3. POPULATION AND BIOLOGICAL PARAMETERS

3.1 Growth of octopus
3.2 Mortalities
3.3 Selectivity and mesh size assessments (octopus and cuttlefishes)
3.4 Length measurement for stock assessment purposes

The Group reviewed the scientific information already published or made available at the meeting on the main biological functions of the three groups of cephalopods, namely octopus, cuttlefishes and squids. This is presented in a synoptic form in table 2. This table does not contain data on squids: this reflects the present poor knowledge on this species group. Sane observations on the distribution, population structure and spawning of cuttlefishes have been presented to the meeting (Appendices 4, 5 and 10), but the information available at the meeting on that species group remains too fragmentary to justify an overall review. Actually most of the investigations conducted on the biology and dynamics of the region's cephalopods have been concentrated on octopus. This situation explains why the following section essentially deals with that species.

3.1 Growth of octopus

Growth data were presented to the meeting by Messrs. Guerra and Hatanaka. Their growth curves were compared between each other as well as to the preliminary results of tagging experiments conducted by the Spanish Institute of Oceanography (Santa Cruz Laboratory).

Attempts to use hard anatomic structures to determine the age of octopus have all failed so far. Therefore, both Mr. Guerra and Mr. Hatanaka have used the Petersen method and the analysis of the length frequency polymodal distributions. The results obtained by Mr. Guerra are presented in Appendix 10; those of Mr. Hatanaka are unpublished as yet.

Mr. Hatanaka studied separately the growth of octopus caught - and supposedly born -in the Cape Garnett-Cape Barbas and the Cape Blanc areas, in spring and autumn respectively. No appreciable differences were observed between the growth of the spring and autumn-born cohorts, but the growth rate is higher for the Cape Blanc octopuses than for those from Cape Garnett-Cape Barbas (K.65 and.47 respectively). These results were derived from the analysis of quarterly LF distributions of the Japanese commercial catches.

Mr. Guerra used length frequency distributions recorded during several surveys conducted by Spain (R.V. "Cornide de Saavedra") on the northernmost fishing grounds as well as from commercial catches (Factory vessel "Galicia"). The distributions collected over the period 1971-1975 were then pooled together by months in order to obtain a simple annual cycle on which the progression of modal lengths was analysed. The first determination - by the Ford-Walford method - of the parameters of the Von Bertalanffy equation led to unlikely values of L_w. Subsequently, the Beverton method was applied using assumed values of L_w.selected on the basis of the length of the biggest specimens observed in the fishery. The Group was of the opinion that the most likely value of Lw, was between 35 and 40 cm (mantle length). A value for K in the region of.7 -.8 corresponds to these assumed ranges for Lw.

Both authors agree about the timing (spring and autumn), the age (6-8 months) and the number (2) of annual recruitments - or rather of the peaks of recruitment as the spawning season extends over more than half of the year. Their disagreement essentially refers to the rate of growth after recruitment and subsequently to the absolute ages at length. Japanese findings conclude to an absolute growth rate appreciably lower than that calculated by the Spanish scientist. When these curves are compared with the results of tagging experiments made by Spain, it appears that the additional weight gained by two tagged specimens^1/ during their release (8 and 3 months respectively) compare better with the Spanish growth curve than with the Japanese one.

^1/ The other recoveries were made after periods of liberty of very short duration

3.2 Mortalities

(a) Natural mortality
(b) Fishing and total mortalities

Some preliminary information on mortality has been presented by Mr. Hatanaka. It all refers to the octopus stocks.

(a) Natural mortality

No reliable estimate is available for natural mortality. Studies on the biology of this species, local observations on its growth and on the population structure indicate that the pattern of natural mortality against age is probably very complex. Combination of sex ratio against length and length frequency distributions of the populations shows that the females rapidly disappear in the catches between 80 and 110 cm (TL). Actually three periods, with very different levels of mortality, are suspected to exist throughout the life span:

- before recruitment, high mortality of juveniles, partly in relation with cannibalism and predation;

- moderate natural mortality during the exploitation phase (octopuses are carnivorous animals)

- disappearance of the females, which would die after their first spawning; the males probably survive one year or two after the first breeding, but it is possible that an appreciable proportion disappear at each spawning season.

(b) Fishing and total mortalities

Mr. Hatanaka presented the observations he made based on Japanese commercial catches in the Cape Blanc area in 1968 and 1969. By comparing at two different periods the total catch (in numbers) per unit area on the whole fishing ground for the same cohort, he obtained two estimates of total mortality: 1.3 for a period of 7 months and 1.0 for a period of 5 months, i.e. approximately 2 by month. Since 1968-69 the rate of exploitation has still largely increased in the whole fishing area. This is shown for instance by the assessments based on the production model given in section 4.4. Similar assessments made by Hatanaka and referring to the Cape Blanc fishery only lead to the same conclusion.

The Group also determined the length frequency distribution corresponding to the 1976 Spanish catches on the Cape Garnett ground, using for that purpose the two growth curves discussed in section 3.1. The following results were derived:

- with the Hatanaka's growth curve:	Z2+/1+ = 1.5
	Z3+/2+=2.3
- with the Guerra's growth curve:	Z1+/0+ = 3.0
	Z2+/1+ =2.5

Although not very precise, all these computations indicate that at present the total mortality is high, possibly appreciably higher than 2 (annually), (If, as it is supposed, natural mortality changes considerably with the age, the total mortality cannot be correctly expressed by a single coefficient). These preliminary observations nevertheless indicate that at present the exploitation rate is intensive, probably appreciably higher than that of the level corresponding to the maximum average yield.

3.3 Selectivity and mesh size assessments (octopus and cuttlefishes)

At its first meeting (Rome, June 1972) the CECAF Sub-Committee for the Implementation of Regulatory Measures, as well as CECAF at its Third Session (Santa Cruz de Tenerife, December 1972), expressed the wish that all trawls with mesh size smaller than 60 mm be banned in the fisheries for cephalopods. The purpose of that recommendation was to facilitate the implementation of the mesh size limitation (70 mm) for hake and sea breams, and to prevent excessive killing of undersized sea breams in the cephalopod fishery. However, because the data needed for the computations have never been available, no mesh size assessment could have been carried out to check whether the use of a 60 mm mesh size was justified for the rational exploitation of cephalopods. Only some calculations on the likely short term losses were made (Report of the Second Session of the CECAF Working Party on Resources Evaluation, Rome, December 1973).

In recent years, coastal states have enforced a 60 mm mesh size limitation in their cephalopod fleets, as well as to the vessels flying foreign flags but operating under their jurisdiction. As a consequence, most fleets fishing for cephalopods in the area are now using a 60 or a 70 mm mesh size (see table 1 - fleets, recent changes in mesh sizes). The Group therefore specifically addressed itself to that issue.

Two series of data could, in principle be used for mesh size assessments:

- Octopus:	.Cape Garnett-Cape Barbas stock exploited until 1977 with a 40 mm mesh size by Spanish vessels and with 30/50 mm mesh sizes by Japanese boats;
	.the length frequency distribution corresponding to the 1975 Spanish catches is available;
	.a selectivity curve is given in Appendix 6 of the present report.
- Cuttlefishes:	.Cape Blanc/Nouakchott's grounds exploited until 1973 with 30/50 mm mesh size;
	. the length frequency distribution for 1972 Japanese annual catches is given in Appendix 5, Report of the 2nd Session of the CECAF Working Party on Resources Evaluation, Rome, December 1973;
	.selectivity data are available in Appendix 6 of the same report.

However, simple models such as Gulland's one which only requires an estimate of E, the rate of exploitation, could not be applied to these data as the natural mortality of cephalopods does not apparently remain approximately constant throughout the life-span, (In addition, and at least for the Japanese/Cape Blanc's fishery, it is possible that q, and thus F, change also appreciably with seasons). More complex analytical models should therefore be used, unfortunately, they require estimates of various population parameters which, for most, are not available as yet (section 3.2) with a sufficient accuracy.

On a tentative basis, calculations have been made using a Ricker model, the two growth curves available (Guerra and Hatanaka), and the most likely mortality patterns. These computations are summarized in Appendix 12. For all combinations of parametric values, they indicate that, for this area and octopus at least, the use of nets with 60 mm mesh size should in the long run be justified.

However, the Group was of the opinion that the best approach to thoroughly investigate this critical issue would be to undertake a cohort analysis - preferably on a monthly basis -and simulation computations based on the length frequency distributions (monthly summaries) already available for the Spanish catches taken from the Cape Garnett-Cape Barbas stock of octopus. To achieve this assessment, two conditions should be met:

- that catches made by other countries on the same stock can be assessed;

- that the structure of the polymodal LF distributions (Spanish catches) can be clarified (i.e. that the growth pattern can be elucidated).

3.4 Length measurement for stock assessment purposes

Aware that no standard procedures have been adopted by CECAF as yet for that species group, the Group also discussed how specimens of cephalopods should be measured for establishing size frequency distributions. For cuttlefishes and squids, which have an internal skeleton, the Group agreed that the mantle length should be preferred.

However, measurement of octopus raises special difficulties. The Group recognized that weight (on fresh specimens) is more accurate and, whenever possible, should be preferred. Still this is not always the case (e.g. when length frequency distributions are established on "gutted" animals). In such circumstances, it is not clear whether the total or the mantle length should be preferred. With the latter, measurements are made difficult by the mantle contractions which occur for a long time after the animals have been caught. In addition, the limits of the measurements remain to be precisely defined. On the other hand, the use of the total length requires long measuring boards and also extrapolations when the arms of the animals are damaged.

Consequently, the Group recommended that Dr. Guerra undertakes a survey among selected world experts on cephalopod stock assessment. The conclusions of his survey, indicating the advantages and disadvantages of each type of measurement as well as giving precise definitions of the standard distances to be measured (such definitions should also be given for cuttlefishes and squids) would be then circulated, through the secretariat of CECAF, to the members of the CECAF Working Party on Resource Evaluation. It is expected that the CECAF Working Party will then be able to adopt standard measurements for establishing and reporting size frequency distributions of cephalopods.