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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

El Niño and Occurrence of Coastal
Pelagic Fishes Outside the 200-Mile
Zone of Peru in 1979-1991

A. Nesterov

Atlantic Research Scientific Institute of Fisheries and
Oceanography (AtlantNIR_), 5,Dimitry Donskoy,
Str., Kaliningrad, 236000, Russia,

The detection of shelf fish aggregations outside the 200-mile zone of Peru (Northern subarea) in 1978 marked the beginning of a large-scale fishery for the USSR. The basis of the fishery in the Northern subarea is constituted by horse mackerel Trachurus murphyi. Catches of this species between 1978 and 1991 varied from 48500 tons to 403500 tons. The maximum catch of mackerel Scomber japonicus was landed in 1980 (53900 tons), while in subsequent years it was below 10000 tons. The fishery of Peruvian sardine Sardinops sagax was only carried out by small groups of vessels and the catch did not exceed 36200 tons (1986). These species frequently formed mixed aggregations and were fished simultaneously.

Formation of horse mackerel, sardine and mackerel aggregations outside the 200-mile zone from 5º to 20ºS is determined by two factors. Firstly, by the condition of these species stocks in the coastal zone including the entire economic zone of Peru. Secondly by the hydrological conditions, which facilitate migration of these species into open ocean waters.

For the Northern subarea, a seasonal pattern of commercial fish aggregation formation is characteristic. Aggregations form from May/June to November/December (see figure). A necessary condition of commercial fish aggregations is the outflow of relatively cold waters of the Peruvian current outside the Peruvian economic zone boundaries. The stability of these aggregations is determined by the spatial - temporaly stability of the outflow. The normal locations of cold water penetration outside the economic zone boundaries are between parallels 8-10º, 14-15º and 17-20ºS. Water temperature in the fish aggregation regions varies from 16 to 23º C, while the more intensive migrations are found at a temperature of 18º-. The cold waters penetrate westwards to distances of up to 400 miles from the coast and fish aggregations are distributed mainly within a 100-mile band adjacent to the Peruvian zone. In a southern part of the Northern subarea, divergence of the Peruvian current jets forms centres of upwelling which also contribute to the formation of a high productivity zone.

Fig. 1 Periodicity of fishing in the Northern subarea (open ocean, 5-20°S)

The formation of mackerel commercial aggregations may begin in May, when fish migrate outside the 200-mile zone at 8 and 14ºS. At this time a smaller number of horse mackerel is observed in catches. In May - July the fishery was conducted in northern locations between 7-14ºS in mixed aggregations of horse mackerel and mackerel. By August horse mackerel aggregations extended to 20ºS. In July - August sardine appeared in catches. In September-October the fishing area was located between 10 and 21ºS. In December, with the summer season approaching, intensive warming of the sea surface layer was observed. The area of aggregations decreased, and fish migrated south-eastwards into the 200-mile zone.

More than ten -years' experience of the fishery has shown that, in years of maximum El Niño development (1982-1983; 1987-1988) in the Northern subarea, commercial aggregations of horse mackerel and other pelagic species outside the 200-mile zone are actually absent (1983; 1987) (see figure). In the years when El Niño development occurred (1982) and in subsequent years (1984, 1985) the scale of aggregation migration from the zone was insignificant. In these years the area of horse mackerel aggregations was smaller than in other fishing periods.

The period 1978-1981 may be considered as a favourable one for formation of commercial aggregations outside the Peruvian zone, when the horse mackerel catch depended only on the level of fishing effort. The catch in 1981 amounted to 403500 tons, while available fishery biomass was up to 4,3 mln.t. Horse mackerel were distributed along the entire zone of Peru (from 6º up to 18º30') and up to 20ºS. The average effective years of fishery were 1982 and 1988-1990. Horse mackerel catches in these years were within 97500 -125400 tons, while available commercial biomass varied from 0,8 mln.t in 1982 to 2,7 mln.t in 1988 (the catch rates were 15,8 and 4,3% respectively). In the same years, horse mackerel distribution was characterized as continuous along the sea zone.

In 1984-1985 after one of the most intensive El-Nino events in recent decades, the catches constituted less than 30000 tons (23400 and 27900 tons respectively) with insignificant areas of fish concentration and total biomass of 0,6-1,2 mln.t.

The highest values of horse mackerel biomass and catch in the oceanic part of the subarea correspond to the maximum values of this species stock estimated by the Institute of the Sea of Peru (IMARPE) on the basis of echo surveys in the 200-mile zone. Maximum biomass in the zone was found in the period from 1978 to 1983 with values of 8,9 mln.t for horse mackerel, 1,8 mln.t for mackerel and 4,8 mln.t for sardine. Between 1988 and 1990 large stocks of these species were observed in "zonal" waters- horse mackerel biomass was 7,5 mln.t, mackerel 1,5 mln.t and sardine 8,5 mln.t.

According to IMARPE data in the "zonal" part of the area in 1984-1985, after the El-Nino event of 1982-1983, reducted horse mackerel and mackerel biomass was noted (Muck, Sanchez, 1987). These data agree with the information of the Soviet Union fleet operation on fish aggregations and migration outside the zone (in these years the total annual catch of these species did not exceed 28000 tons) and peculiarities of fish aggregation distribution and available fishing biomass.

When arranging research and fishing, special attention should be paid to the time of fish migration into the areas accessible to fishing vessels. The data in the figure show that fish aggregations in the oceanic area form in May - June or July - August. Formation of fish aggregations in May - June 1980 and 1981, 1985 and 1986, was evidently caused by the cool Peruvian current strengthening. These years are characterized by negative SST anomalies (-1º) in the area 5-10ºS during the first half of the year and are not related to the strong El-Nino events (Climate Diagnostics Bulletin, 1991). Formation of fish aggregations in July - August (1979, 1984, 1989-1991) coincides with a situation when, in the first half of the year, positive SST anomalies or their alternation with insignificant (up to - 0,5º) negative anomalies are observed. The final years of intensive El-Nino events (1983, 1988) are exceptional. Prediction of events determining migration of fish aggregations outside the zone is the element of El-Nino forecast.

Thus, the timing of pelagic fish migration into the open part of the subarea and the area of fish aggregations is determined by the size of fish stocks within the 200-mile zone of Peru and by hydrologic conditions. The prediction of periodically repeating strong El-Nino events is of special importance. In AtlantNIRO S.Kuderskiy, an oceanologist, is investigating the long-term forecast of El-Nino using the data on the rate of the Earth's rotation, which, in turn, is related to the regularities of amplitude modulation of long-period Luni-Solar and nutational tides.

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