The two main species are the Atlantic horse mackerel (Trachurus trachurus) (and the Cunene horse mackerel (Trachurus trecae). According to acoustic surveys, the false scad (Caranx rhonchus) has become more abundant over the last few years.
This section has already been described by previous Working Groups (FAO 2001 and 2002). Stock identification studies should be undertaken in the future.
Since the beginning of 2002, the industrial horse mackerel fishery has been located almost exclusively in the Mauritanian zone. This fishery is carried out by foreign industrial fleets, chartered or operating within the framework of bilateral agreements and/or joint ventures. The fleet consists mainly of ships from the Russian Federation, the Ukraine, Lithuania, Latvia, Estonia, and the European Union and of many other countries such as Cyprus, Panama, Marshall Islands, Saint Vincent, Grenadines, etc.
The exploitation has seen major changes over time:
Withdrawal of the Romanian fleet at the beginning of 1993 from the Mauritanian EEZ. This fleet was very active at the end of the 1980s.
Withdrawal of a large part of the Russian fleet from the Mauritanian EEZ in 1994 then its return in 1995.
Entrance into the Mauritanian EEZ of new fleets, in addition to those of the ex-Soviet Union, from Cyprus, Panama, Poland, Malta, Sweden, Saint Vincent and the Grenadines, etc. In general, the activity of the vessels from these countries is irregular (some months a year only).
From 1996 the development of a European trawler fleet and from 1999 the introduction of several hundred Senegalese artisanal fishing vessels in Mauritania. Both fleets essentially exploit sardinella.
Withdrawal of the fleets from the Russian Federation and the Ukraine from Zone C, north of Cape Blanc in 1999 and 2001 respectively. This probably explains the intensification of the fishing effort of the East European fleets directed at horse mackerel in the Mauritanian zone over the last few years.
Promulgation in the Mauritanian zone in 2002 of decree No. 2002-073 stipulating that in the future the small pelagic fishery should be conducted beyond 12-15 miles following the configuration of the coast, in order to preserve the coastal zone.
General characteristics of the small pelagic fleets operating off Northwest Africa
The characteristics of these vessels are given in the 2001 Working Group report (FAO 2001). These fleets are mainly composed of Eastern European vessels that have been present in the zone for four decades, and more recently of European Union vessels. The national industrial fleets are not very developed.
There is no artisanal fishery targeting horse mackerel.
Total catches
The annual catch data of the three species of horse mackerel by country and total for the subregion for the period 1990-2002 are presented in Tables 4.2.1a, b and c and Figure 4.2.1. Note that the catch statistics are provisional for 2002.
To split the total horse mackerel landings data for 2002 in Mauritania into species, sampling on board the European Union boats was used. In general, using the sampling of the Russian vessels would have been better due to the fact that this fleet targets horse mackerel, but, the problem is that the Russian fleet does not include false scad in its sampling.
In Senegal, systematic surveys in the field allow determination at source of the quantities of the two horse mackerel species present in those waters (false scad and Cunene horse mackerel).
From Table 4.2.1 a and Figure 4.2.1 it can be seen that for Trachurus trachurus, total catch showed a decrease in 2002. The landings of this species decreasing from around 70 000 tonnes in 2001 to 32 000 tonnes in 2002. This decline is essentially due to the withdrawal of the Ukrainian fleet from the Moroccan zone.
For Trachurus trecae as well, 2002 saw a net decrease with respect to the previous year. The catch decreasing from 210 000 tonnes in 2001 to less than 170 000 tonnes in 2002. The decline is relative compared with the growth since 1999. The bulk of the catches of this species is from the Mauritanian zone.
For Caranx rhonchus, after a period of low landings from 1995-1998, an increase was observed with a peak of around 46 000 tonnes in 2000. In 2001 and 2002 the landings decreased to 21 000 tonnes in 2002. This decrease mainly concerns the Mauritanian zone where the catch dropped from 43 000 tonnes in 2000 to around 15 000 tonnes in 2002 (Table 4.2.1c).
Fishing effort
Figure 4.2.2 shows the effort on horse mackerel by fleet and year in Zone C. A nominal fishing effort expressed in fishing days shows a certain stability since 1998 for the fleets which target horse mackerel (Russian Federation, Ukraine, etc.). For the European Union fleet the standardized effort has seen a continued increase (Figure 4.2.2).
Two CPUE series were obtained, one based on the standardized effort of the European Union fleet and one on the nominal effort of the other fleets (Russian Federation, Ukraine and others). Figure 4.3.1 shows that, for the fleets present, a large decrease in CPUE was observed from 1994-1996. For the CPUE series based on the nominal effort, a general increasing trend is observed from 1997 to 2002, with the exception of 1999 when the CPUE was comparatively low. The CPUE based on the standardized effort of the European Union starts in 1996 shows a slightly higher level over the last three years (2000-2002).
The time series of biomass estimates of horse mackerel from the acoustic surveys of the R/V DR. FRIDTJOF NANSEN, were finalized by incorporating the year 2002 (Tables 4.3.1 and 4.3.2).
These biomass estimates highlight the fluctuating nature of Trachurus spp. in the subregion (Figure 4.3.2).
The Cunene horse mackerel (T. trecae) is a warm water species, abundant in the zones of significant temperature gradients. In 2000 a high biomass was noted, the highest ever observed by the R/V DR. FRIDTJOF NANSEN in the region (Figure 4.3.2) In 2001, the biomass dropped drastically, followed by an increase in the May-July survey of 2002. It should be noted that this species had practically disappeared from Mauritanian waters in 2001, re-appearing at the 2000 level in May-July 2002 (Figure 4.3.2).
The Atlantic horse mackerel (T. trachurus) is a cold water species. The 2002 acoustic surveys showed an increasing trend for this species (Figure 4.3.2). The absence of the species in the Mauritanian zone in May-June 2002 and in the Cape Cantin-Cape Juby zone in November-December 2002 should be noted (Table 4.3.2).
In general, when comparing the CPUE with the acoustic indices from the R/V DR. FRIDTJOF NANSEN, the trends are similar with the exception of 2000 when the biomass was very high.
Until 2002, the Russian sampling used to only cover the cold season (December-May). This may explain why there was no false scad present in the samples (ref. Section 4.2). Since 2002 the sampling of the Russian fleet is carried out throughout the year in the Mauritanian zone.
Furthermore, IMROP and RIVO have been collaborating to collect monthly samples throughout the year on board European Union boats since 1999. It should be noted that this fleet targets sardinella and that the horse mackerel stock structure probably does not reflect the real dynamic of the stock.
Sampling effort remains very insufficient both in terms of geographical and time coverage of the fleets operating in the subregion. For the different horse mackerel species the sampling effort appears to be more constant for the European Union fleet than for the Russian fleet, but less intensive. In the subregion, sampling is essentially carried out in the Mauritanian zone which is the area most involved in fishing for these species (Tables 4.4.1, 4.4.2 and 4.4.3).
For the Trachurus trachurus stock between 26oN and 10oN and the Trachurus trecae stock between 23oN and 9oN catch at age data are available (based on Russian data), these are given in Tables 4.5.1 and 4.5.2. No data are available for Caranx rhonchus.
In 2001 and 2002, the Russians identified age groups up to the age of 13 for the Cunene horse mackerel and age 11 for the Atlantic horse mackerel. However in order to have a uniform series for the period 1990-2002, an age group of 8+ has been used, grouping together the higher ages.
Data analysis
Firstly, an exploratory approach aimed at evaluating the coherence of the catch at age series before using the VPA was carried out. For each of the two Trachurus species, two graphs are presented: the catch-at-age by year class and the log catch ratio.
The catch-at-age by year class for T. trecae (Figure 4.6.3) shows inconsistencies in the recruitment variability in the fisheries. The log catch ratio (Figure 4.6.4) confirms this and furthermore it shows the large variability in the evolution of the cohorts and in mortality as shown in the fisheries data. This large variability can also be seen for T. trachurus (Figures 4.6.1 and 4.6.2).
Several reasons could be envisaged to explain this incoherence:
Because of these inconsistencies and because this method did not produce satisfactory results during the fifth IMROP Working Group on the assessment of the resources in the Mauritanian EEZ held in December 2002 in Nouadhibou, the Working Group decided not to carry out assessments using VPA.
Assessment using a production model
Due to the difficulty in carrying out assessments using an analytical model, a Schaefer type production model was applied using the combined catch of the two Trachurus species. The CPUE used was that calculated on the basis of a standard effort targeted at horse mackerel in the Mauritanian zone for the years 1979-2001 (the data for 2002 are still provisional).
Grouping the two species, which have different distributions, was dictated by the need to limit the variability. In the catch data, the two species are not separated and fishing effort is aimed at the two species. These two constraints forced the group to consider the two species together.
The BIODYN software (Punt and Hilborn 1996) was used. This model requires the estimation of several parameters.
|
r |
= |
1.67 |
|
K |
= |
1 000 000 tonnes |
|
Binit |
= |
1 000 000 tonnes |
|
MSY |
= |
417 000 tonnes |
|
fMSY |
= |
285 320 hours trawled |
|
SSQ |
= |
0.15 |
|
q |
= |
0.0000029 |
In the time series used two distinct periods of different abundance levels can be observed and the model cannot be well adapted. This partly explains the high level of the MSY whereas more recently the catches of these species have been around 200 000 tonnes a year (Figure 4.4.6).
The results of the assessment model suggest that the stocks of the two species are moderately exploited. However, due to the limitations of the model used, uncertainties on the stock assessment and the diverse nature of the fisheries (especially the industrial fisheries), a prudent attitude should be observed in the management of these stocks. For this reason the Working Group recommends not exceeding a level of effort equivalent to that of the average of the last five years for the two species.
The majority of the recommendations made at the last Working Group have not been carried out. Furthermore, new exigencies in terms of research are now necessary. Thus the Working Group recommends:
