FISHERY BULLETIN: VOL. 74, NO. 1 



was measured before food was added and served 

 as a guide for the quantity of food to be added. 

 Where losses from ingestion or other sources of 

 mortality were high, the density measurements 

 tended to be lower than the level we attempted to 

 maintain. In experiment 5 we attempted to main- 

 tain the density of Brachionus between 50 and 

 100/ml and that ofArtemia nauplii at 2 to 3/ml. 



In all experiments, 15 or more larvae were 

 removed on alternate days and consequently, 

 survival estimates include the effect of this sam- 

 pling. In experiments 1 to 4 no daily counts of 

 dead larvae were made until heavy mortalities 

 occurred after age 20 days. In experiment 5, daily 

 records of dead larvae were begun at age 54 and 

 continued to the end of the experiment (age 74 

 days). At age 54 days 20% of the larvae were alive 

 and at age 74 days, 374 larvae or 12.5% were 

 alive. If the tank had not been sampled survival 

 would probably have been greater because be- 

 tween 54 and 74 days the number of larvae 

 sampled, 151, exceeded the number that died in 

 the tank, 70. A total of 387 larvae were removed 

 during the experiment. A method exists for es- 

 timating mortality in rearing work independent 

 of the effect of sampling (Laurence 1974) but the 

 programming effort required seems unwarranted 

 for the objective of this paper. Collision with the 

 walls of the container was a frequent cause of 

 mortality over the last 3 weeks. 



A survival of 12.5% at 74 days contrasts sharp- 

 ly with the other four experiments where nearly 

 all larvae died by 30 to 40 days. Prior to the study 

 described here, marked mortalities were common 

 after 20 days and in all of the attempts Artemia 

 was used as food. The pattern had become so 

 typical at this laboratory that we have called it 

 the 'Artemia syndrome" for some years. The 

 results of the current study suggest that the 

 cause of the Artemia syndrome may simply be an 

 inability of young clupeoid larvae with straight 

 tube digestive tracts to digest Artemia nauplii 

 but that Artemia may be used once the gut 

 becomes differentiated. 



It is important to call attention to the fact that 

 6% of the anchovy larvae in experiment 3 were 

 able to survive for 42 days on a diet of only 

 Brachionus. Plaice, Pleuronectes platessa, larvae 

 have been reared through metamorphosis on only 

 Brachionus although growth was slower than 

 that on Artemia nauplii (Howell 1973). Howell 

 found that plaice larvae, immediately prior to 

 metamorphosis (12.7 mm), consumed 1,400 roti- 



fers per day. In experiment 3, at age 42 days, the 

 mean length of the anchovy larvae was 21.6 mm 

 and dry weight was 5.5 mg. Assuming a digestive 

 efficiency of 100%, larvae of this weight would 

 have to ingest about 3,800 rotifers per day to 

 meet metabolic requirements (calculation based 

 on caloric value of Brachionus and anchovy respi- 

 ration data given by Hunter 1972). These results 

 illustrate the value of maintaining a high density 

 of rotifers in culture containers long after a larger 

 food has been added. They also suggest that some 

 fish larvae have the ability to ingest large quan- 

 tities of small prey and this could be of consider- 

 able benefit under natural conditions. 



TISBE FURCATA AS A FOOD FOR 

 LARVAL FISH 



The evidence for the use of Tisbe as a food for 

 rearing larval anchovy to metamorphosis is a 

 single rearing experiment. It would be preferable 

 to have additional experiments but none are 

 planned at present because current work is con- 

 cerned with only young stages and other species. 

 Two groups of Pacific mackerel. Scomber japon- 

 icus, have been reared to metamorphosis using 

 Brachionus and Tisbe as foods and this supports 

 the contention that Tisbe is a satisfactory food for 

 pelagic marine fish larvae. The work on Scomber 

 will be reported at a later date. 



That larval anchovy ate Tisbe is supported by 

 records of stomach contents of larvae examined 

 during the course of the rearing work. Seventy- 

 four percent of the stomachs examined in experi- 

 ment 5 contained only Tisbe or Tisbe and 

 Brachionus and 26% contained only Brachionus 

 {N = 69, larval length = 8.6-18.8 mm). The num- 

 ber of Tisbe in stomachs of larvae increased 

 from 2.8 per larva (5.6-8.5 mm) to 18 per larva 

 (17.6-20.5 mm). (Data from experiments 4 and 5 

 combined — Table 2.) The average length of the 



Table 2. — Number and mean length of Tisbe furcata in the 

 stomachs of anchovy larvae in experiments 4 and 5. 



'Includes only larvae that had either Tisbe and Brachionus or only Tisbe 

 In stomachs. 



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