3.1 Introduction.
3.2 Sierra Leone, Guinea and Guinea Bissau.
3.3 Senegal, The Gambia, Mauritania.
3.4 Morocco
Based on their behaviour fish species are generally classified as demersal or pelagic. Although many demersal fish often occur in mid water and pelagic fish near the bottom, this is still a useful classification. In addition, the triggerfish which is mostly found by itself in mid water can easily be recognized in echo diagrams and sampled with fishing gear, and has been separately assessed in this report. The larger species of pelagic fish, tunas and billfishes are for methodical reasons not included in the survey.
The general features of the fish communities on the shelf off NW Africa are well known from the fisheries and from previous surveys. In the relatively cool waters in the north off Morocco the sardine, Sardina pilchardus dominates the pelagic community and is usually found together with mackerel. Scomber japonicus and horse mackerel, Trachurus trachurus. From Mauritania southwards the two sardinella species, round sardinella, Sardinella aurita and flat sardinella, Sardinella maderensis represent the clupeids while the carangids include the horse mackerel Trachurus trecae, the scads, Decapterus rhonchus and D. punctatus and the bumper, Chloroscombrus chrysurus among the most common. The triggerfish, Balistes capriscus occurs in the whole area, but appears in abundance from Guinea Bissau southwards.
The main species among the demersal fish from the Gambia to Sierra Leone include representatives of the grunts (Pomadasyidae), seabreams (Sparidae), croakers (Sciaenidae), catfishes (Ariidae), threadfins (Polynemidae) and others, and of cuttlefish (Sepia).
3.3.1 Small pelagic fish.
3.3.2 Demersal fish.
This area was covered between 17 August and 10 September and Guinea Bissau a second time from 23 to 26 November 1986.
Figure 3.1 shows the distribution of fish as observed with the acoustic integration system for the August - September survey. The registrations are mainly made of small pelagic fish and the dominant feature is the aggregations over the middle- and outer parts of the shelf which for the greater part consisted of triggerfish, Balistes. The highest densities were recorded near the shelf edge in the border area between Guinea and Guinea Bissau, but generally the triggerfish was found in relatively loose formations in mid water and near the bottom. This species dominates, however, in the bottom trawl catches as shown in Datafile Table 6.1, appearing in abt. 70 per cent of all hauls and with 6 catches of abt. 4 tonnes or more per hour, one ranging above 20 tonnes per hour. The size composition of samples of trigger fish from August - September and November is shown in Annex VI. Medium sized fish dominate, but with sizes ranging beyond 30 cm.
Outside Freetown an area with aggregations of sardinellas was located. Samples contained both round- and flat sardinella, but the behaviour with almost exclusively surface schooling fish which extended onto the shallow St. Ann’s shoals during the time of our survey did not permit a proper acoustic coverage of the fish. Counts on horizontal ranging sonar showed 10 schools per nautical mile. A purse seine fleet operated in the area.
Along the inner shallow part of the Guinea shelf several patches of pelagic fish were located. These consisted mostly of bumper (Chloroscombrus) with some scad (Decapterus), pompano (Alectis) and jacks (Caranx). The catches also contained smaller amounts of flat and round sardinellas, barracudas (Sphyraena) and Spanish mackerel (Scomberomorus). In the shallow waters, (20 - 30 m) catch rates of bumper ranged up to abt. 3 tonnes per hour in bottom trawl.
The false scad (Decapterus rhonchus) dominated among the small pelagics on the shelf outside the Bijagos Islands in Guinea Bissau with smaller quantities of barracudas, lookdown (Selene dorsalis), chub mackerel and others. The scad was caught by bottom trawl at intermediate depths, 20-60 m with catch rates ranging up to 4.5 tonnes per hour.
In the very shallow waters north of the Bissagos Islands, African ilisha was the most common pelagic form, but also some hairtails (Trichiurus lepturus) occurred here. Catches were however small for both species.
Size compositions of pooled samples of bumper, scads, jacks, barracudas and Spanish mackerel are given in Annex IV to show the prevailing sizes of these fish in the catches.
The acoustic estimate of the total biomass of small pelagic fish in the area from Freetown to Cape Roxo is shown in Table 3.1.
Table 3.1 Estimated biomass of pelagic fish based on acoustic survey data, 1 000 tonnes. Freetown to Cape Roxo, August - September 1986
|
Clupeids, carangids scombrids etc. |
Trigger fish |
Total |
|
200 |
220 |
420 |
The total biomass of the trigger fish estimated at abt. 220 000 tonnes represents an acoustic assessment from day- and night surveying and may also be an underestimate since part of the fish is found near the bottom at least during daytime as evidenced by the often high catch rates in demersal trawl.
The abundance estimates will be further discussed under Chapter 4 below.
The fish distribution on the Guinea Bissau shelf in the November survey is shown in Figure 3.1. The trigger fish dominated the pelagic community outside the Bijagos Islands. Further north the samples consisted of various carangids, anchovy and Spanish mackerels. Catch rates apart from those for trigger fish were low. Estimates of biomass give 45 000 tonnes for trigger fish and 65 000 tonnes for other small pelagics.
The demersal fish has generally a very scattered distribution and a species’ distribution and abundance will not be properly mapped by the acoustic system. The only demersal species which formed aggregations recorded by the acoustic system, was the silverside grunt (Brachydeuterus auritus), located within 30 nm South of Cape Roxo, see Figure 3.1.
During the first survey random trawl stations were set out to assess the demersal biomass by the swept area method. 73 trawl stations were worked out, of which 38 were in the 5-30m bottom depth stratum, 21 between 30 and 60m, 11 from 60 to 100m and 3 stations from 100 to 200m. The results from the analysis are shown in Table 3.2.
The overall mean density for the shelf is estimated to abt 11 tonnes/nm2. By depth strata the mean densities are: 5-30m: »9 tonnes/nm2; 30-60m: » 1 5 tonnes/nm2; 60-100m: » 9 tonnes/nm2 and 100-200m: 15 tonnes/nm2. The last estimate has a low precision as based on three hauls only.
The dominating species is the flying gurnard (Dactylopterus volitans) making up abt 20% of the biomass of demersal fish. It has its principal distribution between 30 and 60m bottom depth. In regional abundance the species is followed by seabreams (Pagellus bellottii, Sparus caeruleostictus) 13%, cuttlefish (Sepia) 5% and silverside grunt (Brachydeuterus auritus) 4%.
Mean densities and predominant depth ranges for some commercially interesting species groups are as follows:
|
|
Mean density |
Main depth |
|
|
tonnes/nm2 |
m |
|
Flying gurnards |
2.2 |
30-60 |
|
Seabreams |
1.7 |
30-100 |
|
Cuttlefish |
0.6 |
5-100 |
|
Groupers |
0.2 |
60-100 |
|
Emperors |
0.2 |
5-30 |
|
Shrimps |
0.3 |
5-30 |
|
Silverside grunt |
0.5 |
5-30 |
|
Other fish |
5.8 |
|
Estimates of abundance are obtained by multiplying the densities by the area of the shelf. The areas by depth zones have been calculated from seamaps by planimeter and are as follows:
|
0-30m |
30-60m |
60-200m |
|
» 16350 nm2 |
» 10000 nm2 |
» 4700 nm2 |
|
|
0-30m |
30-60m |
60-200m |
0-200m |
|
Flying gurnards |
12250 |
62500 |
1000 |
75750 |
|
Seabreams |
11500 |
29600 |
12300 |
53400 |
|
Groupers |
|
600 |
4950 |
5550 |
|
Emperors |
4700 |
300 |
|
5000 |
|
Cuttlefish |
6850 |
8900 |
2650 |
18400 |
|
Shrimps |
7850 |
100 |
|
7950 |
|
Silverside grunt |
14400 |
1200 |
|
15600 |
|
Other fish |
87850 |
43300 |
29400 |
160550 |
|
|
|
|
|
|
|
Total |
145400 |
146500 |
50300 |
342200 |
3.3.1 Small pelagic fish.
3.3.2 Demersal fish off the Gambia.
The shelf off Senegambia was covered by an acoustic survey between 29 August and 5 September, and a swept area trawl survey was made off The Gambia between 10 and 12 September. The trawl survey was repeated off The Gambia 27 - 29 November and most of the Senegambia shelf was covered by an acoustic survey 2 - 7 December and the Mauritanian shelf on 8 - 12 December. The course tracks are shown in Annex I.
Senegambia
Figure 3.2 shows the fish distribution off Senegambia in August - September. Some silverside grunts which occur in mid water especially during the night, are included, but by far the main part consists of small pelagic fish mostly clupeids and carangids. The behavior of the fish was reasonably favourable for acoustic surveying with limited surface schooling of sardinellas.
Several areas of high densities of pelagic fish were located during this survey. The offshore aggregation in the south off Kasamanze consisted of bumper and false scad with some flat sardinella, barracudas, Spanish mackerel and lookdown. To the north of Kasamanze up to the Gambia river, the inshore concentrations seemed to consist mainly of flat sardinella and bumper, also mixed with some jacks, scads and barracudas. The bumper was the dominant form together with both sardinella species in the area of high concentration off Point Sangomar. Among the accompanying species lookdown, scads and pompano were the most common.
Inshore, north of Cayar, the two sardinella species occurred in dense schools with bumper, sharks and barracudas while the offshore aggregations, further north, was identified as horse mackerel, round sardinella and chub mackerel.
The acoustic survey was supplemented with fishing with both bottom - and mid water trawl for purposes of identification and sampling. High catch rates were obtained for bumper in the shallow inshore parts south of Cape Verde, particularly in the midwater trawl where rates ranged up to about 6 tonnes per hour. In the south, the false scad had an occasional good catch rate, 1.4 tonne per hour, sharks up to 400 kgs per hour, barracudas and pompano up to 115 and lookdown up to 300 kgs per hour.
North of Cape Verde some good catch rates, 0.6 and 3 tonnes per hour of horse mackerel (T. trecae) were obtained at abt. 100 m depth. Catch rates for sharks ranged beyond 300 kgs per hour and barracudas appeared in nearly all catches with rates up to 90 kgs per hour.
Samples of the pooled size compositions of the main species are shown in Annex VI.
The estimates of the standing biomass of the small pelagic fish resulting from the observations from the acoustic integration system are shown in Table 3.2. As mentioned under 1.3 above an attempt is made to distinguish between two groups in these assessments, the clupeids, in this case mainly the sardinellas, and a group consisting of other pelagic fish, the carangids-horse mackerel, jacks, scads etc together with mackerels, barracudas and hairtails. This separation is mostly based on trawl sampling and is only roughly approximate. The estimates for total pelagic fish is thus more reliable than those for each group.
The shelf area between 14°05’ and Dakar was not covered. A total estimate of pelagic fish between Cape Roxo and St. Louis will be about 450 000 tonnes, with by far the major part south of Cape Verde. About 2/3 of this or abt. 300 000 tonnes is assessed to be sardinellas, and abt. 150 000 tonnes carangids etc.
Table 3.2 Estimated biomass of pelagic fish based on acoustic survey data.(in thousand tonnes) Senegambia -September survey.
|
|
Clupeids |
Carangids, |
Total |
|
N. of C. Verde |
15 |
35 |
50 |
|
S. of C. Verde |
260 |
110 |
370 |
|
Total |
275 |
145 |
420 |
Some of the trial fishing hauls gave good catch rates, especially of horse mackerel: up to abt. 3 tonnes/hr in bottom trawl and reaching nearly 5 tonnes/hr in mid water hauls.
The size compositions(ANNEX VI) show that south of Cape Verde the samples contained mostly juvenile and immature fish of the two sardinellas, while adult fish occurred off St. Louis. The horse mackerel samples showed consistently medium sized fish.
The biomass estimates for the parts of the Senegambian shelf covered are shown in Table 3.3.
Table 3.3 Estimated biomass of pelagic fish based on acoustic survey data. 1 000 tonnes. Senegambia - November survey.
|
|
Clupeids |
Carangids, |
Total |
|
North of C. Verde |
100 |
110 |
210 |
|
South of C. Verde |
180 |
60 |
240 |
|
Total |
280 |
170 |
450 |
Mauritania
Figure 3.4 shows the distribution of the small pelagic fish over the Mauritanian shelf in early December as observed with the acoustic integration system. A number of smaller areas of high fish densities were located from St. Louis up to Cape Timeris. These consisted mostly of dense mid water schools of large sized horse mackerel, but also hairtails formed a significant part of the catches in this area. Both of the Trachurus species were caught with a majority of T. trecae in the catches, but their true relative abundance is uncertain since there are great difficulties in obtaining representative catches of these fish with the gear and fishing system used by the DR FRIDTJOF NANSEN.
An unusually dense school area was found off Nouadhibou. A few catches gave a mixture of sardinellas and sardine (Sardina pilchardus), but it is likely that the latter species formed the main part of this registration.
The estimates of the standing biomass of the small pelagic fish on the Mauritanian shelf during the survey are shown in Table 3.4.
Table 3.4 Estimated biomass of pelagic fish based on acoustic survey data. 1 000 tonnes. Mauritania, November survey.
|
Sardine and sardinellas |
Carangids |
Total |
|
980 |
540 |
1 520 |
The catch rates in the 19 trawl hauls made were generally low except in the dense school area in the north where a pelagic haul of abt. 10 tonnes/hr of sardinella and sardine was obtained.
The size compositions of the main species are shown in ANNEX IV. Large size adult fish predominate, but for horse mackerel some juvenile fish were also sampled.
During both surveys random trawl stations were set out in the waters off Gambia in order to obtain a trawl survey estimate. 24 and 23 stations were worked out on the first and the second survey respectively. The positions of the trawl stations are shown in Annex I. The results from the swept-area analysis are shown in Tables 3.4 and 3.5. The average density for the whole shelf is estimated to 21 and 18 tonnes/nm2 for the respective surveys. The dominating species is the silverside grunt during both coverages, followed by the red pandora (Pagellus bellottii). A striking feature is the vertical migration of the fish between the surveys, clearly demonstrated by the grunt which during the first survey had its main distribution in waters shallower than 30m, and which in the November survey had moved to the 30-60m zone, with only fractions in the shallow waters (30 t/nm2 compared to 0.2 t/nm2). The pattern is valid also for the whole fish community as can be seen from the average densities in the bottom depth strata during the two surveys (in tonnes/nm2):
|
|
5-30m |
30-60m |
60-100m |
|
First survey |
27.3 |
7.6 |
20.1 |
|
Second survey |
1.9 |
44.1 |
9.6 |
Table 3.5 Swept-area analysis from the trawl survey off Gambia November 1986.
It is likely that the phenomenon is linked to some change in the environment, likely the oceanographic conditions. But as no oceanographic data were collected during the surveys, we can not make a direct comparison.
The results in Tables 3.4-3.5 have been concentrated into Table 3.6 where the findings concerning most abundant species or commercially important species groups have been summarized.
Table 3.6 Mean densities of most abundant species or commercially important species groups by bottom depth
|
Depth zone |
5-30m |
30-60m |
60-100m |
|||
|
Period |
Sep. |
Nov. |
Sep. |
Nov. |
Sep. |
Nov. |
|
Silverside grunt |
16.5 |
0.2 |
|
30.4 |
|
|
|
Seabreams |
1.3 |
|
2.8 |
8.7 |
11.7 |
6.0 |
|
Snappers |
0.1 |
|
0.1 |
0.1 |
|
|
|
Groupers |
|
|
0.1 |
0.1 |
|
|
|
Emperors |
|
|
|
|
|
|
|
Octopus |
|
|
0.26 |
|
0.07 |
|
|
Cuttlefish (Sepia) |
0.12 |
|
0.18 |
|
0.33 |
|
|
Shrimps |
0.16 |
|
|
|
|
|
|
Total fish |
27.2 |
1.9 |
7.6 |
44.0 |
20.1 |
9.6 |
Absolute biomass estimates are obtained by multiplying the density figures with the areas concerned and are given in Table 3.7.
Table 3.7 Biomass estimates of most abundant species or commercially important species groups by bottom depth str ta. (thousand tonnes).
|
Depth zone |
5-30m |
30-60m |
60-100m |
|||
|
Period |
Sep. |
Nov. |
Sep. |
Nov. |
Sep. |
Nov. |
|
Silverside grunt |
12.5 |
0.1 |
|
14.7 |
|
|
|
Seabreams |
1.0 |
|
1.4 |
4.2 |
2.4* |
1.3 |
|
Snappers |
0.1 |
|
0.03 |
0.02 |
0 |
|
|
Groupers |
0.02 |
|
0.02 |
0.05 |
0.01* |
|
|
Emperors |
0 |
|
0 |
|
0 |
|
|
Octopus |
|
|
0.13 |
|
0.01* |
|
|
Cuttlefish (Sepia) |
0.1 |
|
0.1 |
|
0.07* |
|
|
Shrimps |
0.12 |
|
|
|
|
|
|
Total fish |
20.6 |
1.4 |
3.7 |
21.4 |
4.2* |
2.0 |
* Unprecise, few haulsThe total estimate from the September survey is 28 500 tonnes of which 12 500 is silverside grunt and only 2 100 is of the high market value groups mentioned in the table. The total from the November survey is 24 800 tonnes with 14 800 silverside grunt and 5 570 of the “valuable” fish. Most of the fish is probably stationary, making only vertical migrations. This conclusion is based on the fact that the total shelf estimates by groups are quite close for the two surveys:
|
|
Thousand tonnes |
|
|
|
Sep. |
Nov. |
|
Silverside grunt |
12.5 |
14.8 |
|
Seabreams |
4.8 |
5.5 |
|
Snappers |
0.13 |
0.02 |
|
Groupers |
0.06 |
0.05 |
The density figures given Tables 3.5 and 3.5 can easily be converted to expected catch rates if one knows the width of the trawl gear and the trawling speed. Expected mean catch rates can be calculated from the formula:
where C the expected catch rate in kg/hour, D is the estimated density in kg/nm2, W is the length between the wings of the trawl and S is the trawling speed in knots. For example the estimated density of seabreams during the first survey is 3.37 tonnes/nm2. The width of the gear is 18.5 m and the average trawling speed is 3 knots. This gives an expected average catch rate for the whole shelf of 101 kg/hour for the gear used by Dr. Fridtjof Nansen.
3.4.1 Pelagic fish Agadir to Cape Juby.
3.4.2 Results from repeated coverages.
3.4.3 Fish distribution and abundance Cape Juby - Cape Bojador.
The shelf between Agadir and Cape Juby was covered between 22 September and 5 October, and again between 6 and 12 November. During the first survey experiments with repeated coverages were carried out on the main concentrations of the sardine Sardina pilchardus in order to test the consistency of the acoustic estimates and the day/night variations of the acoustic registrations.
The investigational effort was concentrated on the small pelagic species, and no random bottom trawl stations were set out.
The pelagic community is dominated by the sardine with mackerel (Scomber japonicus) and horse mackerel (Trachurus trachurus) as secondary species. Other pelagic species play only a very minor role in the area concerned. Figure 3.5 through 3.13 show the fish distribution by species and coverages.
The sardine is mainly concentrated in the nearshore waters and south of latitude 29°30' during both surveys. Comparisons with maps of the surface temperature, Figures 3.14 and 3.15, show that areas with highest density of sardine corresponds closely with the zones of upwelling, indicated by pockets or belts of cold water close to the shore. On the basis of the observed fish distribution during the two surveys, no major fish migration seems to have occurred between them. The sardine stock is composed of two size groups with modal lengths around 14 and 18 cm. The modal lengths during the first survey are from 0.5 to 1 cm less than during the second, and the oldest age group clearly dominated the samples, both by weight and number.
The mackerel co-occurs with the sardine mainly in the nearshore waters, but generally at lesser density and abundance. The distribution is limited to south of 29°10’ N during the first survey, while during the second it was sampled north to Agadir. Comparison between the distribution maps and the surface temperature maps suggests that the mackerel prefers water temperatures less than 20°C while the sardine extend its distribution to the 21°C limit. The mackerel in the area investigated seems to consists of one age group with mean length around 17.5 cm.
The horse mackerel tends to have a more offshore distribution than the two previous mentioned species. The species is found in a quite even but very scattered distribution on the main part of the investigated shelf area. The species was mostly located close to the seabed and was never observed at densities of interest for commercial fishing. The investigated part of the stock consist of two age groups with modal lengths around 14 and 19 cm, and with the oldest age group as the dominant in the samples.
All trawling in the area were for sampling purposes. The catches grouped by the three dominant species and other fish are presented in Table 3.8 and 3.9 for the first and second survey respectively. The various groups’ shares of the total catch are as follows (in %):
|
|
Sardine |
Mackerel |
Horse Mackerel |
Other |
|
First survey |
90 |
6 |
1 |
3 |
|
Second survey |
67 |
14 |
7 |
12 |
|
STA. NO. |
Sardine |
Mackerel |
Horse mack |
Other |
|
153 |
|
|
5.00 |
1.60 |
|
154 |
28.00 |
4.60 |
0.20 |
7.00 |
|
155 |
|
|
|
|
|
156 |
|
|
|
|
|
157 |
175.00 |
47.60 |
|
|
|
158 |
|
|
|
|
|
159 |
63.00 |
471.00 |
69.00 |
349.20 |
|
160 |
3816.00 |
|
|
2.16 |
|
161 |
|
|
|
|
|
162 |
254.80 |
298.20 |
98.00 |
399.70 |
|
163 |
8946.00 |
375.80 |
30.00 |
248.20 |
|
164 |
127.00 |
190.00 |
1.00 |
3.40 |
|
165 |
2.00 |
22.00 |
|
|
|
166 |
17420.00 |
580.60 |
|
|
|
167 |
0.10 |
0.60 |
|
|
|
168 |
12.00 |
7.80 |
3.60 |
192.00 |
|
169 |
764.40 |
84.00 |
22.40 |
81-20 |
|
170 |
5.20 |
5.20 |
221.00 |
638.40 |
|
171 |
3109.60 |
114.40 |
|
|
|
172 |
233.80 |
1150.00 |
102.60 |
105.20 |
|
173 |
16860.00 |
|
|
|
|
174 |
1984.00 |
6.20 |
|
|
|
175 |
1352.00 |
182.00 |
31.20 |
83.20 |
|
176 |
248-40 |
36.00 |
0.45 |
4.50 |
|
177 |
78.00 |
14.70 |
|
0.90 |
|
178 |
868.00 |
2.80 |
|
23.10 |
|
179 |
232.00 |
16.80 |
|
|
|
180 |
67.50 |
39.00 |
|
|
|
181 |
8727.30 |
272.70 |
|
|
|
182 |
|
18.80 |
259.20 |
166.20 |
|
183 |
117.20 |
2.80 |
10.80 |
0.40 |
|
184 |
47.00 |
1.80 |
1.20 |
0.20 |
|
185 |
12000.00 |
|
|
|
|
186 |
320.00 |
3.00 |
|
|
|
137 |
4032.00 |
6.00 |
3.60 |
|
|
188 |
239.40 |
674.60 |
91.20 |
135.00 |
|
189 |
405.00 |
513.00 |
18.90 |
669.60 |
|
190 |
1620.00 |
66.00 |
42.00 |
168.00 |
|
191 |
88.00 |
17.60 |
|
|
|
192 |
5387.00 |
590.60 |
|
22.40 |
|
MEAN |
2240.74 |
145.41 |
25.28 |
82.54 |
Total number of stations: 40Table 3.9 Catches of sardine, mackerel, horse mackerel and other fish during the November survey in Morocco.
|
STA. NO. |
Sardine |
Mackerel |
Horse mack |
Other |
|
197 |
|
|
|
|
|
198 |
|
0.10 |
|
|
|
199 |
|
60.00 |
|
0.20 |
|
200 |
69.00 |
13.00 |
|
11.10 |
|
201 |
601.66 |
7.61 |
|
2.58 |
|
202 |
25.20 |
176.40 |
82.80 |
83.40 |
|
203 |
5606.26 |
|
|
3.74 |
|
204 |
207.20 |
72.80 |
|
|
|
205 |
176.25 |
14.14 |
|
|
|
206 |
0.40 |
|
|
5.20 |
|
207 |
|
|
|
426.00 |
|
208 |
85.00 |
1597.00 |
1436.60 |
1831.60 |
|
209 |
15.00 |
1.20 |
0.90 |
85.80 |
|
210 |
485.10 |
4.90 |
|
|
|
211 |
|
385.65 |
12.85 |
15.41 |
|
212 |
500.00 |
76.00 |
28.00 |
22.00 |
|
213 |
3.00 |
7.80 |
22.80 |
145.50 |
|
214 |
13.60 |
|
20.40 |
147.60 |
|
215 |
5.40 |
516.60 |
|
|
|
216 |
3410.00 |
80.00 |
|
10.00 |
|
217 |
4864.20 |
135.80 |
|
|
|
218 |
496.00 |
192.00 |
70.40 |
171.20 |
|
MEAN |
752.83 |
151.86 |
78.40 |
134.61 |
Total number of stations: 22The higher share of the sardine during the first survey compared to the second is partly due to a more intensive sampling on this species during the repeated coverages then carried out.
Although catch rates based on a programme for sampling purposes do not reflect expected catch rates in a commercial fishery, the figures can be used as indicative of the relative importance and catchability of the species. Table 3.10 shows the catch distribution of sardine, mackerel and horse mackerel.
Table 3.10 Catch distribution by size classes of catches of the three dominant pelagic species in Morocco. (number of hauls in Kg/hour or tonnes/hour groups)
|
Species |
Survey |
< 30 kg |
30-100 |
100-300 |
300-1000 |
1-3t |
> 3t |
|
Sardine
|
I |
6 |
5 |
8 |
4 |
3 |
9 |
|
II |
6 |
2 |
2 |
4 |
0 |
3 |
|
|
Mackerel
|
I |
19 |
5 |
5 |
6 |
1 |
0 |
|
II |
9 |
4 |
3 |
2 |
1 |
0 |
|
|
Horse mackerel
|
I |
12 |
6 |
3 |
0 |
0 |
0 |
|
II |
5 |
2 |
0 |
0 |
1 |
0 |
The acoustically-based biomass estimates provisionally given in the preliminary cruise reports, have been corrected and slightly reduced and are shown in Table 3.11.
The estimates of total biomass from the two surveys come out remarkably close with 1185 and 1165 thousand tonnes. And the estimates on the species level are in close agreement as well. This indicates that no greater migration from/to the investigated area had taken place between the two surveys. The total estimate is composed of 83% sardine, 7% mackerel and 10% horse mackerel during the first survey, while the corresponding figures from the second survey are 80, 6 and 13% respectively.
Table 3.11 Estimated biomass of pelagic fish between Agadir and Cape Juby, based on acoustic survey data. (1000 tonnes):
|
|
Sardines |
Mackerel |
Horse mackerel |
Total |
|
|
First survey |
|
|
|
|
|
|
|
A: main coverage |
660 |
85 |
115 |
860 |
|
|
B: detailed coverages averaged |
905 |
70 |
15 |
990 |
|
|
A and B combined |
985 |
85 |
115 |
1185 |
|
Second survey |
935 |
75 |
155 |
1165 |
|
During the first survey in Morocco considerable amounts of sardine was located in the shallow waters, and the main survey net did not survey these aggregations properly. To compensate for this a detailed coverage was run twice to assess the biomass in the shallow waters. In addition, a selected course track was run repeatedly under day and night light conditions, to see how the acoustic registrations varied under the influence of light. The two detailed coverages gave a biomass of 820 and 990 thousand tonnes, average 905 thousand tonnes. The two sets of day/night comparisons gave a ratio of 1.38 and 1.42 between average densities observed during night and during day. The night readings thus give at average about 40% higher density figures than during daylight. This difference has not been compensated for in the estimations on biomass and gives a bias towards underestimation in the abundance estimates given above. Comparisons between successive sets of day runs give a ratio of 1.12 and the night runs give a ratio of 1.09. For further details see Strømme and Sætersdal 1987.
To conclude, both the repeated surveys on the major aggregations of the sardine, and the comparisons under similar light conditions show that acoustic surveys can give high consistency in the estimates on this species.
The shelf between Cape Juby and Cape Bojador was surveyed during the second survey only, between 13 and 15 November. The cruise track with stations are shown in Annex I. Nine trawl stations were worked out to identify the acoustic registrations.
As in the region Agadir - Cape Juby the area is dominated by the sardine, with mackerel and horse mackerel only of minor importance. The distribution of these species are shown in Figures 3.16 to 3.18. In the trawl hauls carried out the sardine makes up 93% of the total catch, mackerel 5%, horse mackerel 1%, and other fish 1%. The sardine has, as further north, a rather coastal distribution, and they can in both regions be considered as belonging to the same stock.
The total biomass in this area was estimated to 1140 thousand tonnes, with 1080 thousand tonnes sardine (94%), 40 thousand tonnes mackerel (4%) and 20 thousand tonnes horse-mackerel (2%).
