No new result was given. In fact the present standard recommended by the Committee of measuring octopus by mantle length is now applied correctly, and therefore the recommendations made in previous Working Groups remain applicable.
This ratio was calculated for octopus, cuttlefish and squid in the northern sector (Dakhla stock) in each of the Hispano-Moroccan campaigns. The results are given in Table 8 and on Figure 1. Considerable seasonal and geographic variations of parameters a and b of the length/weight ratio in octopus were found, related to variations of the conditions factor which itself was linked, among other things, to spawning. Results on the length/ weight ratio of Dakhla octopus were presented by Pereiro (1980). These results, obtained on a sample taken from a very small space-time stratum, showed a wide variability of the length/weight ratio, even in a particular stratum.
The Group recommended that the inter-zonal and inter-seasonal variability of different sizes of octopus captured during the Hispano-Moroccan cruises be analysed.
An average size/weight ratio was obtained for Senegalese cuttlefish stock:
Males p = 0.24 L2.74 (from 5 to 44 cm, n = 612)
Females p = 0.26 L2.75 (from 7 to 37 cm, n = 471)
p = fresh weight in grammes.
All cephalopod statistics were given in weight after unfreezing, since the individuals were gutted before freezing. The conversion factors between fresh and unfrozen weight have not been determined. The relation between live and gutted weight was calculated by Hatanaka (1979) and remains valid.
The Group therefore recommended that these factors be determined and that, while waiting for the result, it be borne in mind that the production statistics concern gutted individuals.
No new datum on cephalopod fertility was obtained in the northern sector.
The Group recommended that fertility studies be undertaken during scientific campaigns in the Dakhla and Cape Blanc regions (and also studies on size at first maturity).
In the Senegal area, a cuttlefish fertility study produced the following result:
F = 2.59 L1.77
(13 to 35 cm mantle length)
The L50 size is 13 cm
A female of 13 cm spawns 250 eggs, one of 35 cm spawns 1 400
(or 0.52 eggs/g weight).
During the Moroccan-Spanish campaigns new results were obtained for cuttlefish, octopus and squid. These three results are given in Tables 9 and 11. They confirm the classic result of progressive disappearance of females among large-size individuals (Figure 2). A slight predominance of females was found in octopus of less than 10 cm. A seasonal, geographic and bathymetric variability of sex ratio according to size was also observed. The comparison of these results obtained in 1980 with those of Cort and Pérez Gándaras (1973), obtained in 1972 for octopus, revealed a considerable difference in sex ratio by age, large females being considerably fewer in the 1972 campaigns than in those of 1980. It is difficult to interpret this difference. It may be related to the stock exploitation level which varied during the period. It can also be explained more simply by the heterogeneousness of the space-time size and sex distribution, due to which the results of these two series of campaigns cannot be directly compared and are not representative of the stock's size structure.
Numerous results concerning octopus and squid were presented by Morocco (Appendix 9) and for octopus by Spain (Ariz and Fernández, 1980). These studies referred to the Dakhla stock. These similar results were obtained with a Spanish type 60 mm mesh trawl, a gear in use in present fleets. The hauls of 30 mm1 duration are shorter than the hauls of the professional fishermen which generally have a duration of several hours. The results are shown in Appendix 9.
A fairly high seasonal variability of selectivity was observed; in all cases the results, covering a large number of trawl hauls, were significantly different from earlier results presented to the Working Group (Guerra, 1979).
The Group discussed the best method to be used for determining the selectivity curve. Due to the marked asymmetry of the selection curves for octopus, the Group considered that an adjustment “by sight” of a selectivity curve to all the points observed may be preferable to a logistical adjustment or an adjustment by the Gulland method.
The Group recommended that a study be conducted with longer trawl hauls which would therefore be more representative of the conditions of the fisheries and of trawls of different mesh sizes so as to analyse the selectivity of meshes larger than 60 mm.
The working hypothesis adopted previously for octopus in the northern sector, accepting the presence of two separate stocks in the Dakhla region and in that of Cape Blanc, was retained by the Working Group since no new datum had emerged to contradict it. Concerning cuttlefish in the southern sector the data presented indicate the presence of a single stock from Casamance to Cape Verde (Appendix 10).
The octopus size frequency data obtained during Moroccan-Spanish research campaigns did not justify making new growth estimates at present. However, these results seemed to confirm earlier estimates. Perhaps the results of future cruises could improve the information in this field, especially through inclusion of the class recruited in November 1981 which seems to be very abundant. The earlier hypotheses on growth presented at previous Working Groups were therefore maintained.
A new growth estimate of male cuttlefish from 0 to 25 cm in the Senegal area was obtained by the Petersen method (Appendix 10). Growth was estimated at about 2 cm per month.
The octopus size frequency data obtained by the Spanish fleet from 1976 to 1980 could enable us to estimate mortality if we were to change catch by size structures into catch by age. This method could be applied mainly during the 1976 to 1979 period during which the Spanish trawlers constituted the main fleet exploiting the Dakhla octopus stock. The calculation was made by the cohort analysis method proposed by Jones (Section 5.2). The Z calculated this way fluctuated between 2.33 and 2.96. Catch by age curves can also be obtained from research vessel catches which makes it possible to estimate to some extent the age structure of the underlying population and therefore the apparent total mortality rate (Z = F + M). These catch vectors are represented on a logarithmic scale in Figure 3. The annual Zs calculated vary between 1.50 (Garcia and Cabrera, 1968) and 2.84 (Pereiro and Bravo de Laguna, 1978). They were estimated at the intermediate level in 1980 (Z = 1.88) and in 1981 (Z = 2.32). In fact the figure calculated during the 1980 cruise can be compared to other values because the Z calculated in that year was based on the data from both Cape Blanc and Dakhla, areas in which the exploitation rates seemed to be very different (Section 5).
No new information concerning natural cephalopod mortality was submitted to the Working Group. Among cephalopods sex ratio by size generally reflects a disappearance of females among large-sized individuals. For cuttlefish and squid various observations, in nature and in nurseries, indicate massive mortality of large females after spawning. As far as octopus are concerned, proofs of such mortality due to spawning seem to be tenuous and this hypothesis would have to be confirmed (or invalidated). The natural mortality rate accepted for octopus by the Working Group is close to 0.5 (or above that value). Such a natural mortality level, much higher than the M = 0.2 estimate maintained by Hatanaka (1979), is certainly logically acceptable in the case of a short lifespan (even in the case of males) and rapid growth specie like octopus. The ratio between total Z (calculated for the five years 1967, 1972, 1976, 1978 and 1980 during research campaigns) and estimated fishing effort during the same years would suggest an M value of the order of one year. This estimate, although of an acceptable order of magnitude, is nevertheless doubtful due to:
the nature of the data and the method of calculating Z (calculation not stratified by zone, variable coverage in time and space, etc.);
high variance in the aspects observed (Figure 4) and, in particular, the fact that the estimate of M is based mainly on an estimate of Z calculated for 1967, this point being the only value located on the left side of the point cluster, thereby in itself practically determining the intercept of the relation with the mortalities axis and therefore with natural mortality.
