versão On-line ISSN 0717-7178
Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002
Effects of "El Niño" 1997-98 on the
Schooling Behaviour of the Peruvian
Anchovy (Engraulis ringens)
Marilu Bouchon, Erich Diaz,
Sandra Cahuín, Manuel Ochoa
Instituto del Mar del Perú (IMARPE), Apartado 22,
Callao, Perú, E-mail: firstname.lastname@example.org
The pelagic ecosystem in the Peruvian sea is characterized by high productivity which is sustained by the development of large biomasses of few species, the most important of which is anchovy. This predominance is related to typical environmental conditions which are affected by the occurrence of "El Niño" events. The presence of "El Niño" 1997-98 in the Peruvian sea affected the general situation of pelagic resources, mainly the anchovy, decreasing its population from 5.8 million tons in May 1997 to 1.2 million tons in September 1998.
The goal of this work is to determine the effects of "El Niño" on the anchovy´s schooling behaviour in the Peruvian sea with emphasis on landing levels, distribution and concentration, size structure and industrial purse seine activity, as main indicators.
Landings of anchovy showed a considerable decrease during "El Niño" 1997-98 (Fig. 1), with two stages in the development of the fishery. The first, at the beginning of the event, with predominance of anchovy landings, due mainly to the approach of the fish to the coast and consequent accessibility to the purse seine. The second stage presented a drastic decrease of landings of anchovy and the appearance of other pelagic species like sardine, jack mackerel, horse mackerel and long nose anchovy.
During "El Niño" 1997-98 the anchovy showed a variation in distribution, concentration and spatial availability of schools. By fall 1997 the presence of warm conditions produced a fold of the resource along the coast which became vulnerable to industrial purse seine action. Later, in spring 1997, the anchovy remained concentrated within 20 nautical miles of the shore, showing a southward displacement (Fig. 2). Next, in fall 1998, a change in the migratory process of the anchovy´s schools was observed, with movement from south to north but in low densities.
As an effect of marine environment warming during "El Niño" 1997-98 a deepening of schools in the north-center region was observed. In normal conditions, anchovy are located in the surface layer between 0 and 30 metres. Coincidentally with the deepening of the thermocline, the schools were detected 10 meters below this range even reaching an average depth of 60 meters off Pisco in May 1997.
Anchovy size structure showed a predominance of adults throughout the event. In relation to the variability of sizes according to hour of the day a homogenization of individuals between 15 and 16 cm (average length) was observed at the beginning of "El Niño". Possibly they grouped to concentrate inshore and start the migration process southward. This situation changed with the months, showing a variability of sizes during the daytime and stability of sizes during the night (Fig. 3).
|Fig. 3 Variability of anchovy size structure according to hour of the day during April and June 1997|
Finally, with respect to the industrial purse seine activity, a change in fishing strategy occurred, fluctuating from day work tasks to night work task periods, when the bigger catches were obtained (Fig. 4).