Fish larvae
Vertical distribution

Fish eggs and larvae of the majority of species with known early development are distributed mainly in the upper 200 m of the oceans (Ahlstrom, 1959; Röpke, 1989; Zelck and John, 1995). However, they commonly occupy discrete depth intervals in the water column, only a few species being evenly distributed throughout the column.

The following patterns of vertical distribution may be defined:

—Surface pattern
Species mainly between the surface and 50-60 m, and very scarce below. Eggs and larvae of many species of the genus Engraulis exhibit this pattern (E. anchoita, Matsuura and Kitahara, 1995 ; E. capensis, Shelton and Hutchings, 1982; E. encrasicolus, Palomera, 1991; E. mordax, Ahlstrom, 1959; E. ringens, Gorbunova et al., 1985). Pilchard (Sardina pilchardus and Sardinops spp.) and horse-mackerel (Trachurus spp.) eggs and larvae also adhere to this pattern (Ahlstrom, 1959; John, 1985; Gorbunova et al., 1985; John and Ré, 1993).

—Subsurface pattern
Species mainly below 20 m, as in Merluccius capensis and Sufflogobius bibarbatus (Olivar et al., 1992). This pattern is also shown by larvae of other Merluccius species (Kendall and Naplin, 1981; Röpke, 1989) and other gobies (Southward and Barrett, 1983). The larvae of some gonostomatid species (Vinciguerria poweriae, Ichthyococcus ovatus, Valenciennellus tripunctulatus, Gonostoma atlanticum, G. elongatum) also follow this pattern (Loeb, 1979).

—Deep pattern
Species at between 100 and 200 m, such as eggs and larvae of Bathylagidae (Olivar et al., 1993), Stomiiformes and Argentinidae (Ahlstrom, 1959; Masó and Palomera, 1984; Olivar, 1990).

—Broad vertical distribution
Shown by some species. Early stages of the Soleidae found in the northern Benguela region spanned almost the entire water column of the coastal area (surface to 80 m). Eggs and larvae of the lanternfish Lampanyctodes hectoris and the lightfish Maurolicus muelleri were fairly widely distributed within the upper 200 m of the water column (Olivar et al., 1992). However, there is evidence of an even wider (up to 400 m) vertical distribution of M. muelleri eggs and larvae (John and Kloppmann, 1989).

Even in rather highly productive areas of the ocean, food levels over most of the volume of the habitat may not be high enough for fish larvae to meet their trophic needs (Bakun, 1996). Lasker (1975) first recognized that the average concentration of appropriate food particles in the spawning habitat of the California Current northern anchovy is too low to support successful early larval feeding. Therefore, good survival of anchovy larvae depends on concentration of food organisms in the subsurface layer, corresponding with the chlorophyll-a maximum layer below the thermocline.

Subsequently, many authors have acknowledged the ecological importance to larval survival of processes concentrating food organisms. Fortier and Harris (1989) studied the ontogenetic migrations of five species of fish larvae in the western English channel. They observed that yolk-sac and non-feeding larvae were concentrated in the surface layers, whilst larger larvae were highly correlated with that of their prey, concentrated in bottom layers. In their study, fish and zooplankton species preying on copepods were distributed in proportion to copepod concentrations, whilst species feeding on non-copepod prey did not conform to this pattern. They calculated that larval fish alone cannot significantly deplete their prey by foraging, but the entire foraging community of fish plus zooplankton could exert an impact.

Shelton and Hutchings (1990) demonstrated that enhanced microzooplankton and chlorophyll-a concentrations associated with a strong thermocline during the summer season on the Agulhas Bank, provided conditions favorable for anchovy larvae. Viñas (1991) observed that feeding incidence of anchovy larvae (Engraulis anchoita) in the tidal front off the Valdes Peninsula was higher in the transition and stratified zone, where higher concentrations of microzooplankton were observed in the subsurface layer. Matsuura and Kitahara (1995) showed that, at the post-flexion stage, anchovy larvae (E. anchoita) moved to deep water below the thermocline, where high chlorophyll-a concentration was observed. And their survival rates were higher than those of pre-flexion and flexion stage larvae, which were distributed in the upper mixing layer.