LOEB and ROJAS: ICHTHYOPLANKTON COMPOSITION AND ABUNDANCE 



>50% of cruises) and/or abundant across the 10 

 sampling periods (Table 4). These taxa (four PL 

 species, seven myctophid taxa, two other meso- 

 pelagic species, and five coastal forms) made up 

 99.69; of the identified larvae; they also con- 

 tributed 97-1009r of the identified larvae (91.2- 

 99.6% of total larvae) and included the top 9-12 

 ranked taxa within each sampling period (Table 

 5). 



Table 4— Ichthyoplankton species collected in July-September 

 samples oft northiern Chile (18 -24 S, 70 -72 W), 1964-83. Rela- 

 tive abundance (ROA) and percentage composition based on 

 summed cruise mean abundances (no. 10 m2) of all identified 

 forms. Frequency is number of total 10 sampling periods when 

 taxon was caught. Categories are PL (commercially important 

 pelagic species) and other taxonomic components (M = myc- 

 tophids: OM = other mesopelagic taxa; C = coastal forms). 



INTERANNUAL VARIATIONS IN 

 ABUNDANCE AND COMPOSITION 



The ichthyoplankton demonstrated extreme in- 

 terannual variations in abundance and composi- 

 tion (Tables 3, 5; Fig. 3A, B). Most obvious are the 

 1) total larval and PL abundance peaks of 1966, 

 1972, and 1973 and 2) shift from anchoveta to 

 sardine dominance between 1973 and 1983. The 

 maximum total larval abundance values in 1966, 



1972, and 1973 were 2.4-30 x higher than those 

 of other years; the PL had 3-71 x higher abun- 

 dances during these vs. other years. Interannual 

 abundance variations during the seven years of 

 lower abundance were also large for total larvae 

 (to 6.9 X ) and the PL (to 12.7 x ). Anchoveta domi- 

 nated the ichthyoplankton through 1973 (29.0- 

 88.8% ) and was directly responsible for the ex- 

 treme abundance variations; anchoveta were 

 absent from the 1983 samples, and sardine larvae 

 (72.0%) contributed to the moderately high total 

 larval and PL abundances (Table 5). 



The OL fraction had less extreme abundance 

 variations than the PL: maximum 1966, 1972, 



1973, and 1983 mean abundance values ranged 

 from 1.4 to 9x those of the other years; mean 

 values within the six years of lower abundance 

 varied to 3.8 x. Unlike the PL, OL abundance 

 fluctuations were not attributed to any one taxo- 

 nomic component (Fig. 3B): the 1966 peak was 

 largely due to coastal taxa (73% of OL); myc- 

 tophids and other mesopelagic taxa equally domi- 

 nated the 1972 peak (48%); myctophids domi- 

 nated (79%) in 1983; and all three components 

 were relatively abundant (29-40%) in 1973. Myc- 

 tophids were generally the dominant component 

 (50-90% ) during the six years of lower OL abun- 

 dance. Abundance fluctuations (as range of mean 

 abundance values) across the 10 years were less 

 extreme for myctophids (8.1 x ) than for the other 

 mesopelagic taxa (32.3 x) and coastal forms 

 (160x). 



In contrast to the ichthyoplankton, July-Sep- 

 tember zooplankton biomass values were rela- 

 tively constant between years and exhibited only 

 a 4x range in values (Table 3). 



Despite large between-year variability in rela- 

 tive proportions of the PL and OL, there is a sig- 

 nificant agreement of their ranked mean abun- 

 dances across the 10 years (p = +0.81, P < 0.01). 

 There is also a general agreement of the ranked 

 mean abundances of PL and the three OL compo- 

 nents across the 10 years (W = 0.44; P = 0.05). 

 These categories were generally more abundant 



9 



