HUNTER and KIMBRELL: EARLY LIFE HISTORY OF PACIFIC MACKEREL 



95 



90 - 



80 



V. 70 



UJ 



8 60 



^ 50 



UJ 



^ 40 



I- 



g 30 



^ 20 



• • 



PREY 



O Enq raulis mordox eggs 

 (minor axis = 0674 mm) 



# Artemio solino nauplii 

 (0238 mm) 



5 = 4 322-9580 Log 

 S= % in probits 



10 



_L 



_L 



4 5 6 7 8 9 1.0 1.5 



PREY WIDTH /MOUTH WIDTH (R) 



Figure 8. — Relation between average feeding success (probit 

 scale) and average relative prey size (prey width/mouth width), 

 for larval groups fed Artemia salina nauplii (closed circles) and 

 northern anchovy eggs (open circles). Each point is the percent- 

 age success of fish within a mouth width/prey size class, where 

 n>9. Line is the regression of percentage success probit on log^^ 

 of the prey-width to mouth-width ratio, for A. salina nauplii and 

 anchovy egg data combined. The LD^p for the combined data was 

 0.85 (95<7c confidence interval 0.79-0.91). 



0.79-0.91) and the 95% threshhold was 0.57 

 (0.47-0.70), nearly all Pacific mackerel larvae 

 (95% ) were able to ingest a prey when it was 57% of 

 the width of the mouth and 50% were able to do so 

 when it was 85% of the mouth width. 



Nearly all prey eaten by Pacific mackerel larvae 

 in the sea fell within the range of sizes predicted 

 from the laboratory work; few prey exceeded the 

 width of the mouth (Figure 9). Fifty-nine percent 

 of all identifiable food items in the stomachs of 

 sea-caught larvae were stages of copepods; other 

 items included cladocerans, oikopleurans, gas- 

 tropods, invertebrate eggs, diatoms, fecal pellets, 

 and one fish larvae. 



Although laboratory data indicated that 50% of 

 Pacific mackerel larvae were able to ingest prey 

 having a width of 85% of the mouth width, the 

 mean diameter of prey eaten in the sea was 

 38±2% (2 SE) of the mouth width. Thus, a sub- 

 stantial number of prey eaten by larvae in the sea 

 was much smaller than the maximum size of prey 

 they were capable of ingesting. This may reflect a 



shortage of larger prey in the sea. Larger prey 

 probably are important nutritionally. If one as- 

 sumes the prey given in Figure 8 to be spherical, 

 then 50% of the prey items accounted for about 

 85-90% of the total volume of food, depending on 

 larval size. Conversely, the small prey items that 

 contributed 50% by number, contributed only 

 10-15% of the total volume of prey eaten. This 

 calculation underestimates the volume of the 

 larger prey because they are more elongate or less 

 spherical than smaller ones. Nevertheless, it indi- 

 cates that prey less than the mean size eaten con- 

 tributed relatively little nutritionally to the diet of 

 Pacific mackerel larvae, and that the relatively 

 large, but more rare, prey probably made the 

 major contribution to growth. 



Ration, Growth Efficiency and Metabolism 



Pacific mackerel larvae (age 3-5 d) fed actively 

 throughout the day; the gut was filled within the 

 first hour of feeding and it remained full through- 

 out the remainder of the 12-h feeding day, despite 

 a high rate of gastric evacuation. Evacuated 

 Brachionus plicatilis were well digested; only the 

 lorica remained after digestion. Our measure- 

 ments of evacuation rates indicated that about 

 half the gut contents was evacuated in 2 h (Figure 

 10). Growth of larvae used for ration estimates 

 was about the same as that for other groups reared 

 at 19° C (Figure 3). To grow at this rate in the 

 laboratory, Pacific mackerel larvae (age 3-5 d) 

 consumed an average of about 87% of their dry 

 body weight per day, or about 165-538 rotifers/day 

 (Table 3). This estimate of ration was based on the 

 dry weight of the mean number of rotifers in 

 stomachs, adjusted for the rate of evacuation 

 (Stauffer 1973). The mean gross growth efficiency 

 in dry weight was 33%, which falls within the 

 range of estimates for fish larvae and young fishes 

 (Pandian 1967; Stepien 1976). 



Our respiration experiments indicated that 

 Pacific mackerel larvae at 18.0° C consumed 

 6.1 ± 1.4 (2 SE) Ml Oa/mg per h (n = 24) and at 22.0° 

 C they consumed 11.4±3.0 ^tl Og/mg per h (n = 

 14). By interpolation, the rate at 19° C, the tem- 

 perature of the ration experiments, is estimated as 

 7.4 ^x\ Oj/mg per h. This metabolic expenditure, 

 converted to calories per day (footnote 6, Table 3) 

 was, on the average, about 18% of the mean daily 

 ration for larvae given in the table. This is proba- 

 bly an underestimate of their metabolic require- 

 ment because the activity of larvae confined in 



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