by
S.M. OVERKO
(Atlant-NIRO, Kaliningrad, USSR)
SUMMARY
The first description of Caranx rhonchus was made by Geoffrey Saint-Hilaire in 1809. For a long time many authors considered that this species belonged to the genus Decapterus and its scientific name was D. rhonchus. Later on, however, it was approved that the representatives of genera Decapterus and Caranx differ in a series of morphological features.
C. rhonchus inhabits the area between 22°30' N and 8°N. Small numbers of the species may be found southward of this area. Morphological and biometrical differences have been observed between specimens from the south and from the north (Table 1). The depth range of occurrence is 15-200 m. Optimal temperatures where dense near-bottom concentrations observed are 15-22°C.
The sexual maturity is reached during the second year of life. Absolute fecundity of females of 29-35 cm in length varies between 480-990 thousand eggs. Spawning occurs from April to July. In July it has been observed between 9°30N and 9°45N (30 to 50 m) and in April near Dakar (10 m). The spawning grounds are located in shallow waters. C. rhonchus is a partial spawner. Spawning takes place at a water temperatures of 18.7°C. The young of C. rhonchus inhabit the inshore shallow water, south of 19°N.
The growth rate of C. rhonchus is relatively high. Lengths are: 12.2 cm at age one year, 17.9 at two, 22.3 at three, 26.0 at four and 29.7 at five years old.
C. rhonchus is a predator. Euphausiids, shrimps, squids and fish are its main prey. Food composition markedly changes with age. The euphausiids, larvae and fry of fish make up the bulk of the food intake of individuals of 21-24 cm. The individuals larger than 25 cm become typical ichthyophages (Figure 1).
The trawling fishery essentially exploits fish older than 4.5 years. In 1969 and 1970, on the shelf of Senegal and Mauritania and off the Sahara coast, respectively, the catches primarily consisted of fish of 5 years and older. In 1970, however, one year old C. rhonchus were abundant (about 65 percent) in the catches made south of Cape Verde (Table 2).
Table 1
CATCH COMPOSITION OF CARANX RHONCHUS (IN
PERCENTAGE BY LENGTH CLASSES)
|
Area |
Year |
Length classes (cm) |
total number |
|||||||
|
6-10 |
10-15 |
16-20 |
21-25 |
26-30 |
31-35 |
36-40 |
41-45 |
|||
|
20-25°N
|
1968 |
|
|
|
2.0 |
48.1 |
43.4 |
6.4 |
0.1 |
1 709 |
|
1969 |
|
|
1.8 |
30.2 |
61.2 |
6.8 |
|
|
595 |
|
|
1970 |
|
|
|
|
1.5 |
73.5 |
25.0 |
|
201 |
|
|
15-20°N
|
1968 |
|
|
|
|
46.1 |
51.6 |
2.3 |
|
641 |
|
1969 |
|
|
|
|
3.5 |
81.9 |
14.6 |
|
200 |
|
|
1970 |
|
|
|
5.4 |
52.5 |
35.9 |
5.2 |
|
7 035 |
|
|
12-15°N
|
1968 |
|
|
|
0.1 |
37.7 |
61.8 |
0.4 |
|
1 300 |
|
1970 |
55.0 |
4.9 |
0.7 |
2.0 |
21.3 |
9.7 |
1.0 |
|
582 |
|
Table 2
BIOMETRIC DIFFERENCES OBSERVED BETWEEN CARANX
RHONCHUS CAUGHT IN THE NORTH AND IN THE SOUTH OF THE SENEGAL MAURITANIA
AREA
|
Biometric characters |
11°01N |
19°27N |
||
|
Range |
Mean and |
Range |
Mean and |
|
|
Fork length (cm) |
27.0 - 34.8 |
29.95 - 0.36 |
27.0 - 30.9 |
28.65 ± 0.13 |
|
Body height (cm) |
22.6 - 26.6 |
23.95 ± 0.21 |
21.4 - 25.1 |
23.39 ± 0.13 |
|
Head length (cm) |
24.6 - 28.1 |
26.11 ± 0.19 |
25.3 - 27.9 |
26.47 ± 0.08 |
|
Pre-anterior dorsal length (cm) |
30.4 - 32.6 |
31.38 ± 0.14 |
29.6 - 32.1 |
31.09 ± 0.08 |
|
Preanal length (cm) |
51.6 - 55.5 |
53.67 ± 0.21 |
49.5 - 53.2 |
51.22 ± 0.11 |
|
Preventral length (cm) |
26.0 - 29.3 |
27.67 ± 0.21 |
24.5 - 28.6 |
26.65 ± 0.10 |
|
Eye diameter/head length (%) |
22.2 - 26.7 |
24.19 ± 0.23 |
21.0 - 25.4 |
23.49 ± 0.15 |
|
Number of scutes on the lateral line |
76 - 87 |
82.84 ± 0.45 |
70 - 84 |
76.68 ± 0.51 |
|
Number of rays in the second dorsal fin |
31 - 36 |
32.80 ± 0.25 |
30 - 34 |
32.31 ± 0.16 |
|
Number of rays in the anal fin |
27 - 31 |
29.52 ± 0.27 |
26 - 31 |
28.67 ± 0.16 |
|
Number of branchiospines |
50 - 58 |
53.70 ± 1.70 |
53 - 59 |
55.41 ± 0.23 |
