ARTHUR DISTRIBUTION AND ABUNDANCE OF MICROCOI'KI'MDS 



100 



A A CLARKE-BUMPUS 



O O MICR0PLANKT0N 



TRUNCATED NET 



o 



20 



Microcopepodids 



FIGURE 8. — Comparison of ogives for 

 abundance of nauplii and micro- 

 copepodids for all sizes retained by each 

 of the three samplers in the onshore 

 zone corrected for volume and escape- 

 ment. 



J 



.001 .003 .010 .032 .100 .316 1.000 3.162 10.00 31.622 100.00 

 CONCENTRATIONS (Number/ liter) 



DISCUSSION 



Microcopepod Size and Feeding Habits 

 of Three Larval Fishes 



Feeding habits of larvae of Pacific sardine, 

 Sardinops sagax; northern anchovy, Engraulis 

 mordax; and jack mackerel, Trachurus symmetri- 

 ca, as reported by Arthur ( 1976), may have been 

 associated with spawning distribution of the adult 

 fish as well as with the distribution of micro- 

 copepods and nauplii during the years of this 

 program. Jack mackerel spawned mainly in the 

 offshore zone, as can be determined by comparing 

 the Mecynocera boundary with the distribution 

 of jack mackerel larvae (Anonymous 1953:36). 

 Jack mackerel larvae first start to feed when 

 3.0 mm long and ingest mostly 60- to 70-/um wide 

 (total range 50 to 200 /xm) copepod nauplii. How- 

 ever, when they have grown to 3.5 mm their food 

 is primarily about 125-/xm wide copepodid stages 

 of small copepod species and when 9.0 mm long 

 they eat 250- to 450- / u,m wide copepodids of larger 

 species. The quick change from nauplii to cope- 

 podids, which is facilitated by their relatively 



large mouths, may be related to the low nauplii/ 

 copepodid ratio of the offshore zone. 



Most anchovy larvae were caught inside the 

 Mecynocera boundary (Anonymous 1953:34). The 

 more omnivorous 3.0-mm long first feeding an- 

 chovy larvae select food from the 25 to 100 /xm 

 range with little preference for any size within 

 this range. Food size increases to 125 /xm when 

 larvae are about 4.0 mm after which, though there 

 is some increase, food size does not increase iso- 

 metrically with the increase in length of larvae. 

 This curious slow increase in food size appears to 

 be common to early larval stages of the genus 

 Engraulis, as can be observed in food-size/larval- 

 length graphs for Japanese anchovy, E. japonica 

 (Yokota et al. 1961), Argentine anchovy, E. an- 

 choita (Ciechomski 1967), Peruvian anchovy, 

 E. ringens (Rojas de Mendiola 1974), and can be 

 calculated for northern anchovy, E. mordax, from 

 data presented by Berner (1959) and Arthur 

 (1976). This lack of selecting for the largest in- 

 gestible food size may be related to the high 

 nauplii/copepodid ratio of the inshore zone and 

 may also account for the importance of copepod 

 eggs in the diets of anchovy larvae as reported 

 by the above authors except Yokota et al. (1961). 



607 



