FISHERY BULLETIN: VOL. 70, NO. 3 



it did ingest them along with the larger Artemia 

 when both were present in the water. However, 

 the dry weight of the nauplii ingested would be 

 about 0.1% of the dry weight of the adults in- 

 gested. It appears, also, that the uptake of 

 nauplii was only about 4% as efficient as the 

 uptake of adult Artemia, i.e., the number of 

 nauplii per fish represents 4.4 liters of water in 

 the pool, while the number of adults per fish rep- 

 resents 106 liters. 



Yasuda (1963) obtained results comparable 

 to the above for S. japonicus 120 to 130 mm in 

 length. He found that the mackerel did not eat 

 brine shrimp (0.38 mm in length and presumably 

 nauplii) as did anchovies {Engraulis japonica) 

 and even horse mackerel {Trachurus japonicus) 

 of approximately the same length. The spacing 

 between gill rakers and gill raker processes was 

 shown to be smaller in the latter two species than 

 in the Pacific mackerel. 



RESPONSE TO ARTEMIA ADULTS 



The 28 trials in which Artemia adults were 

 the only available food showed that feeding was 

 particulate at the two lower density levels and 

 filtering at the three higher density levels. Par- 

 ticulate feeding is the capture of individual or- 

 ganisms by directed biting. Filter feeding is the 

 process of straining organisms from the water 

 as it passes through the gill rakers while the 

 mouth remains open. The duration of mouth 

 opening was 1 to 3 sec, and such mouth openings 

 occurred almost rhythmically 15 to 20 times/min. 

 Filtering, however, was sustained for only a lim- 

 ited time, and this time varied inversely with 

 density level of the food. It lasted 30 min at 

 22 Artemia /Mier, 20 min at 66 Artemia /Mier, 

 and about 15 min at 112 Artemia /Wier. At these 

 times the fish noticeably reduced swimming speed 

 and shifted to particulate feeding. Though par- 

 ticulate feeding appeared to be less vigorous 

 under these circumstances than at the two lowest 

 food densities, complete cessation of feeding did 

 not occur in any of the trials. 



The trial groups of four fish evidenced some 

 discomfort upon encountering the highest food 

 density, 112 Artemia /Mier. Swimming speed 



and coloration showed less increase than at other 

 densities, and filtering intervals were shorter 

 and less rhythmic. A school of about 70 fish in- 

 troduced to this highest density, on the other 

 hand, exhibited strong rhythmic filtering ac- 

 companied by marked intensification of color and 

 increased swimming speed. Though digestive 

 tract contents from the fish of this group were 

 not significantly greater than for the four-fish 

 group after 5 min of feeding, the larger group 

 virtually eliminated the available food in this 

 time while the smaller group did not noticeably 

 afl^'ect its density. 



Preliminary analysis indicated that there was 

 no confounding of size of fish with density levels 

 and suggested that the larger fish tended to con- 

 sume slightly more food at all densities. Hence, 

 estimates of total digestive tract quantities were 

 standardized to the average adjusted weight of 

 all fish (147.3 g) by simple proportion. The 

 means and standard errors of the standardized 

 numbers are shown for each trial in Table 2. 

 The greatest change in a trial average resulting 

 from standardization was 20% of the original 

 estimate, and the change was 5% or less for half 

 of the trials. Standardization also aflJ'ected the 

 estimates of variability, but only to the extent 

 that the coefficients of variation averaged 1% 

 higher. 



A separate analysis for each of the food den- 

 sity levels indicated that digestive tract contents 

 increased proportionately with time at the two 

 lower densities, but increased exponentially to- 

 ward an asymptotic value at the three higher 

 densities. It was also evident that the density- 

 specific rates of increase varied directly with 

 density for the two lower levels and with the 

 square root of density for the three higher levels. 

 The modes of food accumulation for the two den- 

 sity ranges can therefore be expressed as 



and 



Np = atD 



fl ^e-^'^A 



(1) 

 (2) 



where Np = number of organisms in the di- 

 gestive tract after t minutes of 

 particulate feeding, 



976 



