Fishes characteristic of an upwelling area are not caught outside it. As the stocks are maintained within it from year to year, it is reasonable to suppose that an upwelling area is a biological unit. The fact that the water necessarily drifts through the area does not mean that the biological structure is a temporary one, but that the fish stocks do maintain their position in it from year to year. Off California, hake spawn in the early spring off the southern area of upwelling (Ahlstrom 1966). The older fish appear off the coasts of Oregon and Washington in June (Alverson, 1967). As they live mainly in deep water (i.e. 200 m), they nay migrate north in the countercurrent. To spawn off Baja California, the hake may return south in winter in the surface drift, perhaps at night when the hake are known to migrate towards the surface. Presumably the eggs and larvae drift south from Baja California and metamorphose fairly quickly to sink into the countercurrent, which persists until the summer. Then the nursery area nay lie at a middle depth off Central California. So the fish might live in the upwelling area all through their lives. Although this argument is in detail speculative, the main points emerge from a figure in Alverson (1967).
The Californian sardine spawns right in the coastal upwelling zones, as physically defined, off Punta S. Eugenio and between Point Conception and San Diego. The anchoveta in Peru spawns between Punta Aguja and San Juan in late winter to early summer, when the Ekman transport is most intense (Wooster and Reid, 1963). The greatest densities of eggs are found within 100 km of the coast where the radiocarbon values are highest (Forsbergh and Joseph, 1964). So the Californian sardine and Peruvian anchoveta spawn near the line of upwelling. They live in or near the thermocline and so cannot make use of the Countercurrent in their migration. However, there is sometimes a very narrow coastal Countercurrent during the season of upwelling (Reid, Roden and Wyllie, 1958; Wyrtki, 1963). The fish spawn in late spring and early summer. Hence one might expect them to drift away from the spawning ground close to the coast in the coastal Countercurrent. Then the fish can return to the spawning ground in the surface drift of the California Current. There were three groups of sardine off California, perhaps one in each upwelling section. (Clark and Marr, 1955).
There is a remarkable phenomenon in the Peru Current upwelling. It is that the spawning anchoveta (as shown in the egg distributions) appear to exclude the zooplankton, Figure 16. The inverse correlation is not exact, which may suggest that there is a time lag in its generation. There are alternative explanations: (a) that the spawning anchoveta eat algae (as its relative, Centengraulis mysticetus does in the Gulf of Panama, Bayliff, 1963) and that the anchoveta aggregate on the algae, competing with the zooplankton; (b) that the anchoveta, like the European anchovy, selects small copepods, but traps algae on its narrow gill rakers; then the spawning anchoveta has eaten the zooplankton animals before they spawn. The latter explanation implies that the anchoveta is at the third trophic level. The stripping of zooplankton from the water by the anchoveta may allow a new production cycle to start after the fish disperse from the spawning ground. This process resembles that postulated in an intermittent upwelling area, as suggested above.
Figure 16. The distribution of anchoveta eggs and of zooplankton off Peru. (Flores, 1967; Guillen and Flores, 1967)
Figure 16 (a). Distribution of zooplankton in ml/haul, displacement volume
Figure 16 (b). Distribution of anchoveta eggs in numbers/haul with a Hensen net
The vertical distribution of animals in an upwelling area has a special interest. Figure 17 shows the distribution of echo traces off Peru, presumably of anchoveta, in the thermocline (Flores and Elias, 1967). The thermocline is rather weakly defined, but the fish remain in the upper part of it.
Figure 17. The distribution of echo traces in the thermocline off Peru (Flores and Elias, 1967)
Sardines, sardinellas and anchovies live at or just above the depth of the thermocline. Below the thermocline live horse mackerel, probably in extensive and rather dispersed shoals. On the bottom at any depth of water from 50 fathoms outward hake live close to the bottom, perhaps depending to some extent upon euphausiids, the biology of which has been little studied in an upwelling area.
In this section, some of the biological problems in an upwelling area have been posed in a speculative manner. They have been grouped together because it is thought that any upwelling area is a biological unit, worthy of investigation in itself. Because the fisheries in the upwelling areas are large and important, the methods of fisheries biology applies to upwelling areas should yield results of wider import.
