For the whole CECAF region, FAO (1968) gives a production model for the years 1958–1966, based on the CPUE of Portuguese vessels, and the total sparid catches. This assessment showed that the MSY corresponds to the effort level of 1963. This assessment however is now out of date and for the more recent years, the Working Group made separate assessments for each statistical division.
For statistical division 34.1.1, a Fox model was fitted to catch and effort data relating to Moroccan coastal trawlers, longliners and gillnetters. The artisanal fishery was not included in the assessment.
The catch and effort data used,and the basic data from which these were derived, are given in Table 1, and some notes about these data are given below :
the total tonnage of the coastal trawlers of Morocco is given for the period 1960–1984.
The total tonnage of the gillnetters/longliners is given for the period 1981–1984. For the period 1960–1980 the tonnage of the gillnetters/longliners has been estimated as 30% of the tonnage of the coastal trawlers.
During the period 1960–1974 the total tonnage decreased whilst the total landings and the CPUE increased. Gréboval (1982) assumed that this was due to an increase in fishing efficiency and Table 1 gives the correction factors used.
Catch Per Unit Effort (CPUE) for Moroccan vessels was determined by dividing total Moroccan landings by total Moroccan GRT.
Finally, an index of total effort was derived by dividing the total sparid landing for division 34.1.1 by the Moroccan CPUE. The year 1975 was excluded from the assessment because of the very high landings reported by the USSR for that year.
Figure 1 shows the basic data, and the fitted catch and effort curve and the fitted CPUE and effort curve. The results indicate that maximum catches were obtained in 1979 and 1981 and that the index of total effort was also as its highest in these two years. Since 1981, both catch and effort appear to have declined, suggesting a moderate state of underexploitation in recent years.
The members of the Working Group noted that a reasonably good fit to the data was apparent in this assessment. However, it was felt that this result should be interpreted with caution for the following reasons:
Gross registered tonnage is not a very precise measure of fishing effort
Sparids are fished in the same area as hakes and shrimp, and hakes are considered to the heavily overexploited at present.
Sparids landed are of small size because they are caught in small meshed nets in very inshore fishing areas.
Because of the reservations, it was felt that no firm conclusions should be drawn until a better assessment could be made.
Owing to inconsistencies in the data, and the possibility that some catches reported as having come from division 34.1.2 may have come from other divisions, no assessments were attempted for the division.
Both general production and yield per recruit assessments were attempted for division 34.1.3.
A general production model has been fitted to the catch and effort data for this division by Mennes (1984). In this assessement, catches were based on reported landings of sparids for all countries taken from the CECAF Statistical Bulletin No. 2 (FAO, 1979). Estimates of total effort were derived, based on CPUE data for Spanish vessels. The assessment indicated that maximum landings had been associated with the years around 1970, when total effort had been rela tively low. Since then, total effort has increased and total landings have decreased
The members of the Working Group took note of this asessment, but also noted that several other indices of CPUE are available for this division and that collectively these do not provide a consistent picture of possible under lying changes in abundance. These Inconsistencies in the various indices of CPUE are not entirely surprising in the light of the njmerous shortcomings in the data, listed in section 2.2. Because of the uncertainties about the data, it was flet that it would be better not to draw firm conclusions from general production models applied to the data for this division as a whole.
A production model (Pella and Tomlinson) was also applied to data for the Canary artisanal fleet that fishes untrawlable grounds in division 34.1.3 and for which sparids are target species. The results are summarized in Figure 2, and show that the optimum level of effort is 13, 038 fishing days associated with a maximum sustainable yield of 2518 T (the parameter is being 2.2).
The sparid resources on these grounds are only exploited by the Canary artisanal fleet, and the current level effort (in 1984) is only 3833. fishing days along with a total catch of 869 tonnes of sparids. It appears from theseresults that a further increase in fishing effort in this particular fishery should lead to a further increase en yield.
Mennes (1984) presents yield isopleth diagrams for 11 species of Sparids based on research vessels data, collected in 1980–1982, within the latitudes of 20°–26° North in o–100 m depth. The results for the 11 species are plotted in Figure 3,based on values of 1c/Loo (where 1c – 50% length at first capture) and F/Z.
For each value of F/Zther is a value of 1c/Loo for which the yield per recruit is a maximum. The locus of these points is shown by the curve A…A. Also, for each value of 1c/Loo there is a value of F/Z for which yield per recruit es a maximum. The locus of these points is shown to the curve B…B Between the curves A…A and B…B is an area, within which combination of 1c/Loo and F/Z may be thought of as being associated with “optimum” yields per recruit.
In Figure 3, the position of the curves A…A and B…A was determined using information in the Tables of Beverton and Holt (1964) for a value of M/K= 2.0. The results shor clearly that the estimated values of 1c/Loo and F/Z are such as to locate points for every species except P. acarne to the right of the area of “optimum” yield per recruit. The implication is that, except for P. acarne. it should be possible to increase yield per recruit by reducing effort and/or increas ing mesh size.
Figure 3 also includes two points (for Sparus caeruleostictus and Dentex canariensis) based on data collected in 1984 off Mauritania from the R/V ALMORAVIDE (Josse et Garcia, 1986). For these two species, it also appears that yield per re cruit could be increased by reducing effort and/or increasing mesh size.
An assessment of the MSY of the Pagellus bellottii stock off Senegal has been made based on data for 1972–1984, by Franqueville (Appendix 5, Figure 6). This gives an MSY of the order of 10 036 T for an effort of 36 527 standardized days at sea of a 200 HP red mullet trawler. The corresponding CPUE is about 295 kg per days–at–sea.
The members of the Working Group also attempted to apply a Fox model to the total sparid catches from the Senegalese continental shelf, using days at sea by pair trawlers as an index of CPUE (Figure 4). In the opinion of the Group, however, the results were not satisfactory. Also, it was noted that for division 34.3.1, it was possible to determine 10 different indices of CPUE, and that these did not give a consistent picture of changes in abundance with time.
The members of the Working Group considered that these inconsistencies were most likely due to shortcomings in the data of the kind listed in section 2.2. It was felt therefore that no useful conclusions could be drawn for division 34.3.1 by the application of general production models.
Franqueville (1983) applied a Ricker yield per recruit model (Ricker, 1958) to data for Pagellus bellottii from the Senegal area. He considered that an increase in the yield per recruit of this species is to be expected if mesh size is increased. He also considered that although this species does not appear to be overexploited in Senegalese waters, fishing effort should not be allowed to increase so long as the present exploitation strategy remains the same.