cannibalism has been investigated by Brownell 

 (1985) and Folkvord and Hunter ( 1986) and found 

 to be an important source of mortality. In this 

 paper the vulnerability of sardine larvae will be 

 compared with that of anchovy larvae and differ- 

 ences in the biology of sardines and anchovies 

 will be discussed. 



Our approach was to observe the avoidance be- 

 havior of Pacific sardine larvae in response to 

 predatory attacks by northern anchovy adults. 

 Adult northern anchovy were chosen as a preda- 

 tor because the northern anchovy was the most 

 abundant pelagic fish in the California Current 

 region during the waning years of the sardine 

 fishery and because its planktivorous diet in- 

 cludes fish eggs and larvae (Loukashkin 1970; 

 Hunter and Kimbrell 1980). 



Materials and Methods 

 Experimental Fishes 



The Pacific sardine larvae used in the experi- 

 ments were reared from eggs spawned in the lab- 

 oratory. Adult Pacific sardines were collected off 

 San Diego and held in 175 m"^ aquarium for six 

 months. Males and females with developing go- 

 nads were isolated in spawning tanks and in- 

 jected with 250 mg human chorionic go- 

 nadotropin and on the following day injected with 

 200 units pregnant mare serum and 20 mg 

 salmon pituitary extract. On the third day fertil- 

 ized eggs were collected from the spawning tank. 

 Larval rearing procedures follow those described 

 by Hunter (1976). Temperature in the rearing 

 tanks was maintained at 21°C. 



Apparatus and Procedures 



Experimental apparatus and procedures were 

 the same as those described by Folkvord and 

 Hunter (1986) but will be briefly outlined here. 

 Experimental predators were two groups of 5 

 adult northern anchovy (range of standard 

 lengths 8.4-9.2 cm). Predators were maintained 

 in two rectangular fiberglass tanks (0.75 x 2.15 

 X 0.83 m = 1.35 m"^) and fed adult brine shrimp 

 except on days of experimental observation. Sea- 

 water was supplied continuously to the tanks ex- 

 cept during experiments. The, temperature in the 

 observation tank ranged from 16.2° to 22.8°C 

 (mean = 20.1°C). Two 100 W incandescent lamps 

 produced 2,000-3,000 mc at the surface of each 

 tank. A black plastic tent enclosing a window on 



one side of the tank provided a darkened observa- 

 tion chamber. 



Each trial consisted of the encounter of three 

 prey with the predators. Prey were introduced 

 into the observation tank with a clear glass 

 beaker. Initial feeding behavior of the predators 

 is quite variable but becomes less variable as the 

 predators become accustomed to prey in the tank. 

 For this reason, prior to each experiment adult 

 Artemia were introduced as prey for five consecu- 

 tive trials to standardize predator behavior. After 

 the preliminary trials with Artemia, three trials 

 with sardine larvae were alternated with one 

 trial with brine shrimp until 18 trials with sar- 

 dine larvae were completed. Each experiment 

 was concluded with a trial of brine shrimp to test 

 for satiation. 



The number of observations for each larval size 

 class was the total number of predator-prey inter- 

 actions observed among larvae in that size class. 

 The mean standard length was determined from 

 20 larvae sampled randomly from the rearing 

 tank on the day of each behavior experiment. The 

 numbers of observations for each size class (mean 

 SL) were 8.0 mm, 41; 11.3 mm, 51; 12.1 mm, 114; 

 12.7 mm, 46; 14.1 mm, 81; 17.6 mm, 104; and 19.6 

 mm, 69. Experiments were not extended to larger 

 sizes due to insufficient numbers of larvae. 



Classification of Behavior 



Prey behavior was scored only when the preda- 

 tor attacked a prey. Four measures of predator- 

 prey interactions were calculated: predator at- 

 tack distance, the distance from which the 

 predator responded to the prey and initiated its 

 attack; frequency of avoidance response; fre- 

 quency of escapes; and predation rate (percentage 

 of larvae captured during the 5-min trials). An 

 avoidance response was a change in speed or tra- 

 jectory of a larvae before the predator had com- 

 pleted its attack by closing its mouth. An escape 

 was defined as a larval response in which the 

 predator failed to capture the larvae in a single 

 attack. Typically adult anchovy make a single 

 attack on a prey item and do not pursue a prey 

 that escapes (Folkvord and Hunter 1986) but 

 rather continue searching the tank. Thus attacks 

 on one prey item were recorded twice if the first 

 attack was unsuccessful. Although predator at- 

 tack distance was recorded, this measurement is 

 highly subjective and comparison with the meas- 

 urements of other observers is suspect. We did not 

 analyze predator attack distance for this reason. 



164 



