Global Eastern Boundary Currents

Parrish et al. (1983), review the four major global eastern boundary currents in terms of physical characteristics and factors influencing reproductive success in sardines and anchovies, species of which are present in all EBCs. The California, Peru-Chile, Canary and Benguela Current Systems feature similar environmental dynamics, characterized by narrow continental shelves, equatorward wind stress, coastal upwelling, meandering equatorward surface flows, subsurface poleward undercurrents and cool, highly productive coastal waters. Despite the fact that these are large systems, usually with more than one region of maximum upwelling, separated by regions of lower upwelling, often due to coastline variations (the Southern California Bight, the change in coastal orientation off southern Peru, the region between southern Portugal and Northwest Africa), Parrish et al. (1983) were able to make a number of statements which generalize EBC systems.

Collectively, EBC systems account for approximately 35% of the total marine fish catch (FAO Yearbook of Fishery Statistics, 1990). The dominant fisheries are listed in the table below, from Parrish, et al. (1983). The primary analogous stocks that are discussed in the literature are the clupeoids (sardines, anchovies, i.e., Sardinops, Sardina and Engraulis), although there are also similarities between the Merluccius (hake), Trachurus (jack mackerel), Scomber (mackerel) and Sarda (bonito). The last two groups are not as numerous. In contrast, there is little similarity between demersal fish species in eastern boundary currents, except for the Merluccius.

Similar to the fish assemblage, the copepod assemblage of eastern boundary current ecosystems is often dominated by two or three small and one or two large calanoid species. Small species are: Pseudocalanus mimus, Acartia hudsonica and A. longiremis (Oregon); Paracalanus parvus and A. tonsa (southern California); P. parvus and A. tonsa Peru/Chile; P. parvus, P. crassirostris and Ctenocalanus vanus (Benguela). The large calanoid in each system is a Calanus species: Calanus marshallae (Oregon), C. pacificus (California), C. chilensis (Peru, Chile), and C. australis (Benguela, Agulhas Bank). Conspicuous in all systems is the cyclopoid, Oithona similis s.l., and often an Oncaea spp.. In addition, an intermediately sized "omnivorous" genus, Centropages, is abundant off Oregon (C. abdominalis), Peru, Chile and South Africa (C. brachiatus), Northwest Africa (C. chierchiae), but is missing off California. Sometimes in the CCS, other large calanoids (e.g., Metridia or Eucalanus spp.) can be locally abundant. The South American and African systems each contain an additional dominant genus, Calanoides-C. patagoniensis (Chile) and C. carinatus (Northwest Africa, West Africa, Benguela, Brazil?, Arabian Sea).

Parrish et al. (1981) argue that the fisheries of the CCS are not limited by the system's carrying capacity, but by environmental fluctuations. Lluch-Belda et al. (1989) have shown that the long-term temporal variability of the sardine catch during the 20th century is similar for the California and Peru-Chile systems, as well as for the Japanese system, with some similarity to the mean ocean temperature, as well. The Pacific sardine catches do not covary with the North Atlantic sardine catch, however. Increases in the yield of Chilean sardines in the early 1980s are attributed to increases in primary and secondary productivity associated with water mass shifts in the Humboldt Current (Sherman, 1992). As described above, a decline in Pacific sardines in the California Current and a subsequent increase in anchovy abundance are believed due to natural environmental variability rather than species competition. Likewise, abundance shifts in horse mackerel and sardines in the Iberian Coastal Ecosystem, and horse mackerel and pilchard in the Benguela Ecosystem, are attributed to natural environmental fluctuations associated with changes in horizontal circulation and upwelling. Thus, for the most part, the abundances of major fish populations in EBCs are determined by large-scale environmental factors, not density-dependent factors.

Eastern boundary currents are forced by both large-scale 'local' wind systems and by basin-scale circulation patterns. Where the forcing is dominated by the atmospheric surface forcing, these ecosystems may reflect climate changes that develop in the atmosphere (e.g., wind forcing, coastal cloud and fog formation, air-sea heat exchange) long before direct changes occur in the ocean (e.g., large-scale changes in stratification due to global warming (Bernal 1993), changes in basin-scale circulation, etc.). Although the CCS appears to be more strongly driven by the winds over it, this may not be true of all subregions of all EBCs. Thomas et al. (1994) use satellite color imagery to compare surface pigment concentrations in the Peru-Chile Current System and the California Current System. The strong upwelling region off Peru shows high values when upwelling winds are strong (winter), as expected, while off Chile at 40 deg S, there are low surface pigments in summer when winds are most upwelling favorable (but weaker than off North America), and higher values off Chile in winter. In the Benguela System, high pigment values are more closely in phase with the winds, as they are off Northwest Africa. They are out of phase, however, off Portugal, where winds are upwelling favorable, but again are weak in summer, like off Chile.

In the Peru-Chile Current System, there is a poleward countercurrent (often called the undercurrent, though it often extends to the surface) off southern Peru and northern Chile (Fonseca, 1989; Huyer et al. 1991), that lies offshore of the narrow, equatorward current next to the coast. Fonseca (1989) comments that this countercurrent is often the dominant feature off northern Chile. Off Peru at 10 deg S, Huyer finds only a very shallow equatorward current, inshore of both an undercurrent and a countercurrent. Lukas (1986) has shown both the undercurrent and countercurrent off Peru at 6 deg S to be connected to the equatorial undercurrent. Thus, the region off Chile may be more strongly forced by basin scale currents than by local winds, which are weak (compared to Peru and North America). Since climatic changes such as global warming or the natural oscillations already discussed may change both basin-scale currents and local winds, comparisons between present global EBCs may be an efficient way to examine the responses of EBC ecosystems to different levels of these two types of forcing.

Bakun and Parrish (1982), Parrish et al. (1983) and Hutchings (1992) make the case for comparative studies in EBCs from an ecosystem standpoint. Comparative studies may reveal the environmental factors controlling reproductive success of these fisheries and the potential impact of climate change. They consider the dominant physical factors for reproductive success in sardines and anchovies to be transport, turbulence and temperature (seasonal or upwelling). There are also biological factors, such as food, predation and population density that may be important for some species, but are argued, above, to be less important for sardines and anchovies. This allows Parrish et al. (1983) to use a comparative analysis approach that seeks to differentiate between these few factors, by looking at common patterns of variability in eastern boundary currents. The use of more than one system increases the significance of trends and patterns over the "noise" level of individual ecosystems, giving more robust results (more degrees of freedom). An example is their attempt to differentiate between turbulence and offshore wind-driven Ekman transport effects on fish populations. In most cases, winds are equatorward in these systems, so high winds lead to both high offshore transport and increased turbulence, making it difficult to distinguish their effects. They are able to find instances, however, where the sardines choose to spawn in regions of increased turbulence-in those cases the transport is directed onshore, suggesting that turbulence is less detrimental than offshore transport. This demonstrates the power of comparative studies to provide enough instances to differentiate between factors.