Fish larvae
Geographic distribution and abundance

Fish eggs and larvae are only temporary members of the plankton, and their occurrence in samples is related to spawning activity in adult fish. But the geographic distribution of adults, eggs, and larvae do not always coincide. In many species spawning areas are strategically selected to assure survival of offspring, and juveniles and adults may migrate extensively to more productive feeding areas. Adult distributions and spawning seasonality are principal influences on the taxonomic composition of larval fish assemblages, as are also such hydrodynamic features as fronts, and water-mass properties (Moser and Smith, 1993). Obviously, ichthyoplankton distribution patterns are limited by the temporal and spatial coverage of sampling in any area. Surveys in the South Atlantic are dominantly from three oceanographic provinces: the Benguela Current, the Malvinas (=Falkland) Current, and the Brazil Current. The geographic distribution of fish larvae in the entire South Atlantic (between 0°-60°S) has been divided into 22 areas (22 geographic zones); references to the larval records in these 22 sectors are given in the Tables.

Most marine fish eggs are planktonic, with incubation times ranging, according to species and water temperature, from less than one day (22 hours) for Sardinella aurita in tropical seas (Matsuura, 1976) to 16 days for Theragra chalcogramma in the Gulf of Alaska (Blood et al., 1994). Similarly, the planktonic stages of fish larvae vary according to species and local temperature. For example, tropical coastal fish larvae attain the transforming stage in less than two weeks, but the pelagic larval stage of the european eel, Anguilla anguilla, takes more than one year to attain the glass eel stage (Bruun, 1963). On the average, we can assume that the incubation time of fish egg takes a few days, and the planktonic larval stage about one month. During this short period the early stages of marine fish are members of the plankton community.

Densities of fish eggs and larvae in plankton samples also vary considerably according to species, season, and location. For example, sardine (Sardinella aurita) eggs ranged from less than 0.1 mö-2 to 13,608 mö-2 in the southeastern Brazilian Bight during the 1981 spawning season (Matsuura, 1996). Mean densities of fish larvae integrated over the entire water column are not high, especially when compared with other zooplankton components. For example, mean larval densities of 0.17, 0.12, and 0.11 ind. mö-3 for Merluccius bilinearis, Limanda ferruginea and Scomber scombrus respectively were reported in the Sable Island area; while those of their food organisms, Oithona similis and copepod nauplii, were 2048 and 1642 ind. mö-3 respectively (Fortier and Villeneuve, 1996).

Although ichthyoplankton studies in the Eastern Atlantic started early in this century (Gilchrist, 1903, 1904), information on egg and larval distribution first became available in 1972 with the start of the SWAPELS project off Namibia, covering spawning locations and distributions of larvae of the most important commercial species (O'Toole, 1977a, 1977b, 1978a, 1978b). Investigations in this area between 1979 and 1986 by Spanish scientists also provided information on temporal and spatial distribution patterns of non-commercial species, and contributed to the knowledge of ichthyoplankton assemblages (Olivar, 1987d).

The CELP project of 1977-1978 off the western coast of South Africa supplied the best seasonal coverage and the best concurrent environmental data (Shelton, 1986). The coastal region between Angola and the southern tip of Africa is affected by the Benguela Current, which flows NNW and is responsible for upwelling of deep South Atlantic Central Water from 100 to 300 m. Nutrients from this upwelled water result in seasonally high productivity (Shannon, 1985). Variations in the upwelling regime strongly affect assemblage composition and abundances of component taxa. In the northern Benguela system upwelling is more intense during winter and spring. In the central Benguela system it is intense throughout the year, and in the southern region strongest in spring and summer. In general, chlorophyll a and zooplankton concentrations are lower offshore than in the upwelling zones (Kruger and Boyd, 1984; Shannon and Pillar, 1986; Estrada and Marrasé, 1987). Spawning strategies for the commonest species in the area tend to minimize the offshore Ekman transport caused by the upwelling and to maximize coincidence with potential food supplies (Olivar and Shelton, 1993).

The Benguela region is bounded by the Angola system to the north, and by the Agulhas retroflection area to the south. Waters from adjacent systems penetrate the Benguela region with different intensity depending on the season, reflected by the presence of larvae of species characteristic of warmer regions. Higher numbers of species are present along water mass boundaries, in offshore areas, and in the confluence of Benguela waters with those of boundary systems.

Distribution patterns of the most abundant ichthyoplanktonic species in the Benguela Current region are shown in Distribution and abundance ABC and Distribution and abundance DEF (Olivar and Shelton, 1993). Of six species in the figure, two are not economically important (Fig. A, B), but their larval abundance in some seasons outnumbered those of economically important species. Larvae of Sufflogobius bibarbatus are more abundant in the northern part of the Benguela Current during most of the year, while larvae of Lampanyctodes hectoris are abundant in the entire Benguela Current, mainly from 150 to below 1000 m (Shelton, 1986; Olivar and Fortuño, 1991). The dominant clupeiform species, including Engraulis capensis, Sardinops ocellatus, and Etrumeus whiteheadi, are distributed throughout the Benguela region, but spawn minimally in the principal upwelling area around Lüderitz (Olivar and Shelton, 1993). Apparently there are two spawning stocks of pilchard and anchovy: one in the northern region, and another in the southern region, including the Agulhas Bank. In the southern Benguela region the majority of species spawn off the west coast of South Africa during quiescent upwelling periods. However E. capensis spawns during spring and summer upwelling months, but the area of reproduction is the Agulhas Bank. Larvae are then advected by the jet current to the more productive west coast (Shelton, 1986; Shelton and Hutchings, 1990). Horse-mackerel, Trachurus trachurus capensis, is very abundant in the northern region and moderately abundant in the southern (Crawford et al., 1987). The main concentration of eggs and larvae is from 17°30' to 22°S, with lower concentrations off the southeastern coast of South Africa (Olivar and Shelton, 1993).

The western boundary current system in the southwest Atlantic is characterized by onshore Ekman transport and the subtropical convergence, where the southward flowing warm Brazil Current encounters the cold Malvinas (=Falkland) Current from the south (off Río de la Plata, around latitude 35°S). Based on analysis of meteorological and hydrographic data, Bakun and Parrish (1991) identified three oceanographic settings along the southwestern Atlantic coast (22°-47°S; see a coastal indentation, a wide continental shelf, a shelf-sea front system).

The geographic configuration of the southeastern Brazilian Bight (23°-29°S) is characterized by coastal indentations and coastal upwelling at the northern margin of the bight. During late-spring and summer the cool, deep, nutrient-rich South Atlantic Central Water (SACW) intrudes over the continental shelf, coming close to the coast. Penetration of the SACW into the euphotic zone enhances subsurface primary production. The strong thermocline between the cool SACW (near the bottom) and warm Coastal Water (at the surface) leads to fine-scale concentrations of food particles in the subsurface layer (a coastal indentation). Because of this oceanographic configuration, the southeastern Brazilian Bight provides excellent survival conditions during late-spring and summer, resulting in intensive spawning activities of not only sardine and anchovy, but also most marine fish (Matsuura, 1990).

The commonest ichthyoplankton species in the southeastern Brazilian Bight are Sardinella aurita, Engraulis anchoita, and Maurolicus muelleri. Distributions and abundances of these species during the survey cruise of January 1976 are shown in larva of Maurolicus muelleri, larva of Sardinella aurita, larva of Engraulis anchoita (Ribeiro, 1996). The former two species are concentrated in coastal and neritic regions, while M. muelleri is offshore near the shelf-break area. Eggs and larvae of S. aurita and M. muelleri are concentrated during late-spring and summer, whereas those of E. anchoita occur throughout the year. Total numbers of larvae of S. aurita, E. anchoita and M. muelleri collected during the survey cruise were 3.83 x 1012, 2.34 x l012 and 0.75 x 1012 respectively. Since the survey area did not cover the entire spawning area of M. muelleri, the real abundance of this species must be considerably higher than this, suggesting its ecological importance in the offshore pelagic habitat (Ribeiro, 1996).

The second subarea is found along the coast between Rio Grande and Baía Blanca (32°-41°S), where the surface Ekman transport tends to be directed onshore (a wide continental shelf). The onshore Ekman transport would serve to carry the shelf-break upwelling-based nutrients and enriched production onto the inner shelf habitat, where the continent itself serves as a shield from the large-scale westerly winds, resulting in relatively low wind-mixing rates. Under these conditions, relatively strong water column stability is favored by the confluence of different water masses associated with the various longshore boundary flows, by strong seasonal surface heating, and by substantial freshwater input (Bakun and Parrish, 1991). The area of water mass confluence, including fronts associated with freshwater outflows, represents convergence zones that may serve to concentrate distributions of larval food particles. Certainly the onshore-directed surface Ekman transport facilitates the retention of eggs and larvae within the shelf habitat.

The third subarea is found on the very wide continental shelf of the Patagonian region (41°-47°S). Strong tidal mixing occurs in the shallower areas over the continental shelf. This results in shelf-sea frontal formations of the type found in the Gulf of Maine-Georges Bank, and in the North Sea, leading to: (1) enrichment of the mixed zone from deep-water sources, (2) concentration of food organisms and other particles in convergent frontal zones, and in the zone of stability in the layer moving slowly offshore within the mid-depth thermocline, and (3) retention of larvae able to migrate vertically so as to use onshore flow in surface or bottom layers. On a larger scale, the general onshore wind-driven surface Ekman transport would prevent major loss of larvae from the shelf habitat (Bakun and Parrish, 1991; Bakun, 1993) (a shelf-sea front system).

Along the coasts of Uruguay and Argentina at least two subpopulations of Engraulis anchoita can be distinguished (Sánchez, 1989), corresponding to the second and third oceanographic configurations. The northern subpopulation extends its southern Brazilian range to the north, but not to exceed 41°S. Spawning takes place throughout the year, but more intensively during spring and summer, whereas the southern subpopulation spawns only during summer in the Patagonian region. Over its extensive range (spawning grounds,anchov. sardin) E. anchoita spawns successfully within three different environmental configurations, involving transport, water column stability, and trophic enrichment. This variability of spawning behavior is unique among small pelagic fishes.