4.1 Stock identity
4.2 The fisheries
4.3 Catch and effort data
4.4 Biological data
4.5 Abundance indices
4.6 Assessment
4.7 Management recommendations
4.8 Future research
In this section we shall only be dealing with two species of horse mackerel, the Trachurus trachurus and the Trachurus trecae. The importance of the false scad (Caranx rhonchus) in commercial fisheries in the sub-region has become negligible over recent years.
The Atlantic horse mackerel (Trachurus trachurus)
This species prefers more temperate waters and can be found off the Atlantic coasts up to Norway. It can also be found in the Mediterranean, but rarely in the Black Sea. In Africa it is distributed between the Straits of Gibraltar and Senegal. It spreads from the coast to more than 300 metres and prefers the deepest areas of the continental shelf (depths of 100 metres plus). In Mauritania the highest concentrations are generally to be found between 18° and 20°N.
In the Northern CECAF zone two Trachurus trachurus stocks can be identified (Table 4.1.1):
The Cunene horse mackerel (Trachurus trecae)
A tropical species, the Cunene horse mackerel is found in all the east Atlantic from Cape Bojador (26°N) to the south of Angola. Whilst present all year round in the whole zone, it is between Saint Louis (16°N) and Nouakchott (18°N) that the highest concentrations are generally to be found. The Cunene horse mackerel is more coastal than the Atlantic horse mackerel. The bulk of its concentration is in depths of less than 100 metres. The movements of this species in the zone vary according to the movement of the intertropical front. During the cold season it is mostly to be found in the south, moving north during the hot season in relation to the advance of the warm waters.
4.2.1 The Eastern European trawlers
4.2.2 The European Union trawlers
Several fleets of different nationalities fish for horse mackerel in the Northern CECAF zone. In Mauritania, in addition to some national vessels, up to 23 foreign nationalities have operated in the zone during the last decade, notably fleets from the ex Soviet Union (Russia, Ukraine, Lithuania, Latvia and Estonia) from Europe (The Netherlands, France, Great Britain, Malta, Sweden, Poland and Cyprus) and from numerous other countries (Panama, Ghana, Marshall Islands, Saint Vincent and Grenadine, etc).
In Senegal, horse mackerel do not constitute a significant part of the catch. Over the last few years the largest catches have been recorded by Russian vessels operating offshore.
In the Moroccan Northern zone, the Atlantic horse mackerel (Trachurus trachurus) is fished by a national fleet composed of purse seiners and coastal trawlers. The purse seiners target mainly sardine, whereas the trawlers target mainly cephalopods and demersal species.
General characteristics of the small pelagic fleets operating in the Northern CECAF zone
The fleets are composed of long range trawlers which follow the concentrations of fish and process the catch (freezing, canning, fish-meal). These fleets are made up of vessels from Eastern Europe that have been in the zone for four decades, and more recently of European Union vessels. The national, industrial vessels are not very developed.
Following FAO classification based on the gross registered tonnage (GRT), the active boats in Mauritania can be classed in twelve categories. Class 10 (2000 to 3999.9 GRT) and class 11 (4000 to 9999.9 GRT) are the most common (Ould Beye, 1998).
Table 4.2.1 shows the general characteristics of the Eastern European pelagic trawlers operating in Mauritanian waters. These vessels are grouped into different categories based on size and gear. Some have been designed for the tropical Atlantic waters such as the BMRT, RTMA and RTMS, whilst others, like the BKRT and RKTS have been built for cold seas. Others still have been designed for fishing and onboard processing of the arctic shrimp. The newest boats, are the BATM and RTMKS. One can see over the last few years a growth in the percentage of vessels that are newer, more powerful and more multipurpose (freezing, canning, fish-meal). Thus vessels with a engine power greater than 7000 HP, which were not present in the zone in 1991, represented 54% of vessels in 1996 (Sepia, 1997).
To reach the deep mackerel and horse mackerel shoals, the pelagic trawls used by the Eastern European vessels have a vertical opening of 72-80 metres. Therefore they operate at a certain depth and for this reason bring back demersal and other neritic species which are found in the intermediate depths swept by the trawls.
The fish are loaded directly onto freighters which transport the cargo to the destination markets.
Most of the European Union trawlers come from the Netherlands. They are relatively recent and found principally in the Mauritanian zone and occasionally in the Moroccan zone. The vessels have similar characteristics (Table 4.2.2). They use large pelagic trawls with a vertical opening of 30-40 metres and a horizontal opening of 60-95 metres. In shallow waters, the vertical opening is reduced to twenty metres. The sardinella shoals situated near the surface, constitute the main target, whilst the deeper horse mackerel are secondary targets. The vessels return to Europe from the beginning of October to fish for their herring quotas in European Union waters (Corten, 1999).
The catch is sucked onboard with the help of a pump attached to the bag of the net. These vessels have a large, daily processing capacity of up to 300 tonnes (Anonymous, 2000).
Catch data from the sub-region for each of the three species of horse mackerel are presented by country in Tables 4.3.1a,b,c for the years 1990-1999. Historical series for 1979-1989 for Atlantic and Cunene horse mackerel are also available (Tables 4.3.2a,b). The standardised fishing effort (calculated on the basis of the criteria explained in section 4.5.1) is also available for 1979-1999 (Table 4.3.3).
For the Mauritanian zone, catch and effort data are available for horse mackerel from 1979-1999.
For Senegal, monthly and annual catches for the artisanal fishery for 1974-1999 and for the industrial fishery for 1966-1999 are available.
For the Northern Moroccan zone, horse mackerel landings are accidental and low. The horse mackerel catch for 1999 was 10 940 tonnes.
The data collected onboard industrial vessels refer to a mix of the three species (Trachurus trachurus, Trachurus trecae and Caranx rhonchus). The proportions of the three species in the biological samples of the Russian and European Union fleets were used to obtain the catches by species in the Mauritanian zone. For the Senegalese zone the breakdown was carried out following proportions obtained for the species from the trawls carried out by R/V Dr. Fridtjof Nansen during its surveys.
For the Trachurus trachurus stock between 26°N and 10°N and the Trachurus trecae stock between 9°N and 23°N the following data are available: age composition for 1979-1999 (Tables 4.4.1 and 4.4.2); length composition for 1992-1999; average weight-at-age for Trachurus trecae; natural mortality; fishing mortality; growth parameters; maturity ogives; and age-length key.
For more details see Appendix II - Russian data.
4.5.1 Catch per unit of effort
4.5.2 Acoustic surveys
On the basis of logbook data, an attempt has been made to standardise fishing effort. This fishing effort relates to several species.
Fleets from the European Union and certain other countries (Norway, Malta etc.) are considered to have comparable characteristics and similar strategies oriented more towards sardinella. The other fleets which in general target horse mackerel, come from Eastern European countries and are thus similarly comparable (Ould Taleb Ould Sidi, 2000).
For the choice of standard vessel type, several factors were taken into consideration: the presence of this type of vessel for the greatest number of years possible; the number of vessels of this type operating; the degree to which horse mackerel are present in the catches. Taking all these into account, class 10 Russian vessels of which the percentage in the catch of horse mackerel is greater than or equal to 30% were selected as standard. This effort is considered to be a standardised fishing effort on the basis of which the CPUE is calculated. Following this approach a series for 1991-1999 was constructed.
To have the longest series possible, the CPUE data provided by Russian researchers to the working group organised by CNROP in 1993 (FAO, 1995) were used. This series was based on a percentage of 50% horse mackerel in the catches.
The CPUEs of the Trachurus sp for the period 1979-1999 show large variations due to a change in strategy and natural stock variations such as those highlighted by the acoustic surveys (Figure 4.5.1). There seems to be a consistent drop in this CPUE series from 1994-1996, with lower levels in the years 1996-1998, followed by an increase in 1999.
The improvement in the CPUEs has not been reflected in the abundance indices of 1999 for the two research vessels. Nevertheless, it should be noted that in 2000 R/V Dr. Fridtjof Nansen detected some very large quantities of horse mackerel.
The time series of the acoustic surveys of the R/V Dr. Fridtjof Nansen of horse mackerel covers 1992-2000 (Table 4.5.1 and 4.5.2 and Figure 4.5.2). The Cunene horse mackerel (T. trecae) is a warm water species which reaches high abundances near oceanographic frontal areas. The Atlantic horse mackerel (T. trachurus) is a cold water species. The general pattern in November-December, when the Nansen surveys 1995-2000 took place, has been a concentration of Cunene horse mackerel in the Cape Juby - Cape Blanc area and in Mauritania. The Atlantic horse mackerel has mainly only been found in the Cape Juby - Cape Blanc area.
Russian research vessels have worked off Morocco and Mauritania for many years. 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 from these surveys are presented in Table 4.5.3 and Figure 4.5.2
BIODYN
A catch and effort series for horse mackerel has been constructed for the years 1991-1999. There is one already available for 1979-1990 in the report of the 1993 working group in Nouadhibou, Mauritania. On the basis of this data, the BIODYN software was used (Punt and Hilborn, 1996).
Several combinations were tried. Firstly on the basis of standardised fishing efforts for the two horse mackerels for the period 1979-1999. Later only on the basis of the more recent period 1991-1999. In both cases the same procedure was used.
The results of these two runs were not conclusive, possibly because of inconsistencies in the input data. An example of the various outputs from the different runs is shown in Figures 4.6.1 and 4.6.2.
VPA
The data relative to age composition and mean weight-at-age of catches by year, used by the 1998 working group, have been completed for recent years. The available series covers the period 1979-1999.
The following parameters were used in the calculations:
A first analysis using a simple VPA was made for T. trecae.
Other VPA analysis were tuned with age-acoustic biomass and CPUE indices for horse mackerel for recent years.
The results highlight large discrepancies for the two species both in the biomass dynamic model and the analytical model (VPA). Neither will it be possible for the working group to determine the mean biomass level for the two species of horse mackerel applying the usual stock assessment methods.
As for the Atlantic horse mackerel off northern Morocco, the working group was not able to carry out an assessment due to the lack of data and the limited catches carried out in the area.
The horse mackerel fishery has undergone very rapid changes over the last few years, including frequent changes in fishing strategies. The change in the system of operations which followed is one of the main reasons for the present difficulties in obtaining a reasonable stock evaluation. The reliability of the statistical and biological data and the large variation in data from different sources also represent a great limitation.
The present assessment is not reliable, probably due to the inconsistencies in the input data. In addition, the CPUE series and the acoustic surveys do not show the same trend.
Therefore, the Working Group recommended that a precautionary approach, avoiding any increase in fishing effort, should be adopted.
The Working Group recommends an improvement in the biological and statistical data, which should be one of the first priorities. Furthermore, the following points should be carefully considered:
1. Monthly samplings on industrial boats to monitor species composition, age and length composition, etc.Finally, a detailed preparation is necessary before every working group. This allows above all for a preliminary analysis to be carried out.2. Growth studies (age reading)
3. Better use of historical series on catch, effort and biological parameters
4. The establishment of a series of fishing effort remains one of the main priorities. The basic difficulty here is the often very irregular presence of foreign industrial vessels with physical characteristics and fishing strategies that evolve rapidly based on the opportunities that the resources offer
5. The scientific surveys should be continued, and the acoustic estimates should be split into age groups.
