The question of sardine stock identity has been posed in the different working groups on sardine. The research results, based on different morphometric, meristic and biostatistical methods and acoustic findings have remained inconclusive (Furnestin 1950; Krzeptowski 1975; Bravo de Laguna, Fernandez and Santana 1976; Belvèze and Rami 1978; FAO 1978). However in the most recent Working Groups three separate stocks have been assumed: the northern stock (35°45' - 32°N), the central stock, Zones A+B (32°N - 26°N) and the southern stock, Zone C (26°N - to the southern extent of the species).
In order to follow the sardine stock dynamics, it may be necessary to further study these stock units using other methods than applied in the past, e.g. techniques which are based on molecular biology. One study on sardine stock identity using two markers (DNA and Allozymes) is being carried out at INRH.
Total catches
The Northwest African sardine was exploited in four fishing zones in 2002: the Northern Zone, 35°45' - 32°N (Cape Spartel-Eljadida); Zone A, 32°N - 29°N (Eljadida-Sidi Ifni); Zone B, 29°N - 26°N (Sidi Ifni-Cape Bojador) and Zone C, 26°N - to the southern extent of the species near Cape Timiris in Mauritania (Cape Bojador-Cape Timiris) (Figure 2.2.1).
The total annual catch (tonnes) of the different fleets operating in the Northern Zone, Zones A, B and C between 1990-2002 is shown in Table 2.2.1a. The corresponding annual fishing effort in Zones A, B and C is shown in Table 2.2.1b. The Moroccan fishing effort is expressed in number of positive trips[1] for the purse seiners and number of fishing days for the RSW[2] seiners.
The trend in sardine catch by zone is shown in Figure 2.2.2. Total catches in Zone B show an increasing trend over the years 1990-2002 which is due to the transfer of a part of the Moroccan fleet to Zone B after the opening of new ports (Tan Tan in 1982, Laâyoune in 1989 and Tarfaya in 1994). The number of purse seiners increased, to reach more than 200 vessels. On the other hand, in Zone C, total catch has decreased following the halt in operation of the Spanish and Russian fleets Sardine catch south of Cape Blanc remains at a low level due to the fact that it is not the target species of the fleets fishing in Mauritanian waters.
Recent developments by country
Morocco
The fishing zones from the northern border of the Moroccan EEZ to Cape Blanc are at present exclusively exploited by the Moroccan fleet. The Spanish and Russian fleets ceased fishing in Moroccan waters after the Morocco-Russian Federation and Morocco-EU fishing agreements were not renewed. The Moroccan fleet operating along the Atlantic coast mostly comprises traditional purse seiners and numbers around 350 vessels. The fleet operates mainly from the ports of Tan Tan and Laâyoune.
Northern Zone
The fleet operating in this zone is composed of about one hundred purse seiners with an average tonnage of 40 GRT and an average power of 250 HP. Since 1990, fishing effort has remained stable but the catches have decreased, decreasing from an average of close to 17 200 tonnes for 1990-1996 to an average of around 6 000 tonnes between 1997-2001, then an increase was registered in 2002 with a catch of about 18 500 tonnes.
Zone A
The fleet operating in this zone is composed of about 150 purse seiners (40 GRT, 250 HP). Fishing effort decreased between 1990-1996, the number of positive trips decreasing from almost 7 000 in 1990 to about 570 in 1996. The catch has fallen drastically from around 48 800 tonnes in 1990 to only 3 500 tonnes in 1996. Since 1997, fishing effort is increasing with an average of around 4 000 positive trips a year and an average catch of 28 000 tonnes for the period 1997 - 2002.
Zone B
The fleet operating in this zone is composed of almost 200 purse seiners of an average tonnage of 55 GRT and power of between 250 HP and 300 HP. Fishing effort is increasing, rising from 7 000 positive trips in 1990 to more than 28 000 in 2002. The catch has also been consistently increasing.
Zone C, north of Cape Blanc
About ten coastal purse seiners of a tonnage less than 120 GRT have moved towards the Dakhla zone to exploit the southern sardine stock. The catch has increased from 3 000 tonnes in 2001 to more than 21 000 tonnes in 2002. Three RSW type boats with a tonnage of up to 1 000 GRT also operate in this part of Zone C.
Mauritania
Zone C, south of Cape Blanc
Sardine catch in the Mauritanian zone has increased from 11 500 tonnes in 1996 to more than 37 500 tonnes in 2002. The catch is seasonal carried out by pelagic trawlers, mainly from the EU.
For Zones A and B, catch per unit of effort (CPUE) has been calculated for the years 1990-2002 for the Moroccan fleet and for 1990-1995 for the Spanish fleet. For the Moroccan fleet the effort is expressed in positive trips, for the Spanish fleet in fishing days.
For Zone C, the CPUEs calculated by fleet for the years 1983-2001 during the last Working Group (FAO 2002) have been retained.
CPUE trends for Zones A, B and C for 1990-2002 are shown in Figures 2.3.1a and b.
The trend in CPUE in Zone B for the Moroccan fleet is fluctuating since 1996, with peaks observed in 1997 and 2001. The CPUEs of the various fleets operating in Zone C between 1990-2002 show varying trends.
R/V AL AMIR MOULAY ABDALLAH
In the northern zone (Cape Spartel - Cape Cantin), the results of the surveys carried out in 2001 and 2002 by R/V AL AMIR MOULAY ABDALLAH show that sardine is predominately present in the regions of Larache and Kénitra. The average biomass of sardine in this region is estimated at 22 000 tonnes.
R/V DR. FRIDTJOF NANSEN
At the end of 2002, R/V DR. FRIDTJOF NANSEN, found exploitable densities of sardine in the zone between Cape Cantin and Cape Bojador (A+B), especially in the regions of Safi, Sidi Ifni-Draa and Laâyoune.
Sardine biomass, estimated by echo-integration, showed year to year variations between 1986 and 2002 (Figure 2.3.2).
The biomass in the zone between Cape Cantin and Cape Bojador (A+B) was estimated at almost 1.2 million tonnes in the autumn of 2001 and 2002, a 25 percent decrease compared to the estimated values in 2000 (1.5 million tonnes). In November-December 2002, the population structure of sardine in this zone (A+B) is marked by a predominance of juveniles. The main mode of the length distribution was 12 cm (Figure 2.3.3a).
For the zone Cape Juby - Cape Bojador, the length distribution of sardine was bimodal (Figure 2.3.3a).The first mode was at 18-19 cm and the second at 23-24 cm which was similar to the length distribution observed in the zone Cape Bojador - Cape Blanc (Zone C) (Figure 2.3.3b).
As for the fishery in Zone C (Cape Bojador-Cape Timiris), the results of the 2002 acoustic survey demonstrated that sardine was continuously distributed from Cape Bojador to Cape Timiris. The distribution was marked by high concentrations around the areas of Garnet, Dakhla and Cape Blanc.
Sardine biomass in Zone C, north of Cape Blanc, estimated at between 3 and 5 million tonnes between 1986 and 1996, saw a marked drop in 1997. Since 1999, biomass in this region has gradually risen to its previous levels, being estimated at close to 4.5 million tonnes at the end of 2002. In the zone between Cape Blanc and Cape Timiris, sardine biomass was estimated to be around 670 000 tonnes in November-December 2002 (Figure 2.3.2).
The structure of the stock in November 2002 was dominated by adults, that is, over 70 percent of the stock measures over 20 cm in length. The number of individuals measuring less than 16 cm was relatively low compared to that observed the two previous years (end of November 1999 and end of November 2001) (Figure 2.3.3c). However, in the zone south of Cape Blanc (Mauritanian EEZ) the sardine stock was mainly composed of juveniles in November 2002, with a mode of 16 cm (Figure 2.3.3d).
Morocco
Biological sampling is carried out five times a week on the daily landings of the Moroccan coastal purse seiners operating in zones A, B and C in each of the main ports. The sampling takes into consideration the area where the catch is taken. The sampling scheme is as follows:
Systematic sampling of the purse seiners at a rate of one in ten.
Random sampling of 3 to 4 kg to measure the length.
Subsampling of five individuals per length group for biological studies and sampling of otoliths for age reading.
Mauritania
Sampling is carried out on board Dutch vessels by IMROP observers. Length measurements are regularly performed on samples of at least 20 kg. Occasionally, biological studies are carried out in which the individuals are weighed to the nearest gram and the fork length is measured to the nearest centimetre. The individuals are examined for sex, sexual maturity, stomach content and fat content.
Sampling intensities of Morocco and Mauritania in 2002 are shown in Table 2.4.1. Analysis of the data for each fishery shows that the sampling intensity is acceptable for Morocco due to the regular sampling at each of the main ports. For Mauritania, the effort of length measurements is considered to be sufficient, however biological sampling is low and age-determination is still lacking.
Three-monthly and annual length compositions for Moroccan sardine landings in Zones A, B and C are available for 2002 (Tables 2.5.1a, b and c). The annual age-length keys are also available for 2002 for Zones A+B and C (Tables 2.5.2a and b).
The weight-length ratios are calculated every three months and annually for Zones A+B and C. Biological data (length, weight, sex, and maturity) are available for 2002.
For Zone C the annual length composition of the Mauritanian fleet landings is available for 2002 (Table 2.5.3a). Fork length is measured to the centimetre below (FL, 1cm). In order to combine this length distribution with the Moroccan one (which is in TL, 0.5 cm), the Mauritanian length-frequency distribution has been converted into the Moroccan format by using the conversion factor LT (0.5cm.inf) = 0.195598 + 1.111756*LF (cm.prox) (CECAF series/PACE 90/50) (Table 2.5.3b).
Length distribution for the entire Zone C for 2002 was found by adding the Moroccan and Mauritanian length distributions (Table 2.5.4).
Data analysis
Zone A+B
The age composition of the catches used during the 2002 Working Group for the 1990-2001 series was used. Only the year 2002 has been updated (Table 2.6.1a).
The average weight-at-age is available for the same series (1990 - 2002) (Table 2.6.1b).
Zone C
The age composition of the catches used during the 2002 Working Group for the 1990-2001 series was used. Only the year 2002 has been updated (Table 2.6.2a).
The data series for 1990-1999 of the average weight-at-age for Zone C used during the 2001 Working Group was used.
For the years 2000-2001, an average was calculated of the two previous years. The weight-at-age in 2002 was calculated using the Moroccan weight-length ratio found for the same zone (Table 2.6.2b).
A preliminary analysis of the catch at age data available on sardine for the A+B and C stocks was carried out in order to check the quality of these data for stock assessment (Figures 2.6.3a and b; Figures 2.6.4a and b).
Analysis of the sardine stock exploitation diagram for the 1990-2002 series both in Zone A+B and Zone C shows that it changes significantly from one year to the next and from one cohort to the next.
Analysis of total mortality by age has highlighted a large variation with the figures alternating between positive and negative.
The anomalies in the data could be due to several factors such as: stock identity, poor or unrepresentative (/) sampling, the lack of age-length keys for some fleets, poor age reading, the change in fishing strategies of some fleets.
Methods, results and discussion
Despite the fact that the preliminary data analysis indicated poor quality of the data, the Group proceeded to make an assessment of stock C, but some problems were encountered in the assessment of sardine stock using the available data (1990-2002).
The method of integrated catch at age analysis (ICA) (Fournier and Archibald 1982; Patterson and Melvin 1995) was used. The analysis was carried out on the 1990-2002 series for Zone C and the period 1997-2002 was used for the separate VPA part. The results obtained by this method were not acceptable. The residuals calculated were very high for certain ages, and a well defined minimum of the target function was not found.
During the previous Working Groups (FAO 2001 and 2002), the assessments did not allow for solid conclusions about the status of the sardine stock or exploitation. Several limiting factors (mentioned above) affecting the quality of the basic data are the cause for these anomalies. Stock identification remains a fundamental problem for the monitoring of pelagic resources and consequently their management. The coherence of the stock dynamic requires firstly the identification of the different stock units.
The assessments based on analytical methods have not provided conclusive enough results to be able to make forecasts.
However, the results of the acoustic surveys show a rebuilding of the stock and as a consequence an increase in the mean lengths of sardine in Zone C since 2000.
As Zone B has seen intense exploitation over the last years and Zone C has been under-exploited, the Working Group recommends:
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[1] Number of positive trips =
number of trips with catches of sardine [2] RSW = Refrigerated Sea Water |
