O'CONNELL and ZWEIFEL: FEEDING OF PACIFIC MACKEREL 



Table 1. — The array of trials with respect to food density 

 and feeding duration. Number/liter is the nominal den- 

 sity for the weight introduced. 



' The multiplication factors show the number of times, at equal inter- 

 vals, that the specified weight was introduced during the trial to maintain 

 tha nominal density. See text. 



sity over long periods at the lower food density- 

 levels. Introduction of food during the trial, 

 and even brief dispersion with the wide-mesh 

 net, did not appear to distress the fish and scarce- 

 ly interrupted their feeding activity. 



The intervals of food introduction for the low- 

 er density levels were determined from a prelimi- 

 nary study. During a 30-min trial with 20 g 

 of Artemia, e.g., feeding activity declined sharply 

 after 15 min, and average digestive tract con- 

 tents did not differ from that for a trial of 15 

 min. A trial of 30 min with two introductions 

 of 20 g showed approximately double the digest- 

 ive tract contents of the above two. The 10-min 

 interval for 10 g introductions was established 

 from similar considerations. Food availability 

 remained high throughout the trials at the three 

 higher density levels. 



The nominal densities of Artemia shown in 

 Table 1 are based on the volume of the pool and 

 a conversion factor of 500 Artemia adults/g wet 

 weight. The conversion factor is an average 

 derived from samples from the several batches 

 of Artemia delivered to the aquarium. Portions 

 of these samples also indicated that individuals 

 averaged 4.5 mm in length and 0.48 mg dry 

 weight. 



The quantities of A^'temia in the digestive 

 tracts of the fish were estimated separately for 

 the mouth, the esophagus and stomach combined, 

 and the anterior and posterior halves of the in- 

 testine. The contents of the esophagus and 

 stomach were weighed. Numbers in the various 

 sections were estimated by total count where 

 quantity was low, but by counts from aliquots 

 where the quantity was large. These data indi- 



cated that the weight of ingested Artemia, de- 

 termined 2 to 6 months after preservation, was 

 25 to 50 9^ lower than the weight of the live 

 organisms. Digestion and leaching by Formalin 

 may have been responsible for such loss. Since 

 the weights of food in the digestive tracts could 

 not be considered representative of the weights 

 of food consumed, the analysis was carried out 

 on the numbers of organisms in digestive tracts. 

 Values for all sections of the digestive tract 

 were pooled to obtain estimates of total digestive 

 tract contents, the primary entity in analysis. 

 The numbers found in the mouth varied from less 

 than 1 to 53% and averaged 18% of total diges- 

 tive tract contents. This percentage, further- 

 more, varied with food density, increasing from 

 4% at the 10 g level to 41% at the 1,010 g level. 

 It is presumed that the contents of the mouth 

 were accumulations to be swallowed and/or ma- 

 terial regurgitated during capture. Proportions 

 in the intestine were considerably lower and will 

 be described later. 



RESULTS 



RESPONSE TO ARTEMIA NAUPLII 



Three trials were carried out with newly 

 hatched Artemia nauplii as the only available 

 food. In two cases the average densities of 

 nauplii were about 190/liter and 230/liter, and 

 groups of four fish showed no recognizable feed- 

 ing activity during 30-min periods of exposure. 

 In the third case density was a little more than 

 200/liter and a group of approximately 50 fish 

 showed no feeding activity. The stomachs of five 

 fish from these trials contained no nauplii, al- 

 though a few nauplii were found on the gill 

 rakers of two of them. 



One trial was carried out with Artemixi adults 

 and nauplii both present in the water. The den- 

 sity of adults was 22/liter and the density of 

 nauplii was 185/liter. After feeding for 5 min, 

 four fish averaged 2,334 Artemia adults and 823 

 Artemia nauplii (635 to 1,083) in the mouth, 

 stomach, and esophagus combined. The bulk 

 of both sizes of organism was in the stomachs. 



From these results it is evident that while the 

 Pacific mackerel did not respond to nauplii alone, 



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