Rapp. P.-v. Réun. Cons. int. Explor. M er, 180: 356-358. 1982.

Kinds and distribution of fish eggs and larvae off Northwest Africa in April/May 1973

I. Palomera and P. RubiésInstituto de Investigaciones Pesqueras, P° Nacional s/n, Barcelona, Espana

Introduction

During the oceanographic cruises ATLOR III (April- May 1973), ATLOR V (April-M ay 1974), and ATLOR VI (October 1975), on board RV “Cornide de Saavedra”, intensive sampling was carried out, directed to the study of the ichthyoplankton of the con­tinental shelf between Cape Bojador and Cape Blanc. In this paper, results from the first expedition, con­ducted between Cape Bojador and Point Durnford, are presented. The samples taken during the other two cruises are at present in course of analysis.

The hydrographic conditions and other oceano­graphic data are reported by Cruzado (1974) and Cruzado and Manriquez (1974). Experimental bottom- trawl hauling data are given by Lloris et al. (1978).

Material and methods

A total of 46 oblique hauls were made with a 60 cm diameter Bongo device fitted with 333 |.im and 505 |xm mesh size nets. The hauls were made from 300 m to the surface, or from near the bottom at stations not reach­ing that depth. The ship speed during the haul was 2 knots, and the fishing gear was hauled at 10 m/min when the station depth was 100 m or less, and at 20 m/min in all other cases.

The amount of filtered water was estimated by means of TSK flowmeters placed at the mouth of each net.

The samples were preserved in 6 % formalin, and fish eggs and larvae were sorted out at the laboratory under stereo-microscope. In the samples in which fish eggs and larvae were very abundant only one aliquot has been studied.

Kinds and distribution of eggs and larvae

Some 65 taxa could be distinguished. However, only a small number of species were well represented in the samples.

As a prevailing component of the ichthyoplankton in that area and at that time, the sardine (Sardina pilchar-

dus) represents nearly 94 % of all larvae examined. Its distribution and other aspects have been studied with more detail in a previous paper (Rubiés and Palomera, 1977).

It is possible to distinguish fairly clearly between fish with neritic spawning as opposed to oceanic habits, as follows.

Neritic species

The eggs and larvae of neritic fish are usually found along the continental shelf, with a maximum abund­ance located in most cases between the 50 and 100 m isobaths. The eggs and larvae of these fish are also frequently collected over the edge and farther beyond it, especially the more advanced larvae. This is due to the drift of larval stages caused by the Ekman trans­port, as has been stated previously (Blackburn and Nellen, 1976; Rubiés and Palomera, 1977).

Besides the sardine the main ichthyoplankton com­ponents of the neritic fauna are: the anchovy (Engraulis encrasicolus), which ranks second in abund­ance after the sardine, the Sparidae (several kinds of larvae that we did not try to distinguish), two species of Trachurus (T. trachurus and T. sp.), mackerel (Scomber japonicus), Centracanthidae, and the Sol- eidae species Buglossidium luteum, Microchirus azevia, and Microchirus ocellatus. The early stages of the last two species have been described previously (Palomera and Rubiés, 1977).

Among the rarely found components of the neritic ichthyoplankton the following should be cited: Merluc- cius Sp., Capros aper, Serranus cabrilla, Mullus sur- muletus, Scomber scombrus, Sphyraena sphyraena, Microchirus variegatus, and unidentified species belonging to the families Labridae, Ammodytidae, Gobiidae, Callionymidae, Blenniidae, and Soleidae, and to the order Anguilliformes.

Oceanic species

In the oceanic group we include the species whose eggs and larvae are collected over the continental edge or

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farther. Special mention must be made of the speci­mens of Gonostomatidae and Myctophidae, which con­stitute a fundamental link in the pelagic ecosystem.

Gonostomatidae: The most abundant species has been Maurolicus muelleri, which ranks third among the total larvae caught. Also abundant were species of the genus Vinciguerria (V . poweriae and V. attenuata), fol­lowed by Cyclothone braueri, Gonostoma sp., and other unidentified species.

Myctophidae: The rule here is a high specific diver­sity, in which no species is clearly dominant. In decreasing order of abundance we found: Cerato- scopelus maderensis, Notoscopelus sp., Diogenichthys adanticus, Hygophum reinhardti, Lobianchia gemel- lari, Benthosema suborbitale, M yctophum punctatum, Lamp any ctus sp., Benthosema glaciale, Symbolo- phorus veranyi, Hygophum sp., Hygophum benoiti, and Myctophum sp., with other unidentified indi­viduals.

Apart from these two groups, two Trichiuridae (Trichiurus lepturus and Lepidopus caudatus) and a Bothidae (Arnoglossus thori) appeared frequently. Other species which have been caught only sporadi­cally are: Argyropelecus hemigymnus, Chauliodus sloani, Bathylagus sp., Scopelosaurus sp., Lestidiops sp., Belone belone, Scomberesox sp., Macrorham- phosus sp., Zeus f aber, and Ophidion barbatum, with unidentified species belonging to the families Scopelar- chidae, Paralepididae, Macrouridae, and Triglidae, and to the order Beloniformes.

Ecological remarks

Considering the presented results from a general point of view and relating them to the environmental condi­tions found during the cruise, two main aspects should perhaps be pointed out: a) the specific diversity in the samples, and b) the general distribution pattern of fish eggs and larvae in the area.

a) Even disregarding the sardine, whose high abund­ance reduces to a minimum the specific diversity in the samples when it appears, the diversity is extremely low at the more shallow stations, as has also been remarked by Nellen (1973) near Cape Blanc.

Diversity increases slightly in an offshore direction, but its values remain moderate all over the continental shelf, increasing considerably at the edge, mainly owing to the high diversification introduced in the sys­tem by the Gonostomatidae, Myctophidae, and other mesopelagic fish.

b) Spatial distribution of fish eggs and larvae shows a strong coincidence in almost all species: the presence of fish eggs and larvae is really important in the northern part of the area (between Cape Bojador and Pena Grande); a less important nucleus is observed in the south (between Point Leven and Point Durnford). There is a very poor area between both abundance

nuclei. This pattern has two exceptions: Engraulis encrasicolus and Arnoglossus thori are more abundant in the south than in the north, but these species are also practically absent from the central area.

Although the upwelling in this region is an active phenomenon throughout the year, it has no definite geographical location with respect to time, moving its nuclei in latitude. During the A TLO R III cruise, the upwelling was located in the coastal zone between Point Leven and Point Durnford, as is shown by the temperature and chlorophyll distributions (Cruzado, 1974), roughly concurring with the area poor in eggs and larvae. The isopycnes also suggest that another upwelling center could exist to the north of Cape Bojador, beyond the area studied.

It seems obvious that most fish seek ecologically advantageous reproductive conditions, looking for places with a favourable spawning environment. These places are found at the outer boundaries of the intense upwelling regions, where sufficient time and water sta­bility allow the blooming of important phytoplankton and zooplankton populations, which are essential for the survival of early larvae (Lasker, 1975; Biester and Jönsson, 1975). Actually, the highest densities of eggs and larvae occur in the areas where the highest plank- tonic biomass has been found.

Consequently, we may conclude that in a system as unstable as the present one, there is little sense in try­ing to delimit in space or time the fixed spawning areas of a given species, and it seems quite probable that the location of the spawning grounds depends upon the geographical fluctuations of the system.

References

Biester, E., and Jönsson, N. 1975. Ichthyoplankton in the fishery area off Cap Blanc 1971-1974. ICES CM 1975/J: 15, 6 pp. (mimeo).

Blackburn, M., and Nellen, W. 1976. Distribution and ecol­ogy of pelagic fishes studied from eggs and larvae in an upwelling area off Spanish Sahara. U. S. Fish. Bull., 74: 885-896.

Cruzado, A. 1974. Coastal upwelling between Cape Bojador and Point Durnford (Spanish Sahara). Téthys, 6: 133-142.

Cruzado, A., and Manriquez, M. 1974. Datos hidrogrâficos de la campana ‘ATLOR III’ en la regiôn de afloramiento entre Cabo Bojador y Punta Durnford. Res. Exp. Cient. B/O Cornide, 3: 89-115.

Lasker, R. 1975. Field criteria for survival of anchovy larvae: the relation between inshore chlorophyll maximum layers and successful first feeding. U. S. Fish. Bull., 73: 453-462.

Lloris, D., Rucabado, J., Fusté, X., Allué, C., and Bas, C. 1978. Ärea de afloramiento del NW. de Africa. Campanas ‘ATLOR III’ (1973) y .‘ATLOR V’ (1974) - Cabo Bojador (26°10'N) a Cabo Blanco (21°45'N) - Pescas de arrastre de fondo. Datos Informativos Inst. Inv. Pesq., 4: 1-247.

Nellen, W. 1973. Kinds and abundance of fish larvae in the neuston of patches of upwelled water off West Africa. ICES CM 1973/J:25, 11 pp. (mimeo).

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Palomera, I., and Rubiés, P. 1977. Descripciön de huevos y larvas de Microchirus ocellatus y M. azevia (Pleuronec- tiformes, Soleidae) de las costas del NW de Àfrica. Res. Exp. Cient. B/O Cornide, 6: 211-220.

Rubiés, P., and Palomera, I. 1977. Abundance and distribu­tion of sardine eggs and larvae off Northwest Africa, April- May 1973. ICES CM 1977/L:7, 8 pp. (mimeo).

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