Habitat and Biology
Climatic zone: Tropical. Bottom type: Unspecified. Depth zone: Coastal (0 m - 50 m); Shelf (50 m - 200 m); Slope (200 m - 1000 m). Horizontal distribution: Neritic; Oceanic. Vertical distribution: Pelagic.
Jurisdictional distribution: Shared between nations
Considered a single stock: No
Whilst waiting for more detailed information on stock identity of the carangid species in the southern CECAF region, the Working Group decided to consider five stocks: the northern stock (Guinea-Bissau, Guinea, Sierra Leone and Liberia), the western stock (Côte d’Ivoire, Ghana, Togo and Benin), the central stock (Nigeria and Cameroon), the Sao Tome stock and the southern stock (Gabon, DR Congo, Congo and Angola).
Catch and effort data for the five stocks with total observed catches between 1990 and 2008 are shown in Tables 5.2.1a,b and c and Figures 5.2.1.a,b,c.Total catch
It can be seen that, for Decapterus
spp., the catches are basically all from the industrial fishery in the northern stock (Guinea and Sierra Leone) with an annual average of about 4 000 tonnes. The majority of catches in the southern CECAF region (mainly Decapterus rhonchus
) are taken in Guinea. A large decrease can be seen in all catches from around 5 700 tonnes in 1996 to around 2 000 tonnes in 2004 followed by a recovery in 2005 and 2006.
For the group of Trachurus
spp. species, most of the Trachurus trecae
are caught in the stocks of the northern, western and central regions and the Trachurus
spp. are caught in the southern stock (notably in the Angolan zone). A worrying decrease can be seen over the whole southern CECAF region, going from over 65 000 tonnes in 1990 to less than 7 000 tonnes in 2004. It should however be noted that an increasing trend in catches can be seen from 2003 in the western stock.
The other carangid species that are generally exploited in the region are Selene dorsalis
, Chloroscombrus chrysurus
and the Caranx
spp. species. Total catches of these species have seen an increasing trend, rising from 1 300 tonnes in 1993 to 15 000 tonnes in 2008.Effort
In Guinea and Sierra Leone these species are mainly fished by encircling gillnets and driftnets in the artisanal fishery. In Ghana, Togo, Benin, Nigeria and Cameroon, small carangids are mainly exploited by the beach seine and the purse seine.
Most of the industrial fleets’ effort is concentrated in the Guinean EEZ. The large pelagic trawlers that target horse mackerel come from eastern European countries (Russian Federation and Ukraine). The nominal effort of this fleet (fishing days) decreased overall from over 600 fishing days in 1996 to around 400 days in 2004, then recovered increasing to 900 days in 2005 before falling again to 600 days in 2007 (Table 5.2.2).
The model requires a time series of total catch and abundance indices of the stock.
The estimates of total catch obtained by summing catches from different fleets from countries were used.
The global model was applied to the Decapterus
spp. data from the northern stock (Guinea-Bissau, Guinea, Sierra Leone and Liberia) using the CPUEs of the industrial pelagic trawlers of Guinea (period 1995– 2007).
For Trachurus trecae
, the northern stock (Guinea-Bissau and Liberia) and the southern stock (Congo, Gabon and Angola) were considered. In the northern region, the model was applied to the CPUE of the industrial trawlers of Guinea (period 1995 stock 2007) and to the CPUE of the artisanal fishery of Liberia (1997–2008). In the southern region, the model was applied to the stock data using the biomass indexes of acoustic surveys of the R/V DR. FRIDTJOF NANSEN as the abundance index.
spp. the model was applied to the Sao Tome stock using the CPUE of the artisanal fishery.
The Schaefer dynamic production model was used on an Excel spreadsheet (model described in Appendix II).
Schaefer dynamic production model
The global models applied to the data for the Decapterus
spp. species in the northern stock, T. trecae
species in the northern and southern zone were not considered because the indices used are not representative of the change in biomass of these different zones. No diagnosis can thus be established on the basis of these models.Discussion
Based on these models it was not possible to draw conclusions on the state of the stocks of the different species studied. In the case of the northern stocks, the species are the most exploited by Guinea-Bissau and Guinea. In Guinea-Bissau, the data collection is irregular while in Guinea, an estimated proportionality coefficient based on the data for one survey was applied to the landings with a view to estimating the catches of each of the species. Additional surveys are therefore necessary. Moreover, the sharp declining trends in the catches of the two species (Decapterus
spp. and Trachurus trecae
) at least over the last three years suggest a precautionary approach in the exploitation of the stocks.
As concerns the southern stocks, the most exploited Carangid species is T. trecae
in Angola. However the landings of this species are not fully recorded. Nevertheless, the biomass and average length series in Angola which began in 1985 show a decline. The estimated MSY using the Cadima formula is lower than the catch of the last year and furthermore, 90 percent of the biomass recorded in 2009 is made up of undersized individuals (length <21 cm, pers. comm. 2009). All these observations also indicate the adoption of a precautionary approach.
Scientific AdviceFuture research
In order to reduce uncertainties surrounding the assessment, the Working Group recommends carrying out the following research:
for Management consideration
- Support sampling programmes in order that they cover total catch of all the main carangid species, including horse mackerel, for all fleets in all countries of the southern CECAF region.
- Continue biological data collection for biological studies (growth, reproduction, feeding) on the main carangid species and make these data available to the Working Group by the next meeting.
- Begin sampling catches and discards onboard all vessels fishing horse mackerel due to problems arising from under-declaration of catch, especially of juveniles.
Due to limitations in the models obtained and uncertainties surrounding the data, whilst waiting for more investigations on the fisheries and survey data, the Working Group suggests that the following precautionary measures be adopted:
The catch of the Decapterus spp. species group in the northern countries should not exceed the 3 000 tonnes level (last year’s value).
The catch of the T. trecae species should not exceed the level of 10 000 tonnes (last year’s value) in the northern countries and in the southern countries the effort targetting this species should be reduced.
Biological State and Trend
Trachurus trecae - North (Gabon, Guinea, Sierra Leone and Liberia)
Trachurus trecae - South (Gabon, Congo, DR Congo and Angola)
Decapterus spp. - North (Guinea-Bissau, Guinea, Sierra Leone and Liberia)
Source of information
Report of the FAO/CECAF Working Group on the Assessment of Small Pelagic Fish – Subgroup South Accra, Ghana, 19–28 October 2009./Rapport du Groupe de travail FAO/COPACE sur l'évaluation des petits poissons pélagiques – Sous-groupe Sud Accra, Ghana, 19-28 octobre 2009. http://www.fao.org/docrep/018/i2909b/i2909b.pdf