FISHERY BULLETIN: VOL. 78, NO. 1 



weight per day and weight increased from 0.034 

 mg to 7.5 mg over the larval period. To capture 

 sufficient numbers of prey to support such rapid 

 grovvi;h requires that the size of the prey and the 

 size of the mouth increase rapidly. Our analysis of 

 sea-caught Pacific mackerel larvae showed that 

 the maximum size of prey did increase rapidly, 

 more or less, in proportion to mouth size. The 

 mean and minimum size of prey eaten by Pacific 

 mackerel changed more slowly but the smaller 

 prey, those less than the average size, may consti- 

 tute <15% of the volume of food eaten. A similar 

 pattern of rapidly increasing prey size with length 

 also has been documented for Scomber japonicus 

 larvae by Shirota (1970) and Yokota et al. (1961). 



A dependency on larger prey and fast growth 

 requires faster swimming to increase the volume 

 of water searched for prey because abundance de- 

 clines with increased prey size (Sheldon et al. 

 1972). The swimming behavior of Pacific mackerel 

 larvae appeared consistent with this argument. 

 Cruising speeds increased rapidly with length, 

 roughly to the 1.8 power, and speeds of the larger 

 larvae were at the upper end of the range, typical 

 of larval fishes (3 SL/s) (Blaxter 1969). Higher 

 speeds require a greater metabolic expenditure. 

 The rate of oxygen consumption for Pacific mack- 

 erel (6-11 fx\ Og/mg per h) was above that for other 

 marine fish larvae (Blaxter 1969) indicating a 

 higher-than-average metabolic expenditure de- 

 spite the fact that the rates probably do not reflect 

 the entire cost of high speed swimming. 



Piscivorous feeding was an import".:.;. e- 

 havioral trait in the early life history of Pacific 

 mackerel because larvae were no longer limited to 

 prey sizes equal to or less than the size of an open 

 mouth. In piscivorous feeding, prey were seized, 

 manipulated and the mouth greatly expanded 

 during ingestion, permitting consumption of 

 much larger diameter foods. In our samples of 

 sea-caught larvae, only one stomach contained a 

 larval fish, but the actual incidence may be higher 

 because larvae are digested rapidly. Cannibalism, 

 a correlate of piscivorous feeding, was common in 

 laboratory groups after the larvae reached 8 mm 

 SL. This also has been observed from stomach 

 contents of the Atlantic mackerel, S. scombrus 

 (Lett 1978). Cannibalism appears to be a common 

 feature of scombroid life history; Mayo (1973) re- 

 marked that Euthynnus alletteratus , Scom- 

 beromorus cavalla, S. regalis, and Auxis sp. be- 

 came cannibalistic at about 5 mm SL. He also 

 noted that cannibalism ceased as the fish became 



100 



juveniles which agrees with our observation that 

 cannibalism ended as Pacific mackerel ap- 

 proached metamorphosis and began to school. The 

 extent that cannibalism affected the form of our 

 laboratory growth curves is unknown. Although 

 cannibalism was high in all groups, survival was 

 higher in groups reared at high temperatures be- 

 cause of the faster growth rate, which meant faster 

 transit through cannibalistic sizes. 



In summary, traits that characterize the early 

 life history of Pacific mackerel are the interrelated 

 characteristics of fast growth, fast swimming, 

 high metabolism, a dependence on increasingly 

 larger prey, and cannibalism. The high food re- 

 quirements of the larvae, and the fact that in the 

 sea they feed upon many prey substantially small- 

 er than they are capable of eating, indicates that 

 growth or survival in the sea might be limited by 

 the availability of larger prey. 



LITERATURE CITED 



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