5.1 Stock identity
5.2 The fisheries
5.3 Catch and effort data
5.4 Biological data
5.5 Abundance indices
5.6 Assessment
5.7 Management recommendations
5.8 Future research
The chub mackerel (Scomber japonicus, Houttuyn,1872) is found in relatively cold waters. In Northwest Africa this species is most abundant from northern Morocco to Senegal. Two stocks were identified in this region:
There is uncertainty concerning the separation of these two stocks.
In southern Morocco and Mauritania the chub mackerel was recorded from the coast to more than 300 m in depth, and the distribution is related to hydrological conditions.
For the assessment of chub mackerel, the group considered only the “southern stock”.
The northern stock is fished by the Moroccan fleet which is composed of purse seiners and trawlers. This species is caught as a by-catch. The total catch in 1999 was around 16 000 tonnes.
The total catch of the former Soviet Union fleet fishing in the southern stock was 41 000 tonnes in 1999, which constituted 20 % of the total catch of Scomber japonicus in the region. Of the total catch, 32 % was taken by Russia, 41 % by Ukraine and 27% by others.
The fishery of chub mackerel is located mainly in the area between Cape Bojador and Cape Blanc, where over three-quarters of the total catch is caught. In this region the fleet was composed of 15 -25 pelagic trawlers (BATA, BATM et BMRTIB RTMS).
In recent years, 25-35 pelagic trawlers have been operating in Mauritania and the catch of mackerel represents 7-33% of total pelagic catch. In Senegal and the Gambia, the chub mackerel is fished by artisanal and industrial fleets. The total catch in this part of the southern area in 1999 was about 9000 tonnes.
The catch data of chub mackerel from 1990 to 1999 are shown in Table 5.3.1. These data were obtained from the official statistics of the countries operating in this area.
Effort data for various fleets are shown in Table 5.3.2. The effort data refer to the total effort data of the pelagic fleet in the area and the fleet do not target mackerel all year. An estimate of total effort directed at mackerel is given in Section 5.5.
The following biological data were available from the Russian catches in the area:
Length composition from 1984 to 1999; Age composition from 1984 to 1999; Mean weight of catch 1984 to 1999; Maturity ogive; Natural mortality 1982-1985 and 1998; Fishing mortality 1992-1999; Growth in length 1995-1999; Growth in weight 1995-1999; Length at first maturity.
For more details see Appendix II - Russian data.
5.5.1 Catch per unit of effort
5.5.2 Acoustic surveys
In order to calculate the CPUE, vessels were selected with catches of 50% or more of chub mackerel per fishing day. The Russian fleet was used. The effort series presented to the Working group was standardised by using the multiplicatif model of Robson (1966) and Gavaris (1983). The total catches, total effort in fishing days of Russian trawlers and CPUE in tonnes per fishing day are presented in Table 5.5.1. The CPUE time series is shown in Figure 5.5.1
A series of acoustic surveys of chub mackerel is available from 1994 to 1999 from the Russian research vessels R/V AtlantNiro and Atlantida. In 1995 and 1998 two surveys were conducted, one in February-March and the other in June-September. In 1994, 1996 and 1999 summer surveys were conducted, while in 1997 a winter survey was carried out. The results of the surveys show a gradual reduction in biomass from levels of around 540 000 tonnes in 1995 to 185 000 tonnes in 1999 (Figure 5.5.2).
Methods
A biomass dynamic model was applied (BIODYN (Punt and Hilborn, 1996)).
Data input
The CPUE series used in the model (1984 to 1999) is shown in Table 5.5.1.
Results and discussion
The results obtained are shown in Figure 5.6.1. The analysis of the results shows that from 1984 to 1990 the level of the predicted CPUE was quite stable with some fluctuations, whereas from 1991 to 1996 we see an increase in the predicted CPUE. This can be explained by the change in strategy of the Russian fleet in Morocco, which after 1993 was more oriented towards mackerel.
The working group recommends applying analytical models (VPA) in the future, using the data that have recently become available. These data should be prepared in the right format before the next assessment meeting.
The results from the BIODYN analysis indicate a reasonably stable situation for the stock, however, acoustic estimates show a consistent decreasing trend (Figure 5.5.2). In the light of this conflicting information the Working Group recommends a precautionary approach and that the effort should not be increased from recent levels.
The working group recommends that to better evaluate mackerel stocks the following measures be adopted in order of priority:
1. Establish an appropriate sampling programme for each country fishing in the Northern CECAF sub-region2. Adopt the same age reading method for Scomber japonicus
3. Carry out supplementary studies to improve understanding of stock identity
4. All acoustic surveys should give a separate biomass estimate for mackerel
5. It is recommended that studies comparing environment with the biomass indices should be present in the next meeting
