3.4 Nutrition and growth 

 3.41 Feeding 



Observations on the feeding habits of larval A. rochei 

 indicate that they are particulate feeders. Yokota et al. 

 (1961) observed that larval A. rochei between 3.0 and 5.4 

 mm in length consumed nauplii ranging in size from 

 0.10 to 10.30 mm and copepodites of about 0.55 mm. 



Auxis have been observed to use their caudal fin to 

 hold their position in swift currents to capture drifting 

 food and to migrate away from swift currents in search 

 for food. They seek food not only by sight but also by us- 

 ing their lateral line sensory system (Imamura 1949; 

 Uchihashi 1953). 



The maximum size of fish eaten and also the total 

 weight of food consumed by different sizes of A. rochei 

 caught in the pole-and-line and set net fisheries have 

 been discussed by Morita (1972). For the relationship 

 between length of prey and body length of the predator, 

 he calculated the following formula: 



Y = -0.015X2 + 0.358X + 9.075 



where X = length of predator in centimeters 

 y = length of prey in centimeters. 



The curvilinear relationship can be seen in Figure 28. 

 The linear regression describing the relationship 

 between weight of stomach contents and body length is 

 shown in Figure 29. The equation is as follows: 



Y = 2.33X - 40.78 



where X = length of A. rochei 



Y = weight of the stomach contents in grams. 



The selection of food organisms above a minimum size 

 apparently is determined by the magnitude of gill raker 

 gaps among mackerels, tunas, and dolphins. Magnuson 

 and Heitz (1971), who calculated mean gill raker gaps of 

 0.74 mm for A. thazard and 0.51 mm for A. rochei, con- 

 cluded that among scombrids Auxis and Katsuwonus 

 have small gill raker gaps. The filtering areas of the two 

 species of Auxis, which measured 570 mm^ for A. 

 thazard and 55 mm^ for A. rochei, were intermediate 

 when compared to other scombrids. Magnuson and 

 Heitz suggested that gill raker gap and the maximum 

 distensibility of the esophagus would set limits on the 

 size ot toed eaten. Usually, however, selectivity in lead- 

 ing among scombrids is masked because of the diversity 

 of food organisms in the size range consumed by them. 



Competition for food appears to be of fundamental 

 importance in inducing young scombrids to feed 

 (Clemens 1956). In the eastern Pacific, juvenile Auxis 

 kept in shipboard aquaria learned to eat unnatural food 

 that was offered them by watching other species — 

 mahimahi, Coryphaena sp.; jacks, Caranx sp.; pom- 

 pano, Trachinotus sp.; threadfins, Polydactylus sp.; and 

 pomacentrids, Chromis sp. — splashing and moving 



FORK LENGTH (cm) 



Figure 28.— Relationship between fork length of Auxis rochei and 

 total length of prey consumed, off Japan (Morita 1972). 



20 22 24 26 28 30 32 34 36 



FORK LENGTH (cm) 



Figure 29.— Relationship between fork length of Auxis rochei and 

 weight of stomach contents, off Japan (Morita 1972). 



about the tank swiftly and striking voraciously at every- 

 thing in sight (Clemens 1956). The food offered included 

 live planktonic organisms, ground flesh of anchoveta, 

 Cetengralis mysticetus, yellowfin tuna, skipjack tuna, 

 and mahimahi, particles of coagulated fish blood, and 

 mahimahi eggs. 



3.42 Food 



Planktonic crustaceans and fishes constitute a Ifirge 

 part of the diet of juvenile Auxis. In Japanese waters, 

 juvenile A. rochei, which were recovered from the 



34 



