HUNTER: BEHAVIOR OF LARVAL ANCHOVY 



extent larvae are adapted in their searching oe- 

 havior to a patchy food distribution. 



SUMMARY 



1. During the first 2 days of larval life (age 

 0-1 day) anchovy larvae spent 95% of the time 

 resting. Rest was interrupted once a minute by 

 a burst of continuous swimming lasting 1-2 sec. 

 By age 4 days, yolk was completely absorbed; 

 85% of the time was spent in intermittent swim- 

 ming, 7% in feeding, and only 4% in rest. The 

 proportion of time spent in each of these ac- 

 tivities remained about the same thereafter. 



2. Intermittent swimming was associated with 

 food search. It consisted of repeated sequences 

 of a single tail beat followed by a glide. The 

 speed of the tail was a constant; thus, tail beat 

 frequency was a function of the glide duration. 

 During intermittent swimming tail beat ampli- 

 tude (A) was the constant proportion of length 

 (L) expressed by the equation, A = 0.112 + 

 0.170L. 



3. In continuous swimming, larvae executed 

 a burst of swimming in which no pauses existed 

 between tail beats. Both tail beat frequency and 

 amplitude were modulated during the burst be- 

 cause the larvae accelerated and decelerated. 

 Larvae had a burst speed capability in excess 

 of 25L/sec for bursts lasting 0.1-0.2 sec, but 

 the typical or average burst speed was near 

 15L/sec. The speed (V), tail beat frequency 

 (F), and amplitude {A) relationship for con- 

 tinuous and intermittent swimming were ex- 

 pressed by the equation, y/A = — 1.11 + 1.59F. 



4. The daily rate of movement of larvae ex- 

 cluding rest was estimated using visual and 

 photographic techniques. Measurements taken 

 from cine photographs gave the relationship 

 V = —0.215 + 1.038L. Visual observations 

 indicated that average tail beat frequency during 

 intermittent swimming was a constant for lar- 

 vae of different sizes, and this value was used 

 to provide an additional estimate of the daily 

 rate of movement. 



5. The size of the reactive perceptive field for 

 larval anchovy was determined from cine pho- 

 tographs taken in vertical and horizontal planes 

 by plotting the position of prey when larvae first 



reacted to them. The limits of the perceptive 

 field were set to include 95% of the prey sighted. 

 The field was elliptical, and increased with length 

 and had a cross section proportional to 0A5L^. 



6. Larvae first reacted to prey by turning the 

 head toward the prey. The larvae then swam to- 

 ward it, stopped swimming, coiled the body into 

 an S-shape, and struck at the prey by thrusting 

 the tail backward and extending the head. The 

 total feeding sequence lasted only 1-2 sec. 

 Throughout the sequence the larva adjusted 

 its body to compensate for movements by the 

 prey and slowly moved toward the prey by vi- 

 brating the finfold or caudal fin. 



7. Only 40% of all feeding sequences were 

 completed. Sequences were ended at all stages 

 up to just before the strike began. The prin- 

 cipal cause of failure to complete a feeding se- 

 quence after a larva began to form the S- 

 shaped posture was the inability of the larva 

 to close the distance between it and the prey. 



8. Under conditions of high food density, 

 larvae fed throughout the day. The average 

 feeding rate for larvae 4 to 27 days old was 1.28 

 completed feeding sequences/min. During the 

 initial filling of the gut in the morning the mean 

 rate was to 1.75/min. 



9. Feeding success of larvae fed Brachionus 

 increased rapidly from 11% success on the first 

 day of feeding (age 3 days) to 50% by age 8 

 days, but the rate of improvement was more 

 gradual thereafter. The relationship between 

 feeding success and age was expressed by the 

 equation, % success = 93.2 (log age) — 33.30. 

 Seventeen-day-old larvae were less successful in 

 capturing Artemia for the first time than they 

 had been in capturing Brachionus, but after 3 

 days of feeding experience on Artemia the larvae 

 regained their former level of success. 



10. The liters of water searched per hour by 

 larval anchovy were estimated by combining the 

 estimate of the reactive perceptive field with an 

 estimate of sustained swimming activity. When 

 the average of the two swimming activity esti- 

 mates was used, the estimate of volume searched 

 was nearly the same as the relationship, liters 

 searched/hr = L^ where L is larval length in cm. 



11. The density of rotifers and dinoflagellates 

 required for larvae to meet metabolic needs was 



837 



