by
S.M. OVERKO and N.I. MYLNIKOV
(Atlant-NIRO, Kaliningrad, USSR)
SUMMARY
Different forms of Trachurus trecae from the area between Cap Blanc (20°54' N) to Bissagos Islands (11°21' N) were studied in order to detect the possible effects of the environment on the intraspecific variability. A number of differences in morphological characters has been evidenced between the northern and southern forms (in maximum depth of body, in the depth of the largest scute on the straight section of the lateral line, in antedorsal, anteventral and anteanal distances, in the length of the head and in gill raker number on the first gill arch).
T. trecae inhabits the area from 24 to 12°N as well to further South. The depth range of occurrence is 30-250m, the optimal temperature of near-bottom water is 14-18°C. Fish are distributed by sizes with depth: the fish of 13-19 cm occur at depths of 30-34 m; the length of T. trecae fluctuates insignificantly between 50 and 200 m, and the fish of 29-34 cm make up the bulk of the catches at depths of 200-250 m.
The species is characterized by early maturation. It is a partial spawner; the spawning season extends from October to April. The sex ratio is about 1:1. The minimum absolute fecundity of the individual of 32-42 cm in length is 151 thousand eggs, the maximum is about 773 thousand eggs. The relative fecundity extends from 347 to 951 eggs, with a mean of 663 eggs. The pre-spawning and spawning concentrations of T. trecae are found south of 19°N as a rule. The young inhabit the shelf zone from Cape Verde to Cap Blanc.
The growth rate; of this species is higher than that of other horse mackerel species in the same area. The annual ring formation on the otoliths, scales and rays of pectorals takes place in the spawning period.
This species feeds on organisms such as crustaceans (Mysidacea copepoda), fishes and squids.
One major area of concentration is the shelf of Mauritania (21°-16°N) (Fig. 1). In June-August and December-January the horse mackerel concentrations are centered in two sectors (Arguin Bank and 19°40-16°20N), while in February-March the species is distributed all over the shelf, however, with lesser density. The size structure differs markedly by season: in December-March one-year old fishes made up 90 percent of the samples (Fig. 2). The horse mackerel stocks off Mauritania are characterized by significant seasonal fluctuations of abundance and biomass (see table below). In December 1976-March 1977, in particular, the population number amounted to 759-1 832 million individuals and the biomass reached 26-53 10 tons, while in summer (July-August 1977) the population was 3 965 million individuals and the biomass 73 thousand tons.
Minimum biomass (103 tons) and
population
(10 individuals) of T. trecae in the
Mauritanian area
|
|
1972 |
1974 |
1975 |
1976 |
1976-77 |
1977 |
1977 |
|
Population Nbr. |
66.8 |
768.4 |
1 288.8 |
889.0 |
1 891.5 |
759.0 |
396.5 |
|
Biomass |
5.3 |
26.3 |
79.2 |
28.7 |
52.8 |
26.2 |
73.0 |
The shelf of Guinea-Bissau (10°-12°20N) is another area of high occurrence of T. trecae. The stock distribution does change markedly with seasons (Fig. 3). The bulk of the population is made up of 2 year old (15-17 cm) fishes. Appreciable fluctuations in biomass and number are observed. The indices are the lowest (25-30 million individuals and 1-2 thousand tons) in the feeding period. Then increase manyfold (225-350 million individuals and 7-13 thousand tons) in the pre-spawning period (January-March).
From time to time small concentrations of T. trecae were observed off Sierra Leone (December 1976) and Benin (April-May 1977). However, the biomasses were low.
Fig. 1 Distribution in space and time of concentrations of Trachurus trecae in Mauritanian waters
Fig. 2 Seasonal variations in size and age compositions of Trachurus trecae in Mauritanian waters
Fig. 3 Geographical distribution of concentrations of Trachurus trecae in December 1976 in Sierra Leone waters
