Appendix 5. Spawning season of the cuttlefish, Sepia officinalis officinalis LINNE, off the northwestern coast of Africa

by

Hiroshi Hatanaka

INTRODUCTION

The cuttlefish, Sepia officinalis officinalis LIME, is a main target species for Japanese trawlers operating off the northwestern coast of Africa (Hatanaka, 1978).

There is little information on the spawning season of this subspecies. Cabrera (1970) observed large quantities of mature cuttlefish from July to August. Ikeda et al (1976) found the highest concentrations of egg clusters from May to September in waters shallower than 20 fathoms. However, the timing and duration of the spawning season is still poorly known. In this note, the author attempts to determine the spawning season of Sepia officinalis officinalis LINNE off the northwestern coast of Africa, using for that purpose samples collected from Japanese trawlers.

MATERIAL AND METHODS

Since 1967, when the laboratory started regular investigations on bottom fish resources of CECAP waters about 3,300 specimens have been collected from Japanese research and commercial trawlers.

For determining the spawning cycle, the seasonal variations in sexual maturity and gonad indices were analyzed. The maturity stages were determined by visual observation of the gonads and accessory organs. The criteria used were the following:

males	immature:	spermatophoric sac without spermatophores
	maturing:	spermatophoric sac with some spermatophores
	mature:	spermatophoric sac filled with spermatophores
	spent:	after release of spermatophores
females	immature:	no egg visible in the ovaries
	maturing:	milletlike eggs in the ovaries
	mature:	yellow and transparent eggs in the ovaries, reddish accessory nidamental glands
	spent:	after release of eggs.

The following equation was used for calculating the gonad index. For males, the testis index had to be calculated without taking into account the weight of the Needham's sac because in the samples available the measurement of the sac mass has not been made in all cases. For females, indices were calculated for ovary and nidamental glands separately. All indices were expressed by the following equation:

where ML is the mantle length in mm and G the gonad weight in grams.

RESULTS

Seasonal variations in the maturity condition. In order to clarify the relation between growth and maturation, the relationship between the maturity condition and the body length was examined, using for that purpose specimens collected in waters shallower than 39 meters and from May to August, where mature specimens were predominant (Fig. 1). The proportion of mature males decreases for specimens smaller than 12 cm (mantle length) caught in the Cape Garnett - Cape Blanc area and for individuals smaller than 14 cm in samples coming from the Nouakchott fishing ground. The rate of mature females decreases for specimens smaller than 14 cm collected in both fishing areas. Therefore, in the following analyses, the author considers only individuals larger than the lengths at first marurity given above.

The frequencies of each maturity stage were calculated by depths and by cuarters (Table 2 and 3) in order to take into account the observations made by Ikeda et al (1976) and according to which the egg clusters of cuttlefishes were more abundant in shallower waters. The results show that, although mature specimens of both sexes are found at all seasons, their proportion is higher from April to September and this is more evident for females. By depths, nature specimens are more frequent in waters shallower than 40-50 meters; in general the proportion of mature specimens decreases with depth.

For the females collected in waters shallower than 39 meters, the proportion of mature specimens reaches about 90 % from April to September, and decreases to about 50 % from October to March (Table 4).

No spent specimen was ever observed in the material examined.

Seasonal variations in the gonad indices. Gonad indices calculated on a 2 months basis for males larger than 12 cm (14 cm for the Nouakchott samples) collected in the waters shallower than 39 meters, are given in Fig. 2. The index increases from winter to spring. High values are only found from June to July and disappear from October to November, whereas low values are observed in August to September.

The seasonal variations of the ovary index were examined after having eliminated the individuals smaller than the size at first maturity (Table 4) regardless of the depth of capture. The sane patterns are observed with both the ovary and the nidamental gland indices; no difference is observed between fishing grounds (Fig. 3). The indices increase from autumn to spring, and most females have a high index from June to September; then the indices decline in October. This suggests that from June to September almost all females are completely mature, whereas some of the individuals identified as mature in other seasons could be not completly so.

DISCUSSION

The material available did not permit an analysis of the year to year variations. Also samples viere scarce for some months, especially in the Nouakchott sector. However, the timing and duration of the spawning season off the northwestern coast of Africa are now better determined. If spawning may occur all the year round, there is a distinct season of more intensive reproduction from may to September.

This conclusion is in agreement with the results given by Ikeda et al (1976) and by Cabrera (1970). These are also similar to the observations, reported by Mangold-Wirz (1963) regarding the spawning season of the same species in the Mediterranean Sea.

SUMMARY

The spawning season of cuttlefish, Sepia officinalis officinalis LINNE, off the northwestern coast of Africa has been determined using material collected from Japanese trawlers. The results obtained are as follows:

(1) The size at which most cuttlefishes mature is 12-14 cm for males and 14 cm for females (mantle length).

(2) Spawning takes place all the year round, but there is a distinctly more intensive season from Hay to September.

(3) Spawning occurs mainly in waters shallower than about 40 meters.

REFERENCES

Cabrera, R.C.G. 1970: Espèces du genre Sepia du Sahara espagnol. Rapp. P. -v. Réun. Cons, int. Explor. Mer. 159, 132-139.

Hatanaka, H. 1978: Geographical distribution of two subspecies of Sepia officinalis LIME in the northwestern coast of Africa. Appendix 4, present report.

Ikeda, I. et al 1976: Observations on Egg Cluster of Sepia spp. Caught by commercial Trawlers in the CECAF Waters. FAO Fish, Rep. 183, 65-71.

Mangold-Wirz. K- 1963: Biologie des céphalopodes benthiques de la mer catalane. Vie et Milieu, Suppl. 13. 285 pp.

Table 1 Size and origin of samples, used in this report

Table 2 Maturity stages of adult males by seasons and by depths

Table 3 Maturity stages of adult females by seasons and by depths

Table 4

Seasonal variations of maturity stages for adult males collected in fishing grounds shallower than 35 meters

Sectors	Dec. - Jan.			Feb. - mar.			Apr. - May			June - July			Aug. - Sept			Oct. - Nov
	imm	ing	mat	imm	ing	mat	imm	ing	at	imm	ing	mat	imm	ing	mat	imm	ing	mat
Gape Garnett-Cape Barbas	2	3	1	17	1	16	-	2	16	-	-	10	6	1	27	26	5	40
Cape Blanc	1	5	11	4	10	27	-	-	-	-	-	13	-	-	6	24	12	33
Nouakchott	-	-	-	-	-	-	-	-	-	3	1	35	-	-	-	16	4	4
Total	3	8	12	21	11	43	-	2	16	3	1	58	6	1	33	66	21	77
(%)	13	35	52	28	15	57	-	11	89	5	2	94	15	3	83	40	13	47

imm: immature, ing: maturing, mat: mature.

Fig. 2 Seasonal variations of the gonad (testis) index for adult males collected at depths shallower than 39 meters.

Fig. 3-A Seasonal variations of the gonad (ovary) index for mature females.

Fig. 3-B Seasonal variations in the nidamental gland index for mature females.

Fig. 3-C Seasonal variations of the ovary + nidamental gland index for mature females