Introduction
During the present survey anchovy was caught in pelagic trawls (during night time) and in some of the shrimp bottom trawls (during day time).
Using “FAO Species Identification Sheets, 1983” (W. Indian Ocean) the following species were identified:
Stolephorus punctiferIt was not possible to identify one of the species which occurred in some of the bottom trawls. This species will be referred to in the present report as Species A.
Stolephorus heterolobus
Stolephorus indicus
The difference between the two first species, was not detected during the survey. This difference was only detected later, in a sample analysed in the laboratory where the proportion between the S. punctifer and S. heterolobus was 1:2. As we do not have samples from other hauls and the majority of samples analysed could have been a mixture of these two species, this group is always referred to as S. punctifer/heterolobus.
S. punctifer, S. heterolobus and S. indicus occurred both in the pelagic and bottom trawls. Species A did not occur in the pelagic hauls.
Catch rates
8 pelagic trawl hauls (during night time) were made. The following table shows the results obtained.
|
Station number |
Latitude |
Longitude |
Gear depth (m) |
Bottom depth (m) |
Catch rates kg/h |
|
512 |
S 17°34’ |
E 38°03’ |
10 |
31-34 |
30.8 |
|
517 |
S 18°07’ |
E 37°14’ |
21-24 |
21-24 |
41.6 |
|
523 |
S 18°34’ |
E 36°55’ |
8-14 |
28-29 |
480.0 |
|
529 |
S 19°01’ |
E 36°48’ |
10-43 |
36-43 |
127.5 |
|
530 |
S 18°59’ |
E 36°42’ |
20-24 |
34-38 |
488.0 |
|
539 |
S 19°23’ |
E 36°52’ |
10 |
73 |
30.0 |
|
540 |
S 19°12’ |
E 36°07’ |
10 |
25 |
120.0 |
|
545 |
S 19°49’ |
E 36°17’ |
15 |
65-68 |
11.4 |
|
Station number |
Latitude |
Longitude |
Gear depth (m) |
Catch rates kg/h |
|
499 |
S 17°01’ |
E 39°09’ |
13 |
0.5 |
|
501 |
S 17°06’ |
E 38°50’ |
9-10 |
2.2 |
|
504 |
S 17°16’ |
E 38°29’ |
15 |
0.2 |
|
525 |
S 18°17’ |
E 36°55’ |
18-19 |
31.2 |
|
536 |
S 19°05’ |
E 36°03’ |
19 |
2.4 |
|
541 |
S 19°16’ |
E 35°52’ |
20-21 |
3.0 |
|
550 |
S 20°11’ |
E 34°53’ |
5 |
5.2 |
Fig. 8. Stations with catch of anchovy.
Biological analysis of the main species
At each station with a significant catch of anchovy, samples were analysed. For all species length was recorded by 0.5 cm groups. For S. punctifer/heterolobus maturity stages were also recorded using the maturity scale included in Annex 1, Table 2.
Length distribution was analysed by haul. Fig. 9 shows the mean length by haul obtained in pelagic trawls (A) and in bottom trawls (B).
Fig. 9. Mean total length/haul of S. punctifer/heterolobus.
S. indicus (*) and Species A (**).Comparing the distribution areas of the pelagic fish obtained during the present survey (Fig. 7), with the position of the control pelagic trawls (Fig. 1), the samples which seem to belong to the same distribution area were joined. The length distribution, modal size and mean estimates obtained (although not weighted by the stock size of each area) are approximations of the structure of the population of those areas.
A) Pelagic trawl hauls. B) Shrimp bottom trawl hauls.
Some samples were brought to the laboratory to calculate length-weight relationship. For each species this relationship was obtained by applying the functional linear regression to data, transformed in Logarithms.
S. punctifer/heterolobus
This group dominated the catches. It was present from latitude S 17°34’ to S 20°11’.
Total length of 572 individuals caught in 6 pelagic hauls, was measured (Annex 2, Table 1). The sizes ranged from 2.5 to 9.5 cm long, with the modes varying between the classes 3.5-4.0 and 8.0-8.5 cm.
From bottom trawls a total of 313 individuals were measured (Annex 2, Table 1). The size ranged from 4.0 to 9.0 cm long, with the modes varying between the classes 5.0-5.5 and 8.5-9.0 cm.
As already mentioned, the samples which seemed to belong to the same distribution area were joined and the results are shown in the following table:
|
Area |
Length range Class Mark |
Modal size Class Mark |
Mean size |
|
From Quelimane to Zambezi River |
2.75 - 9.75 |
4.25 and 7.75 |
7.8 |
|
In front of Zambezi River |
6.25 - 9.25 |
8.25 |
7.7 |
The relative percentage of each maturity stage, males and females separated, are shown in the following table.
|
|
|
2 |
3 |
4 |
5 |
Total number |
|
|
no |
34 |
97 |
16 |
3 |
150 |
|
% |
22.7 |
64.7 |
10.6 |
2.0 |
||
|
|
no |
32 |
93 |
95 |
|
220 |
|
% |
14.6 |
42.3 |
43.1 |
|
||
|
Juveniles |
12 |
|||||
Stolephorus punctifer
|
log |
W = 3.9178 log LT - 3.0093 |
|
|
n = 277 |
|
|
r = 0.9455 |
Fig. 10. Stolephorus punctifer: Length-weight relationship.
Fig. 11. Stolephorus heterolobus: Length-weight relationship.
Stolephorus heterolobus
|
log |
W = 3.1916 log LT - 2.3273 |
|
|
n = 228 |
|
|
r = 0.94.55 |
This species occurred in a low percentage between S 17º01’ and S 17°34’, in waters shallower than 13 meters.
S. indicus was present in one pelagic and two bottom trawl hauls. The total length of 140 individuals from the pelagic hauls was measured (Annex 1, Table 1). The size ranged from class 13.5 to 16.0 cm long. The modal size was at class 15.0-15.5 cm long and a mean value of 14.8 was estimated.
Length-weight relationship was studied and graphically represented in Fig. 12.
The results are as follows:
|
log |
W = 2.894 log LT - 1.985 |
|
|
n = 80 |
|
|
r = 0.8570 |
Fig. 12. Stolephorus indicus: Length-weight relationship.
Another species, not identified and referred to as species A occurred only in three bottom trawl hauls at depths lower than 20 meters, and in negligible quantities.
The total length of 267 individuals was measured (Annex 1, Table 1). The range of sizes was from 4.5 to 7.0 cm long with the estimated mean of 5.8 cm.
Length-weight relationship was studied and graphically represented in Fig. 13. The results are as follows:
|
log |
W = 3.092 log LT - 2.227 |
|
|
n = 293 |
|
|
r = 0.9752 |
