FISHERY BULLETIN: VOL. 70. NO. 3 



large crustaceans, the latter do not necessarily 

 make an equivalent contribution to their diets. 

 It is probable that the segment of the biota rep- 

 resented by such crustaceans in the sea is a food 

 source of different potential for different fishes. 

 Assessing the food potential of large crusta- 

 ceans for the Pacific mackerel, or any of the 

 above teleosts, requires a knowledge of the den- 

 sity levels of such crustaceans in the sea and of 

 the rates at which the organisms can be cap- 

 tured in relation to density. Leong and O'Con- 

 nell (1969) determined by a laboratory study 

 the rates at which the northern anchovy feeds 

 on Artemia nauplii by filtering and on Artemia 

 adults by biting (particulate feeding), and 

 O'Connell (1972) showed that the two kinds 

 of feeding activity in a small school varied with 

 the relative abundance of the two sizes of Ar- 

 temia. The present study was undertaken to 

 obtain comparable information for the Pacific 

 mackerel, using Artemia as the food. The re- 

 sults are discussed in respect to the densities of 

 large crustaceans in the sea and in respect to the 

 feeding rates of the anchovy. 



METHODS 



Feeding trials were carried out in an arrange- 

 ment of two plastic pools with a connecting 

 trough and gates (Leong and O'Connell, 1969; 

 O'Connell, 1972). The pools were supplied 

 with a continuous flow of filtered sea water and 

 each contained 4.5 m^ of water at a depth of 

 0.61 m (2 ft). They were under a 12-hr day, 

 12-hr night cycle of illumination, but all trials 

 were carried out during the day. Temperature 

 varied from 16° to 19°C. 



A school of about 160 mackerel was acclima- 

 tized in the pool arrangement with the gates open 

 for about 2 weeks and then confined to one pool 

 before the study started. The fish averaged 

 222 mm (202 to 247 mm) in standard length 

 and 152 g (100 to 225 g) in weight. The latter 

 average is based on weights taken at the end 

 of each feeding trial and included digestive tract 

 contents, which were later found to range from 

 0.1 to 21.8 g. These values were subtracted from 

 the measured fish weights to obtain estimates 



of the weights before feeding. The subsequent 

 analysis involves the adjusted weights, which 

 averaged 147.3 g. 



All feeding trials were preceded by at least 

 24 hr without feeding, which was enough to pro- 

 duce virtually empty digestive tracts in prelimi- 

 nary investigations. For each trial the water 

 flow was turned off, and four fish were diverted 

 from the holding pool to the trough and then 

 admitted to the prepared food situation in the 

 other pool where they were allowed to feed for 

 a given number of minutes. Closing of the gate 

 after admission initiated the time period and in- 

 troduction of a hinged crowder, which was rap- 

 idly closed to trap and remove the fish, terminat- 

 ed the time period. The fish were killed in less 

 than 2 min after introduction of the crowder, 

 immediately measured and weighed, then placed 

 in jars of lO^/r Formalin with the body cavity 

 opened. 



Though the study is concerned primarily with 

 feeding on Artemia adults, a few feeding trials 

 were carried out with newly hatched Arteinia 

 nauplii, which averaged 0.65 mm in length, to 

 document the response of the mackerel to very 

 small crustaceans. The densities in the water 

 were estimated from subsamples, and quantities 

 in the digestive tracts were estimated by total 

 counts or volumetric aliquoting, as described by 

 Leong and O'Connell (1969). 



Feeding on Artemia adults is based on digest- 

 ive tract contents of four fish in each of 28 trials, 

 with each trial representing a different combina- 

 tion of food density and feeding time (Table 1). 

 The food situation was established for each trial 

 by placing a given wet weight of Artemia adults 

 in the experimental pool and dispersing it gently 

 with wide-mesh dip nets just before admitting 

 the fish. The weights given under food quantity 

 are the wet weights introduced at the start of 

 each trial. These were selected after prelimi- 

 nary observation to include the highest and the 

 lowest levels that could be managed conveniently. 

 The multiplication factors given in certain cells 

 of the table indicate the number of times the 

 specified weight was introduced into the pool dur- 

 ing the trial, e.g., 20 g was introduced six times, 

 or at 15-min intervals, during the 90-min trial. 

 This was done to sustain the nominal food den- 



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