LOEB and ROJAS: ICHTHYOPLANKTON COMPOSITION AND ABUNDANCE 



TURBULENT MIXING INDEX 



350 - 



300 - 



250 - 



200 - 



55 



65 70 



Year 

 STANDARD DEVIATIONS 



4 0- 

 3 5 

 3 

 2 5 

 2.0 



I 5 



1.0 



B 



Std. dev 



12- mo- running meon of std. dev. 



.— Long - Itrtn meon of tid. dev. 



85 



Yeor 



Figure 7.— (A) Surface layer turbulent mixing index values and (B) standard deviations and 12-mo running mean of standard 

 deviations associated with mean monthly sea surface temperature values off of Peru (5°-15°S), 1953-84. Dotted lines represent 

 long-term mean values of each index. From Bakun 1987. 



conditions initiated during the warm 1968-69 pe- 

 riod; these could possibly have persisted and in- 

 tensified again during the 1976 and 1983 El Nino 

 events. 



It is also likely that the 1970 change in OL 

 composition is related to onshore advection of 

 northern or oceanic water masses and associated 

 faunal assemblages, but this cannot be confirmed. 

 All involved species are relatively abundant in 

 coastal Peruvian and south eastern tropical 

 Pacific waters (Ahlstrom 1971, 1972; de Castillo 

 1979; Santander and de Castillo 1979) and in 

 more southern coastal Chilean waters (Table 2), 

 but their wintertime relative abundances in these 

 areas have not been documented. It is also possi- 

 ble that the changes in species composition are 

 related to locally lowered zooplankton abundance 

 (e.g., that the Group II species are relatively more 

 successful than Lampanyctus parvicauda and 

 Triphoturus mexicanus during periods of lowered 

 secondary productivity levels). Alternatively, the 



observed change could be due to altered seasonal 

 spawning activity which is not treated in the 

 present study. However, whatever the cause, 

 there is evidence for an environmental change 

 in the study area, and this may be also implicated 

 in changes occurring within the PL ichthyoplank- 

 ton fraction. 



Increased abundances of sardine and Mycto- 

 phum nitidulum (Pair I species) during and after 

 1972 may be further evidence for a changed 

 marine environment off of northern Chile. Addi- 

 tionally, the significant correlation between lar- 

 val sardine abundance and temperature 

 (Table 10) suggests that elevated temperatures 

 may have been important for increased spawning 

 activity and/or increased success of hatching and 

 early larval survival. Given this observation one 

 may speculate that the increased sardine catches 

 after 1973 (Fig. 4) are related to increased fre- 

 quency of warm-water events in the 1964-84 pe- 

 riod relative to earlier years. Increased sardine 



21 



