by
Hiroshi Hatanaka
Far Seas Fisheries Research Laboratory
1 000 Orido, Shimizu 424
Japan
Common octopus, Octopus vulgaris CUVIER, is an important target species for commercial trawlers operating off the northwestern coast of Africa where in recent years total annual catches for that species have fluctuated between 90 and 130 thousand tons.
Common octopus is encountered in most warm and temperate coastal waters in the northern and southern hemispheres. Many studies on the timing of spawning have been published for various regions (Rees and Lumby 1954; Mangold-Wirz, 1963; Guerra, 19755 Tanaka, 1958; 1967; Inoue, 1969; Itami, 1975). However, no observations on the spawning season off the northwestern coast of Africa have been reported as yet.
This report indicates the timing and duration of the season at which the Octopus spawns off the northwestern coast of Africa. This work is based on samples collected from the catches of Japanese trawlers.
MATERIAL AND METHODS
Since 1967 when the laboratory started regular investigations on the bottom fish resources in CECAF region about 1,000 specimens have been collected. The samples cover several years; they have been combined to analyse the annual cycle which is described in the present note.
The timing and duration of the spawning season were analyzed by observing the variations of sexual maturity and gonad indices. The maturity stages were determined by eye through the observation of the gonads and their accessory organs. The following scale was used for that purpose:
|
malea: |
immature: |
spermatophoric sac without spermatophores |
|
|
mature: |
spermatophoric sac with spermatophores |
|
females: |
immature: |
milky-white ovary |
|
|
maturing: |
yellowish white ovary |
|
|
mature: |
yellow and translucent ovary. |
where TL is the total length in mm and G the gonad weight in grains. Por facilitating the measurements, the Needham's sac and the oviduct were weighed together with the testis and the ovary, respectively.
RESULTS
Seasonal variations in the maturity condition. Fig. 1 shows the relation which exists for males between the total length and the percentage of mature individuals (all seasons combined). As the figure implies no male is mature before reaching the length of 50 cm in total length, whatever season is considered, all individuals this length seem to be able to mate at any season. On the other hand, the maturity condition of females greatly varies with seasons (Fig. 2). The highest proportions of mature females are observed in May. June and September. According to the seasonal variation in the proportion of mature females for length groups larger than 60 cm, these two peaks are found on all fishing grounds. The smallest size observed for a mature female was 37.9 cm (total length) and almost all specimens larger than 60 cm were mature in May-June and September.
Seasonal variations of the gonad indices. In order to investigate further the relationship between growth and maturation, the distributions maturity stages were analyzed in relation to the body length. For that analysis only the specimens collected off Cape Blanc in August-September for males and in September for females i.e., when the proportion of nature individuals is the highest, were used. For males the mean and the variance of the index decrease with size (Fig. 3). However, within the range 50-90 cm, the mean decreased only slightly. Therefore, the index means were calculated after having recombined all specimens between 50 and 90 cm.
No relation seems to exist between the total length of females and the value of their gonad index (Fig. 4). Index values are distributed in two groups, one of immature and one of nature animals. In relation to body length, females can be divided into three groups. The first group consisted of juveniles under 50 cm, the second one from 50 to 70 cm contains both mature and immature specimens and the third one is essentially made up of mature females larger than 7° cm.
Monthly variations of the male average index are shown in Fig. 5. Although the pattern is not clear, two peaks can be observed, one in April-May and one in August-September respectively.
Monthly variations of the female index are given by length groups in Fig. 6. As no difference could be detected between them, samples from the various fishing grounds have been combined in this figure. High index values are observed in May-June and September. This result is similar to the one obtained from the analysis of maturity conditions (Fig. 2). All these observations lead to the conclusion that there are two peaks in the annual spawning cycle.
DISCUSSION
Because the samples axe not large enough for some months and grounds it cannot be ascertained whether the two spawning peaks occur in all fishing areas. For the same reason, the year to year fluctuations could not be analyzed.
Mangold-Wirz (1963) pointed out that nesting females never go out of their nests and are therefore inaccessible to fishing. This observation would imply that the season would terminate at the beginning of the spawning season. In the Cape Blanc area where is the most important Japanese fishery for octopus, two fishing seasons, one in summer and one in winter respectively, are observed (Fig- 7) · In spring and autumn, where as we have seen. spawning probably takes place, the catch rate are markedly lower. This observation is in full agreement with the existence of two spawning seasons.
For males, no seasonal variation was observed in the maturity condition; the differences with body length of the gonad indices were disregarded from the analysis and the relation between the mating ability and the gonad index value is not known. However, an interesting observation on the seasonal variations of the gonad index can be made. The two peaks in the gonad index occur about one month earlier than those observed for females. It seems that the mating season precedes the spawning season as it has already been reported for some other cephalopod species.
In the Mediterranean, according to Mangold-Wirz (1963) and Guerra (1975), the spawning season of common octopus lasts from March to October with a peak from spring to early summer. Although no peak corresponding to an autumn spawning has been observed in the Mediterranean Sea the spawning season extends until autumn as off northwest Africa. In Japanese coastal waters, there are spring and autumn spawning seasons and the autumn spawners largely dominate the spring ones (Itami, 1975). This pattern is the opposite of the one described for the Mediterranean.
SUMMARY
A description of the spawning annual cycle of common octopus, Octopus vulgaris CUVIER in the waters along the northwestern coast of Africa has been drawn from an analysis of samples collected on Japanese commercial trawlers. The main features of the cycle are the following:
(1) Host of the males larger than 50 cm (total length) bear spermatophores all the year round. Almost all females larger than 60 cm are mature during the spawning seasons.(2) There are two spawning seasons, one in spring (May-June), and the other in autumn (September), at least in the Cape Blanc Region.
(3) In other fishing grounds, it is assumed that there are also two spawning seasons such as in the Cape Blanc sector, although the samples available are not large enough to ascertain this suggestion.
REFERENCES
Guerra, A. 1975: Determinación de las diferentes fases del desarrollo sexual de Octopus vulgaris Lamark, mediante un indice de madurez. Inv. Pesq., 39 (2), 397-416.
Inoue, K. 1969: Studies on the Propagation of the Octopus, Octopus vulgaris CUVIER. Suisan Zoyoshoku Sosho, 20, Nihon Suisanshigen Hogo Kyokai.
Itami, K. 1975: Shigen Baiyogiho no Kaihatsuo Chushin toshita Setonaikaigata Tako Gyogyo. Sekaino Ika-Tako Shigen no Kaihatsu to sonó Riyo, Shiryo (5), 179 - 183 Kaiyosuisan Shigen Kaihatsu Center.
Mangold-Wirtz, K. 1963: Biologie des céphalopodes benthiques et nectoniques de la mer catalane. Vie et Milieu, Suppl. 13, 285pp.
Rees, W.I. and J.R. Lumby 1954.: The abundance of octopus in the English Channel. J. Mar. Biol. Ass., 33 (2), 515-536.
Tanaka, J. 1958: On the Stock of Octopus (Octopus) vulgaris LAMARCK on the East Coast of Boso Peninsula, Japan, Bull. Jap. Soc. Sci. Fish, 24 (8).
Tanaka, J. 1567: Yogyogaku Kakuron, 6. Tako. Suisangaku Zenshu, 32, 685-692, Koseisha-Koseikaku, Tokyo.
Table 1 Size and origin of samples used in this report
Fig. 1. Percentage of males with spermatophores in the spermatophoric sac.
Fig. 3. - Frequency distributions of the gonad index by size groups for males collected in the Cape Blanc area from August to September (N: number of specimens, M: mean, V: variance).
Fig. 4. - Frequency distributions of the gonad index and maturity condition by size groups for females collected in the Cape Blanc area in September. Same legend as in Fig. 2.
Fig. 5. - Monthly variations of the mean gonad index for males (50-90 cm TL).
Fig. 6. - Seasonal variations of the gonad index for females and by size group (all fishing grounds combined); same legend as in Fig. 2.
