4.1 General
4.2 Number of vertebrae
4.3 Electrophoresis studies
4.4 Growth
4.5 Length-weight relationships
4.6 Spawning
4.7 Distribution and migration
4.8 Conclusions on stock separation
The 1976 working party meeting considered the possibility that the three zones contained different stocks but agreed that the available information did not allow them to draw conclusions on this question. The assumption of one stock in the area, or of separate stocks, may greatly affect the conclusions on the state of exploitation of the sardine. The Ad hoc Working Group, therefore, in discussing the biology of the sardine, paid particular attention to those factors which might provide further information relevant to stock separation.
The available observations are summarized in Table 1. These data show that, whereas there exists a statistically significant difference between the average number of vertebrae from around Casablanca and from the area further south, no significant differences are observed between the other areas.
Biaz (1976) has developed a biochemical method to identify the sardine populations from the various fishing zones by electrophoresis of the soluble proteins from the nucleus of the crystalline lens of the eye in a gel of polyacrylamide. His preliminary results suggested significant differences between the fish in the Casablanca area and in the area between Safi and Cape d’Aglou (Zone A), and also between the latter fish and the fish in the area between Ifni and El Ayoun (Zone B).
Data on growth parameters in different seasons and fishing zones were presented by Belveze and Rami of the Moroccan Institut scientifique des pêches maritimes, and by Krzeptowski, Poland, The available data show a fairly wide range of values (Table 2). Part of the differences appear to be related with the method of determining length at age, lengths back calculated from scale readings giving lower L¥ and higher K values than direct observations. There seems to be a tendency for samples from more southern areas to give higher L¥ values than those from further north. It should be noted, however, that these values may also be affected by differences in the age composition in the catches from the various areas. Furthermore, a difference in growth parameters was found between various sardine cohorts in the same area. In general, a fairly wide variance is observed in the figures obtained with the same method for the same zone.
In view of these uncertainties, the group decided to use a range of values for K and L¥ in the considerations on potential yield and state of exploitation of the sardine resources.
More detailed studies, comparing also the actual figures of length at age in the different areas, would be helpful to determine the existence of real growth differences as between regions.
The length-weight relationships of the sardine in the various zones are given as follows:
Zone A (Rami, unpublished data for second quarter 1977)
|
W = 1.62 x 10-6 |
L 3.32 |
(N = 700) |
Zone B (Bravo de Laguna et al., 1976. Data from April 1975 till March 1976, based on fish between 10 and 26 cm total length)
|
W = 0.6 x 10-6 |
L 3.47 |
(N = 998) |
Zone C (Krzeptowski 1977, data average 1973-1976)
|
W = 9.76 x 10-3 |
L 2.996 |
(N = 3700) |
According to Domanevsky and Barkova (1976) there are two spawning seasons for the sardine. The major spawning takes place in winter and a weaker peak in spawning occurs in spring. Spawning is more prolonged along the Sahara coast. In general, some mature females (stage VI according to the Maier scale) are found in samples all the year round, especially in the Sahara coast area. Krzeptowski (personal communication) observed three spawning seasons along the Sahara coast, November-December, March - April and August-September.
Information presented to the meeting indicated that in Zone A the main spawning takes place from October to February and a minor spawning peak occurs between April and May.
Two spawning seasons are also observed in Zone B (Bravo de Laguna et al., 1976), the major one between November and February with peaks in December (45%) and January (60% of females mature), and the minor one in May.
An egg and larvae survey conducted in April-May 1973 revealed concentrations of eggs south of Cape Bojador (Rubies and Palomera, 1977).
The results of echo-surveys reviewed in the report of the Third Session of the Working Party on Resource Evaluation appeared to confirm the possibility of a separation between the stocks in Zone A and Zone B. An interruption in the distribution of sardine was observed near Cape Anglou, somewhat south of the Bay of Agadir. During a new survey (Lamboeuf, January 1977) three concentrations of sardines were found, the most important one between 27° N and Cape Juby. The second concentration extended from “Puerto Cansado” to the mouth of the Oued Brâa. The third concentration, covering a small area but representing an important biomass was observed near Sidi Ifni. No interruption of continuity in the distribution was found between the Bay of Agadir and Cape Juby, and dispersed small concentrations were observed between Agadir and Safi. No sardine was found between Safi and Casablanca. There are, thus, substantial differences in the distribution and the number of concentrations observed during different surveys. The available observations do not provide a basis for conclusions on stock separation.
The seasonal differences in the fisheries in the various zones, however, suggest that considerable migrations take place. From the work of Belveze (1972) and Bravo de Laguna et al. (1977), who described the activities of the different fleets operating in the area, it would appear that in spring, young sardines (mainly one to three years old) move north through Zone A. This movement produces seasonal fisheries off the coast of Morocco. The first high catches are obtained off Agadir (May, June and July), and later high catches are made off Essaouira and Safi (July and August). During the second half of the year the sardines seem to migrate southward to the spawning grounds, increasing once more the yields off Agadir (October). The disappearance of the sardine in fishing Zone A coincides with substantial increase in catches of fish of the same zones (23-26 cm) in Zone B. Belveze also considers that due to continuous upwelling off Cape Juby, a part of the sardines remains in the fishing Zone B throughout the year.
Data presented by Krzeptowski, referring to the period 1972-1974, indicate that fishing in Zone C takes place in the southern part of the Zone (between 23° and 25° 30’ N) in January, and starts moving northward in June, to reach the area around Cape Bojador in August. A spawning ground has been found south of Cape Bojador.
The observations on number of vertebrae, on electrophoresis differences and perhaps also on growth suggest that there exists a certain amount of stock separation. The vertebrae counts and the electrophoresis differences of the sardines caught off Casablanca and somewhat further north indicate that these fish might belong to a small separate stock, different from the fish further south in Zone A. The electrophoresis and growth studies suggest that there may also be a certain stock separation between A, B and C. However, there is no clear evidence of such separation and observations on the seasonal changes in catch rates in the various zones, reported in section 3.5, suggest that there may well be substantial migration from one zone to another.
There is need for more information on the migration, mixing and separation of the fish in the various areas. Therefore, the group believed that it would be important to carry out tagging experiments. This would first require the development of feasible tagging and recovery methods.
