LOEB and ROJAS: ICHTHYOPLANKTON COMPOSITION AND ABUNDANCE 



3,0 



1955 



83 84 



Figure 4.— Total catch and catch of dominant species taken in the northern Chilean pelagic fishery, 1955-84. 



negative sea surface atmospheric pressure 

 anomalies occurred off of Arica from 1960 to 1972 

 with a strong negative anomaly occurring in 

 1969; predominantly positive anomalies occurred 

 there after 1972 (Fig. 6A). A similar but less ex- 

 treme change from negative or neutral anomalies 

 to predominantly positive anomalies occurred off 

 Iquique (20"S) in 1970 (Fig. 6B; Kelly and Blanco 

 1983). 



Off Peru (5°-15°S) the wind driven turbulent 

 mixing index of surface waters shows a general 

 increase during and after the 1972 El Nino event 

 (Fig. 7A; Bakun 1987). A probable result of this 

 increased turbulence is an increase in standard 

 deviations associated with monthly temperature 

 values; standard deviations above the 30-yr mean 

 generally persisted throughout the year from 

 1972 to 1984 (Fig. 7B) and suggest increased 

 physical variability and heterogeneity in this 

 later period. Comparable data sets from northern 

 Chile are not available to determine if these lat- 



ter two features were also characteristic of the 

 Chilean area. 



How these observed atmospherically related 

 changes could be related to changes in the marine 

 environment off northern Chile is uncertain. It is 

 possible that the observed changes in atmos- 

 pheric pressure off Arica and Iquique have associ- 

 ated changes in advection of water mass and fau- 

 nal sources. Bernal et al. (1983) discussed El Nino 

 related changes in water mass distribution off 

 Chile in 1972 and 1973 relative to cold-year 1967. 

 These changes involved southerly extensions of 

 oceanic subtropical and equatorial subsurface 

 waters, strengthening of the spring-summer ther- 

 mocline, and cessation of coastal upwelling. 

 These authors did not examine water mass distri- 

 butions in the 1968-70 period. However, lowered 

 zooplankton biomass starting in 1969 and the OL 

 composition change starting around 1970 suggest 

 that the hydrographic conditions attributed to the 

 1972 El Nino may have been an intensification of 



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