings of more than 1000 units per nm2 were frequent. The total biomass of sardines was 1 120 000 tonnes for the whole region. Of this 900 000 tonnes or 80% were in very dense concentrations.
Figure 5. Distribution of sardine, Cap Safi to Cap Yubi.
Figure 6. Distribution of horse mackerel and mackerel, Cap Safi. to Cap Yubi. -
Figure 7. Distribution of sardine, Cap Yubi to Cap Blanc.
Figure 8. Distribution of horse mackerel and mackerel, Cap Yubi to Cap Blanc.
As more north the mackerel and horsemackerel were less abundant and tended to have a more offshore distribution. In the shallow waters the presence of the species could however be heavily shadowed by the dominance of the sardines, and might thus be somewhat underestimated. The biomass estimate for the mackerel and horse mackerel combined is 250 000 tonnes. Most of this or 63% is in scattered distributions. The remaining is only slightly gathered.
Unexpectedly sardinella (S. aurita) was caught in one of the hauls, at st no. 13. Except for one small commercial catch last spring the species has not been reported in the region since 1979.
Cape Juby to Cape Bojador
The sardine was found over the major part of the inner shelf as shown in Figure 7. The biomass was estimated to 625 000 tonnes, of which 50% was distributed as very dense. The modal lengths of the sardine were 14 and 22.5 cm.
Horsemackerel and mackerel was found in a wider but more scattered distribution over the shelf. The biomass of the two species combined was estimated to 125 000 tonnes. Less than 30% of this was found slightly aggregated, the rest was scattered.
Cape Bojador to Dakhla
As further north the sardine was found at most part of the inner shelf- Especially north and west of Dahkla there were heavy concentrations, as shown in Figure 7. The total biomass of sardine in the region was estimated to 4 340 000 tonnes. 3 490 000 tonnes of this or 81% were mapped as very dense and vulnerable for fishing. The sardine consisted of mainly one cohort of 24 cm modal fork length.
Horse mackerel and mackerel was found scattered over the whole shelf with a few and small slightly aggregated patches. The biomass was estimated to 250 000 tonnes, with 13 % as slightly aggregated.
Dakhla to Cape Blanc
The sardine is found at the inner part of the shelf down to abt. 21° 20'N. At 21° 50' N a very dense patch of juveniles was also located more offshore at abt 80 m bottom depth together with anchovies. The sardines south of Dakhla were mainly young, all samples had a mean length around 14 cm. The biomass of sardines was estimated to 465 000 tonnes, of which 100 000 tonnes were mapped as very dense.
The mackerel was caught down to 23° N, while southwards to Cape Blanc adult Atlantic horsemackerel became more prominent in the catches. Close to Cape Blanc the Cunene horse mackerel was also found in one catch. The biomass of mackerel and horsemackerel combined is 230 000 tonnes, of which the main part is Atlantic horsemackerel. On the shelf the biomass was generally in scattered distribution but 115 000 tonnes located along the peninsula of Cape Blanc, could be characterized as dense aggregated. A very dense school of adult horsemackerel was registrated at shelf edge off Cape Blanc. Such schools are typical for the distribution pattern of adult horsemackerel off the Arguin Banc in Mauritania, and will not be representatively sampled with the survey grid laid out during the present survey. A special survey, with a detailed survey grid at the shelf edge, should be laid out to sample this adult stock. There were no time available to carry this out, and biomass at the shelf edge in the south have not been included in our estimates. As it is a shared stock the slope resources are best estimated in a joint survey with Mauritania and Morocco.
Summary on biomass
Figure 9 a, b with tables show the biomass estimates by regions and compared with the estimates from the similar surveys in 1986. Figure 9a shows that the level of the sardines is fairly the same in the three northern regions, while a significant drop has occurred south of Dakhla. The biomass has dropped from 2.2 mill. to 0.5 mill. tonnes. It could be that some of the differences is due to seasonal migration, as the seasons in the comparisons are not the same. The main cause is however more likely the heavy fishing pressure the region has been under for several years.
Figure 9. The biomass estimates by regions, compared with the similar 1986 estimates. - a. Sardines 1989 and 1986
Figure 9. The biomass estimates by regions, compared with the similar 1986 estimates. - b. Estimated mackerel + horse mackerel
The estimates of mackerel and horse mackerel are remarkably similar in the two surveys.
Table 1. The catch summarized by the main pelagic species. In kg/hour.
|
ST.NO. |
DEP. |
Sardine |
Sardinella |
Horse.mack |
Mackerel |
Other |
|
1 |
7 |
15.00 |
|
1.20 |
1.70 |
3.80 |
|
2 |
21 |
18.00 |
|
80.00 |
|
90.00 |
|
3 |
30 |
12.00 |
|
27.60 |
|
24.00 |
|
4 |
18 |
18.86 |
|
|
|
|
|
5 |
7 |
132.00 |
|
24.00 |
|
4.50 |
|
6 |
85 |
|
|
|
|
|
|
7 |
80 |
0.20 |
|
|
372.00 |
14.00 |
|
8 |
12 |
108.00 |
|
0.60 |
12.00 |
22.80 |
|
9 |
12 |
189.00 |
|
|
0.60 |
|
|
10 |
7 |
105.00 |
|
5.60 |
23.80 |
71.60 |
|
11 |
12 |
88.00 |
|
0.40 |
5.20 |
1.20 |
|
12 |
10 |
816.00 |
|
3.40 |
30.60 |
130.80 |
|
13 |
50 |
350.50 |
|
81.50 |
1844.83 |
223.00 |
|
14 |
7 |
0.33 |
|
0.05 |
7.64 |
0.59 |
|
15 |
7 |
247.00 |
|
1.20 |
11.40 |
30.80 |
|
16 |
52 |
5.60 |
|
|
406.00 |
2.20 |
|
17 |
25 |
339.04 |
|
3.90 |
81.60 |
31.20 |
|
18 |
17 |
92.40 |
|
|
8.60 |
16.02 |
|
19 |
49 |
55.65 |
|
69.30 |
16.80 |
194.25 |
|
20 |
12 |
60.00 |
|
0.40 |
1.60 |
30.40 |
|
21 |
45 |
880.80 |
|
53.80 |
391.40 |
273.80 |
|
22 |
69 |
2.60 |
|
129.60 |
64.80 |
372.00 |
|
23 |
12 |
438.60 |
|
20.40 |
6.80 |
1708.80 |
|
24 |
12 |
350.00 |
|
1.40 |
16.80 |
36.40 |
|
25 |
66 |
|
|
|
1.80 |
348.00 |
|
26 |
|
|
|
|
|
|
|
27 |
60 |
28.00 |
|
3.60 |
63.20 |
23.80 |
|
28 |
7 |
420.00 |
|
|
130.00 |
41.60 |
|
29 |
7 |
27.00 |
|
0.30 |
25.40 |
42.50 |
|
30 |
12 |
212.80 |
|
1.74 |
41.30 |
180.18 |
|
31 |
23 |
61.30 |
|
4.80 |
8.40 |
7.04 |
|
32 |
35 |
40.00 |
|
|
30.00 |
18.83 |
|
33 |
7 |
37.33 |
0.53 |
|
4.80 |
29.08 |
|
34 |
10 |
42.00 |
|
|
0.60 |
247.00 |
|
35 |
50 |
46.20 |
|
72.60 |
|
1175.84 |
|
36 |
27 |
1550.00 |
|
27.20 |
|
849.40 |
|
37 |
78 |
353.08 |
|
82.69 |
|
4856.75 |
|
38 |
25 |
|
|
|
|
100.50 |
|
39 |
12 |
|
|
|
|
33.00 |
|
40 |
220 |
|
11.10 |
|
|
948.30 |
|
MEAN |
|
178.57 |
0.90 |
19.96 |
90.29 |
308.63 |
Total number of stations: 40Catch records and length distributions
Complete records from the trawling operations are listed in Appendix 2, and Table 1 summarizes the catches on the most important pelagic species.
All fishing carried out is aimed fishing to identify acoustic registrations, and it was never an aim to obtain high catches. Therefore the catch rates, given in Table 1 and in Appendix 2, can not be used to obtain estimates of commercial catch rates. Used with caution the data can however give information on distribution of species, their relative strength and degree of co-occurence.
A graphic presentation of pooled length distributions on the important pelagic species by two regions is given in Annex 1. In addition Annex 4 lists the rawdata from all length measurements carried out and Annex 5 give the same data processed to show the main summary statistics by samples.
Fishery surveillance
From Dakhla and southwards fishing vessels within 5 nm off the cruise track were counted in 5 nm intervals, as during previous surveys. With 10 nm between the transects, this gives almost a complete coverage of the shelf. The results are presented in Figure 10.
Figure 10. Observations of fishing vessels, within 5 nm. off the course track .
