FISHERY BULLETIN: VOL. 74, NO. 1 



After the squid oriented toward a particular 

 prey, it approached the prey until it was within 

 attacking distance (positioning). This distance 

 was not constant. At times there was no clear 

 separation between the attention and positioning 

 patterns. LaRoe (1970) suggested that the posi- 

 tioning approach is an example of an aggression- 

 fear conflict. This appears to be the case in Loligo. 

 The young squid would sometimes flee rapidly 

 after closely approaching a large prey. 



The prey was usually captured with the tenta- 

 cles (seizure), although occasionally the arms 

 alone were used. The arms were used to maneu- 

 ver the food toward the mouth. At times a new 

 attack began while the squid was holding other 

 prey in the arms. 



LaRoe (1970) reported that for Sepioteuthis 

 sepioidea physical fights over food were rare. This 

 was not true for young L. opalescens. Fighting 

 between squid was never observed when prey was 

 small (brine shrimp nauplii), but if the prey was 

 large and could not be completely enclosed within 

 the arms, other squid would often chase the one 

 which caught the food and try to take the food 

 away from it. Often several (in one case, four) 

 squid held on to the captured prey and all fed on 

 it. The prey would be tugged about until one 

 squid pulled it away from the others. This be- 

 havior occurred even when there was an abun- 

 dance of prey in the tank. This attack on captured 

 prey at times allowed small squid to eat larger 

 prey organisms than they could normally subdue 

 alone. 



Prey Selection 



Unlike Sepioteuthis (LaRoe 1971), young L. 

 opalescens were not extremely selective as to the 

 type and size of prey they would attack. Within a 

 few days after hatching, the young Loligo (2.7 

 mm ML) readily attacked Artemia nauplii (0.7 

 mm long), Artemia adults (5 mm long), copepods 

 (1 mm long), and larval fish (4 mm long). Occa- 

 sionally, squid attacked and ate dead prey (e.g., 

 6.ea.6. Artemia dropped into the tank), but usually 

 the food had to move before it was attacked. An 

 exception to this was that the squid attacked fish 

 larvae which appeared to be motionless in the 

 water. 



When the squid were 17 days old, nine squid 

 from group 2 were placed in a small cylindrical 

 container (8 liters of water) to determine whether 

 a food size preference existed in Loligo. The food 



used was Artemia nauplii (0.6 to 0.8 mm long) at 

 10/ml and small adult Artemia (2 to 4 mm long) 

 at 0.2/ml. After the squid were added, I recorded 

 the number of attacks until a prey was captured 

 and the type of prey being attacked. If no prey 

 was captured in 20 min, I selected another squid. 

 At this age, the squid attacked both large and 

 small prey. During the 164 min of observation, 23 

 nauplii were attacked (9 actually captured) and 

 30 adults were attacked (8 actually captured). 

 These results are different from those given for 

 Sepioteuthis sepioides (LaRoe 1971). That squid 

 only attacked food species in a very limited size 

 range. Within several days, Sepioteuthis would 

 cease to attack the prey it had previously eaten 

 and would only attack larger prey. This seemed to 

 occur when the squid were 1 to IV2 times as large 

 as their prey. Although Loligo captured both 

 large and small prey with about equal frequency, 

 a preference may exist for larger prey as their 

 density in the container was much lower. 



An experiment was run with group 1 when the 

 L. opalescens were 49 days old. In this case the 

 choice was between two different prey species of 

 approximately the same size. Two thousand 

 2-day-old chub mackerel, iScom6erJaponicus, lar- 

 vae were added to one of the rearing tanks where 

 the squid had been feeding on Artemia adults. 

 There were approximately 2,000 Artemia in the 

 tank. The same method was used to record feeding 

 as in the previous experiment. Observation time 

 in this case was 69 min. The squid showed a high 

 incidence of attacks on fish larvae (52 attacks, 6 

 captures) even though the success rate was much 

 lower than when attacking Artemia (4 attacks, 3 

 captures). This may indicate a preference for fish 

 larvae, but without further experiments it is im- 

 possible to say whether this is true. 



Feeding Success 



The ability of the squid to successfully complete 

 an attack sequence depended on the size and 

 species of prey and the age and experience of the 

 young squid. Figure 2 is a record of the percent of 

 successful attacks on Artemia nauplii as a func- 

 tion of the age of the squid. Each point is an aver- 

 age from the squid observed during that day. The 

 number of squid observed per day ranged from 5 

 to 11, with the total daily observation time rang- 

 ing from 25 to 55 min. The attack efficiency in- 

 creased with the age of the squid, but a number of 

 prey were still being lost even after 3 wk. LaRoe 



178 



