6. BIOLOGY OF THE SPECIES

6.1 Senegal-Mauritanian Zone
6.2 Sherbro Division

Differences in point of view sometimes arise as to the biology of the species, especially regarding migration, distribution of stocks, and growth; the various results were presented. Regarding bathymetric distribution it is unanimously considered that the species concerned have one important common characteristic for the management of the resources, namely, that the biotope of young fish is situated in the shallow coastal waters. Stratification of size groups in relation to the depth of the bottom above which the specimens are found shows that during the growing period the preferential biotope shifts from the shore to the steep drop of the continental shelf (Fig. 7, Annex 4).

6.1 Senegal-Mauritanian Zone

6.1.1 Sardinella aurita
6.1.2 Sardinella maderensis
6.1.3 Trachurus trachurus
6.1.4 Trachurus trecae
6.1.5 Caranx rhonchus
6.1.6 Scomber japonicus
6.1.7 Sardina pilchardus

6.1.1 Sardinella aurita

Reproduction: According to the work at CRODT, the sexual cycle of Sardinella aurita in the Senegal-Mauritanian region presents the following characteristics (Boëly et al., 1978):

- Considering the zone as a whole, spawning spreads throughout the year with a maximum reproduction period between June and September (Fig. 1, Annex 4);

- The main spawning area extends over the continental shelf from Gambia to Cape Blanc. Spawning does not take place at the same time throughout the whole area; there is a shifting focal point of maximum reproduction which is situated at the level of Gambia in May and reaches Cape Blanc in September (F. Conand, 1978). These observations are in accordance with the work of Sedletskaïa (Annex 12, Figs. 1 to 4);

- In a given region spawning is quite widespread with two maximum periods, one main and one secondary, separated by a period of sexual quiesence;

- Larvae are found across the full width of the continental shelf. As the embryos develop they reach the coast concentrating in very shallow waters leading to the concentration in two nurseries, of which one is situated on the Petite Cote of Senegal from Dakar to the north of Gambia while the other, less well known, is in the region of Cape Timiris and the Arguin Bank off Mauritania;

- At these sites the young sardinella develop until they reach a size of 20-25 cm, when they are just over one year old. As they grow they move away slightly from the coast preferring to live in depths of 20 m.

The various length/weight ratios recently calculated in the region are given below, where LF = fork length, LT = total length, and P = weight:

P = .00692	LF3.274	(Fréon et al. (Annex 12))
P = .01648	LT2.8386	(Pham-Thuoc and Szypula (1973))

Growth has been the object of study by several authors. Results have varied widely and fall into two groups, one presumes very fast growth, the other a slow growth:

a) Biester and Bu Dinh Chung (1975): (equation not published)
b) Boëly, Fréon and Stequert (pers. comm.) LF_t (mm) = 312.3 [1 - e^{-0.97 (t + 0.21)}]
c) Pham-Thuoc and Szypula (1973): LT_t (cm) = 40.7 [1 - e^{-0.326 (t + 0.6283)}]

Age (years)	a (LT)	b (LF)	c (LT)
1	15.8	21.7	16.7
2	21.7	27.6	22.9
3	26.6	29.9	27.8
4	30.4	30.7	31.4
5	33.2	31.0	34.1
6	35.0	31.2	36.1
7	36.3		37.3
8	37.5
9	38.6
10	39.6

Feeding: The diet is based on plankton, phytoplankton especially for the young, zooplankton - (copepods and euphausids) as they grow older (Pham-Thuoc and Szypula, 1973; Nieland, 1976). This last author has shown the presence in the stomach of large quantities of detritus, the abundance of which seems difficult to understand.

Mortality: There are no available data on mortality.

Migration: The migratory cycle of the majority of Sardinella aurita adults may be summarized as follows: at the end of the hot season and the beginning of the cooling of the water (from October to January) there is a period of scattering in the open sea north of Mauritania (Fig. 1, Annex 4); a phase of rapid migration toward the south in February; a pre-spawning concentration phase in March-April in the southernmost part of the habitat (Guinea-Bissau and southern Senegal); phase of northward migration as far as 24°N to 25°N, with spawning, from May to September (Boëly et al., 1978).

The life cycle of this species seems to be highly dependent upon hydroclimatic variations (Fréon et al., Annex 3; Rébert, Annex 11).

6.1.2 Sardinella maderensis

Reproduction has only been studied on the Senegalese coast (Conand and Fagetti, 1971; Boëly, 1978). It takes place throughout the year with a peak at the beginning of the hot season (May to July).

Fecundity has not been studied. The length/weight ratio has been calculated on the basis of samples taken in the southern coast of Senegal (Fréon et al., Annex 13).

P(g) = .01034

LF3.142 (cm)

The only information on growth covers the young individuals along the Senegalese coast; growth is fast reaching a length of about 18 cm at one year. Feeding has been studied by Nieland (1976) also showing that the amount of detritus in stomach contents largely exceeds that of phytoplankton and zooplankton. No data are available on mortality. Rapid movements along the coast have been observed within the southern nursery, related to temperature variations. No cases of widescale migrations are known (Boëly et al., Annex 4, Fig. 2).

6.1.3 Trachurus trachurus

Reproduction: The differences in reproduction for the two stocks identified in the Senegal-Mauritanian zone are summed up in the following table:

Area of population distribution	Spawning area	Depth (m)	Spawning spread (months)	Spawning peak (month)	Av. length first maturity (L 50) (cm)	Optimum spawning temp. (C)
19° - 25°N	20° - 21°N (main) 18°30 - 19°30N (secondary)	110-300	IX - IV	XII	21	15° - 16°C
14° - 17°N	14° - 16°N	110-300	XII - V	I	16	15° - 16°C

Fecundity: Average fecundity for the two stocks is of the same order (115 000 and 127 000) (Overko and Mylnikov, Annex 5).

Length/weight ratio: The following equations have been established:

P = .01 LT3.03	(Overko and Mylnikov, pers. comm.)
P = .0049 LT3.14	(Wengrzyn, 1975)

Growth: Determined on the basis of number of scale annuli which appear to form during spawning (a: Mylnikov and Overko, pers. comm.) and examination of otoliths, (b: Wengrzyn, 1975):

a) LF_t = 37.98 cm (1 - e^{-0.25 (t + 0.98)}) Cape Blanc/Cape Bojador
b) LT_t = 50 cm [1 - e^{-0.13 (t + 2.32)}]

Mortality: No information.

Migration: According to Overko and Mylnikov (Annex 5) predominant migrations are from the coast toward the open sea. In the hot season fish concentrate in the open sea off the continental shelf, and in depth in two zones of concentration, each corresponding to one stock. In the cold season fish are found near the coast occupying wider areas which partially overlap (Annex 5, Fig. 1).

According to CRODT studies there also seem to be large scale migrations parallel to the coast (Domain, 1976; Boëly et al., Annex 4, Fig. 5).

6.1.4 Trachurus trecae

Reproduction: Three reproduction zones with certain different characteristics have been described. The information is summed up in the following table (Sedletskaïa, 1978):

Area of population distribution	Spawning area	Depth (m)	Spawning spread (months)	Spawning peak (months)	Av. length first maturity (cm)	Optimum spawning temp. (°C)
20° - 21°N	21° - 22°N	100-150	IX - IV	II - IV	31.5	18°5-25°5C
15° - 19°N	15° - 19°N	100-150	IX - IV	II - IV	16.8	18°5-25°5C
Not determined	11°30 - 14°	100-150	IX - IV	II - IV	12.5	17°5-25°5C

Observations made by CRODT and on board Polish vessels show that a peak of spawning moves from February to May along the coast between 13°N and 19°N (Boëly et al., 1973).

Fecundity has been calculated for individuals 32-42 cm (Overko and Mylnikov, Annex 7):

- minimum fecundity: 151 000 ova/spawning
- maximum fecundity: 772 000 ova/spawning

P(g) = .01489 LF2.954 (cm)	(Fréon et al., Annex 13)
P(g) = 0.01 LF2.98 (cm)	(Mylnikov (pers. comm.))

Growth: Growth studies on this species have presented some difficulties, especially when using methods based on otolith and scale readings. Examination of the pectoral fin lines give the following figures (Mylnikov, pers. comm.):

Age (years)	Size (cm LF)
	Cape Blanc-Saint Louis	Gambia-Bissagos
1	14.7	13.0
2	21.3	18.8
3	27.8	25.0
4	31.6	29.6
5	35.0	33.5
6	38.4	36.8
7	40.6

Feeding: Based on euphausids with an addition of Calanoides and cyclopods. After reaching the length of 24 cm they become ichthyoplanktonophagous: 14 percent of their food is made up of Engraulidae, Myctophidae and young Brotula barbata (Overko and Mylnikov, Annex 7; Nieland, 1976).

Mortality: No information available.

Migration: Different hypotheses have been advanced on the migrations of this species and the number of its populations; the existence of several more or less independent populations (Overko and Mylnikov, Annex 6, Fig. 1); and the existence of a single population making very extensive migrations (Domain, 1976; Boëly et al., Annex 4, Fig. 4).

6.1.5 Caranx rhonchus

Work on this species has been carried out by research scientists of the Atlant-NIRO, the RIBASKY Institute and of CRODT.

Reproduction: Three areas of intensive spawning have been defined: the first, between 9°30N and 9°45N where spawning is very active in July (Overko, Annex 14); the second in Senegal from April to November with a peak period in May-June (Conand and Franqueville, 1973); the third near Cape Timiris (Wysokinsky, pers. comm.). In the Senegal nursery juveniles are caught with beach seines, mainly in the hot season, while the young are caught by the sardine fishing vessels in the cold season. Water temperature at spawning time may vary widely, between 15°C and 29°C.

Fecundity: The works of Overko (Annex 14) show an absolute fecundity of between 480 000 to 990 000 ova/spawning, with an average fecundity of 680 000 for specimens of between 29 and 35 cm (LF).

Length/weight ratio: The following equations have been obtained by different authors:

P(g) = 0.02 LT2.88 (cm)	(Overko (pers. comm.))
Log P(g) = .006521 LT3.097 (cm)	(Wysokinski (pers. comm.))
P(g) = .01243 LF3.055 (cm)	(Fréon et al. Annex 13)

Growth: Otolith examination has shown the following results:

a) LT_t = 48.57 cm (1 - e^{-0.16 (t + 0.80)}) cape Blanc/Cape Verde (Overko (pers. comm.))
b) LT_t = 45.3 cm [l - e^{-0.303 (t + 0.515)}] (Wysokinski (pers. comm.))

Feeding: The species has a very wide range of diet, becoming ichthyophagous when they have reached 25 cm, which explains the large catches by Senegalese artisanal fishermen using baited hook and lines.

Mortality: Not studied.

Migration: Soviet work (Overko, Annex 14, Table 1) shows wide biometric differences between individuals from the Mauritanian zone and those found in Guinea. Although there is not much knowledge as to their migration, it is safe to think that if the two populations of the northern zone (already defined) can intermingle, those of the southern zone seem to keep separate. Polish results (Wysokinski, pers. comm.) on biometry following in the same sense show that the parameters vary progressively from north to south.

CRODT studies describe the occurrence of migrations of adults in the Senegal-Mauritanian zone (Domain, 1976; Boëly et al., Annex 4, Fig. 4).

6.1.6 Scomber japonicus

Most of the information on this species is derived from the work of Atlant-NIRO.

Reproduction: Soviet catches rarely find this species in the spawning stage, therefore less is known about its reproduction. Present knowledge is given in the following table (Sedletskaïa, 1978):

Area of population distribution	Spawning area	Spawning spread (months)	Spawning peak (month)	Av. length first maturity (L 50) (cm)	Optimum spawning temp. (°C)
20° - 31°N	30° - 31°N	I - III	-	31.0	17.2° - 22°C
	24° - 25°30N
	22°30
11° - 19°N	15° - 10°N	XII - II	XII	27.5	17.2° - 22°C

Several hypotheses have been advanced as to the depth at which spawning takes place. According to certain authors it is in the upper layer of the coastal zone, while some believe reproduction takes place beyond the continental shelf, toward the open sea. Still others think that spawning takes place in the surroundings of Cape Verde close to shore.

CRODT observations confirm the presence of sexually mature individuals at the beginning of the cold season (December-January) and concentrations of larval fish have been found there in January 1968 as well as outside the Casamance, at the level of the break in the continental shelf (Conand, 1970). Juveniles are found near the shore in April-May at the level of the Cape Verde peninsula. In addition a strong age class (mode 18-23 cm) has been observed in the hot season (1974) at the drop of the continental shelf, south of the Cape Verde peninsula. It may be supposed that this corresponds with the heavy catches of sardine fishermen from Dakar in the cold season in January to April 1975.

Fecundity: Not studied.

Length/weight ratio: The following equation was obtained by CRODT (Fréon et al., Annex 13):

P(g) = .004871 LF^3.278 (cm)

a) LF_t = 44.08 (1 - e^{-0.326 (t + 0.834)}) (Krivospitchenco (Annex 15))
b) LF_t = 55.357 (1 - e^{-0.118 (t + 3.178)}) (Staïcu and Maxim (1974))

Age	a	b
(years)	LF (cm)	LF (cm)
1	20.37 ± 0.13	-
2	26.35 ± 0.12	-
3	31.7 ± 0.20	28.68
4	35.2 ± 0.23	31.62
5	37.4 ± 0.32	34.30
6	38.8 ± 0.51	36.66
7		38.76
8		40.59
9		42.25
10		43.69

CRODT observations on cohorts of young individuals agree with the value of 20 cm at age one year.

Feeding: Feeding consists mainly of copepods and larval Engraulidae among individuals between 17 and 24 cm (Nieland, 1976).

Mortality: Not studied.

Migration: Little is known and available information from CRODT appear in Fig. 7 (Boëly et al., Annex 4).

6.1.7 Sardina pilchardus

This species was studied by a working group which met recently (FAO, 1977) so the essential points of its biology will not be repeated here. The most remarkable point is the southern expansion of the area of geographic distribution (Domanevsky and Barkova, Annex 8, Fig. 2), which is attributed to an intensification of the upwelling.

In addition it seems that the entire biology of this species is highly dependent on environmental conditions. In particular, variations in size and growth rate at first sexual maturity have been correlated with changes in upwelling intensity (Annex 8, Figs. 3 and 4).

6.2 Sherbro Division

Studies are much less advanced in this region, although Soviet work has recently begun. The only published work is that from the FAO/UNDP Project (1973) on sardinella in 1970/71. S. maderensis seems to be abundant and probably constitutes a single stock running from southern Guinea to Liberia. Individuals of 15-18 cm are caught throughout the year and larger ones (21 cm) in February and March. Eggs and larvae have been found in the open sea off Freetown from February to April. Concentrations of S. aurita (LT 17-21 cm) have been observed from January to May in the deepest part of Sierra Leone's continental shelf. During this period they seem to leave this sector moving toward the north.

There is almost no information on the biology of the other species.