Informe Anual de la Comisión Interamericana del Atún Tropical, 19

30 TUNA COMMISSION
feet) below the normal depth for the equatorial area between 2°8 and 5°N, east of 130'W, and also
along the coast ofEcuador (personal communications from D. W. Behringer, Atlantic Oceanographic
and Meteorological Laboratory, V.S. National Oceanographic and Atmospheric Administration,
Miami, Florida).
In July 1986, before the thermocline began to deepen, the sea level started to rise. In Figure
24A are shown data on the sea-leve! anomalies (departures from the normal mean sea level) based on
data averaged for three coastal stations from Colombia to northern Peru. Mter June 1986 the sea
level along the west coast of South America remained aboye normal except for a brief interval in
December 1986 when it was slightly below normal. By February 1987 the sea level had risen to 10 cm
(4 inches) aboye normal. Thereafter, it slowly lowered, reaching normal by the end of1987. In May
1987, 3 months after the sea level reached its maximum, the S8Ts reached their peak of 3°C aboye
normal, and then leveled offat about 1° to 2°C aboye normal through the end ofthe year (Figure 24A).
As an El Niño develops there is a decrease in the strength of the easterly trade winds across
the South Pacific, especially offthe coast ofSouthAmerica south of15°S. This results in weaker-thannormal
wind mixing, which is needed to maintain strong upwelling in the coastal part of the Peru
Current. Aweakening ofthe wind circulation in the South Pacific is related to large-scale changes in
the atmospheric circulations which are usually represented by the Southern Oscillation Index (SOl),
indicated as a difference in sea-level pressures between Darwin, Australia, and an average ofEaster
Island and Tahiti. The differences for individual years or months are compared to long-term mean
pressure differences to calculate anomalies ofthe SOr. Negative anomalies represent weaker-thannormal
surface winds and lower surface pressures in the ETP south of the equator, and positive
anomalies represent the reverse. Data on the variations in the SOl for 1986 and 1987 are given in
Figure 24B. During most of1986 the SOl oscillated about the zero line, but after October it became
negative and remained so until November 1987. Thus by the end of1987 the lowering ofthe sea level
along the coast ofSouth America and the trend ofthe SOl to move back toward normal suggested the
El Niño was weakening in the ETP. However, the 88Ts were still aboye normal and the thermocline
remained deeper than normal (Figure 23). This indicates that residual effects of the El Niño will
remain through the austral summer of 1987-1988.
This El Niño is not expected to regenerate after the end ofthe austral summer of1987-1988.
El Niño and low catch rates
Significant El Niño events occur in the ETP every 4 to 6years, on an average. The intensity
and extent of these events differ markedly, however. Tunas apparently react to El Niño events by
moving away from the area or deeper into the mixed layer, possibly in search of food, and thereby
become less availabJe or vulnerable to capture by purse seiners.
The El Niño of1986-1987, descrihed in the previous section, was ofmoderate intensity, hut its
effect on the ocean environment in the ETP was not widespread. It was restricted during most of
1986-1987 to the ETP south of 5°N, where the catches per unit of effort (CPUEs) were very poor
during 1986 and 1987, especially in the coastal area off Ecuador and northern Peru. In contrast,
fishing was quite successful during those two years in the ETP north of 5°N, where the effects ofthe
El Niño appear to have been minimal. One ofthe regions most often affected by El Niño events is the
heavily-fished area outside the Gulf of Guayaquil, bounded by the equator, 5 0
S, 85°W, and the coasts
of Ecuador and Peru. During 1986 and 1987 the catches of skipjack and yellowfin were generally
below the mean annual catch for the same area between 1965 and 1984. (It should he pointed out,
however, that there has been a downward trend in the catches ofthese two species during the 1961­
1984 period.)
Data on the fishing effort and CPUEs for skipjack and yellowfin for 1986 and 1987 and for
1965-1984 for the 5-degree area described aboye are presented in Figure 25. Throughout 1986 the
fishing effort (Figure 25A) was less than the 1965-1984 mean. In 1987 the fishing effort was below the