Ecosystem variations induced by El Nio events

Shortly after the beginning of the development of industrial fisheries in the coastal regions bordering the Humboldt Current, it was discovered that oceanic variations along the coasts of Peru and northern Chile had a noticeable impact on living marine resources. El Niño phenomena in 1972-73 and 1982-83 revealed the severity of their impacts on fish availability and accessibility as well as on the vulnerability of certain species and the resilience of others. Furthermore, the relatively minor ENSO events of 1977 and 1986 had the marine scientists focus on the long-term effects of oceanic-climatic variabilities on fish population dynamics. Before the question of climate-induced change is addressed, it is necessary to identify the impacts of previous variations on the ecosystem of the Humboldt Current, as recorded in reliable statistics.

Prior to 1972, the areas of major anchovy concentrations along the coast of Peru coincided with upwelling centers located at latitudes 8°, 10°, 11° and 12°S. Since 1973, the centers of concentration have shifted to the south of latitude 14°S and into Chilean waters south of latitude 18°S (Santander & Flores, 1983). At the same time, there has been a noticeable increase in the sardine populations between 5°S and 12°S, an increase which Santander and Flores attributed to a warming of coastal waters and to the migration of high-seas sardines into this warmer than usual coastal environment. In terms of capture distribution, sardines have steadily grown into the mainstay of Peruvian fisheries, except for a temporary contraction between 1983 and 1985 (Fig. 15.6). Furthermore, along with the decrease of anchovies and bonito (Sarda chilensis) after 1973, Peruvian industrial fisheries began to rely increasingly on jack mackerel (Trachurus murphyi) and hake (Merluccius gayi), but at levels far below the traditional high landings of anchovies and sardines (Barber & Chavez, 1983), or the catches from central and southern Peruvian ports.

The spatial and volume variations of captures in northern Chilean waters differ from their Peruvian counterparts, particu-

YEAR

^Anchovy E3 Sordines 0 Jock Mackerel H Mackerel HHoke

Fig. 15.6 Species contribution to Peruvian catch, 1973-87.

YEAR

^Anchovy E3 Sordines 0 Jock Mackerel H Mackerel HHoke

Fig. 15.6 Species contribution to Peruvian catch, 1973-87.

° /o 1974 75 76 77 78 79 8081 82 83 84 85 86 871988

YEAR

R1 Sardines Jock Mackerel ilAnchovy ^ Mackerel

Fig. 15.7 Species contribution to Chilean catch, 1973-87.

larly after the 1972-73 El Niño (Fig. 15.7). The dominance of anchovies in pre-1972 landings was replaced by a surge in jack mackerel, a fish that thrives in habitats with water temperatures between 10° and 18°C, and in sardines that live under the same ocean conditions as jack mackerel, their major predator (Méndez & Neshiba, 1976).

In the early 1980s, the biomass of northern Chilean waters (specifically, the fishing districts of Arica and Antofagasta) was estimated at above 13 million mt, with jack mackerel accounting for 6 to 8 million mt and sardines for about 5.5 million mt (Castillo et al., 1986). This situation was drastically altered by the 1982-83 El Niño which reduced the stock of both jack mackerel and sardines to 3.5 million mt each. By 1986, the captures of jack mackerel had dropped to 100,000 mt, and those of sardines to 600,000 mt. Coin-cidentally, anchovy catches rose to 1.3 million mt, and surveys of

anchovy eggs and larvae revealed a surprising rise in the anchovy stock, the capture of which had been prohibited in 1983 to allow for its recovery (Martinez, 1987).

According to Chilean oceanographers and fisheries specialists, since the warming episode of 1982-83 off the coast of the northernmost fishing district of Arica and Antofagasta, a persistent intrusion of subtropical waters with high salinity and low oxygen content has created strong saline and thermal fronts extending 30-60 km from the coast. Upwelling foci have been restricted to coastal pockets (Martinez, 1987), and the general conditions have been favorable for sardines and jack mackerel, as shown by the volume of their landings. South of this district conditions were normal as reflected in the stability of the fishery's output. This situation continued until the moderate 1987 El Niño warmed the tropical and subtropical Pacific, altering once again the oceanic conditions of the Humboldt Current (Pizarro, 1987). This El Niño, however, did not have serious consequences for the development of the fisheries. Anchovies spawned normally between latitudes 11° and 18°S and adult exemplars were caught between latitudes 10° and 14°S. Sardines were also captured at levels considered normal for the winter season, and spawning occurred mostly at latitudes 13° and 15°S (Zuzunaga et al., 1988).

These oceanic and biological events ceased in the late fall of 1987 (ERFEN, 1987), when surface water along the coast of western South America began to be affected by a severe cooling episode (i.e., an anti-El Niño) that lasted through the spring of 1988, and continued into the southern summer of 1989 (ERFEN, 1988, 1989). This cooling episode terminated the warming trend of the Southeast Pacific that had been observed since the early 1980s.

There is some debate about whether the anti-El Niño of 1988-89 was only a temporary interruption of the assumed warming trend or if such a trend was just one of the many oscillations that characterize ocean-atmosphere systems in the Southern Hemisphere (Ramage, 1987). Disregarding for a moment the discussion about the transitory or long-term nature of such change, it seems evident that the 1972-73 El Niño introduced an element of disruption into the thermal regime of the Humboldt Current and that a return to "normal" (i.e., to pre-1972 conditions) has not yet occurred because of the warming episodes of 1976-77, 1982-83, and 1986-87. The 1972-73 disruption triggered an alteration in the latitudi-

nal distribution of traditional stocks and also in the availability of different species. In the years following the 1972-73 El Niño, coastal upwelling species, such as anchovies, have been replaced by offshore oceanic species, such as sardines and jack mackerel. Confined to isolated pockets of upwelling along the coast from the Gulf of Guayaquil to Valdivia (Chile), anchovies have weathered unfavorable warming episodes and appear to be making a comeback (Sharp & McLain, 1991). A successful resurgence of the anchovy may possibly be enhanced by the generalized cooling conditions off the western coast of South America that began in 1988 (Caviedes, 1989; ERFEN, 1989). Since the arguments cited in favor of a permanent change in the oceanic-climatic conditions (i.e., a warming of the Southeast Pacific) cannot be substantiated after the occurrence of the 1988-89 cold episode, it is necessary to look for alternative explanations for the changing patterns of Peruvian/Chilean fisheries.

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