To compare response and escape behaviors of 

 Pacific sardine with those reported by Folkvord 

 and Hunter (1986) for northern anchovy at simi- 

 lar stages of development, mean lengths were 

 converted to ages using field growth rates back- 

 calculated from otolith increment widths for each 

 species. Confidence limits of the percentage of lar- 

 vae responding or escaping attack were estimated 

 assuming the binomial distribution. 



Results 



Probability of Response to Attack 



The youngest larval stages of both sardine and 

 anchovy were the most vulnerable to predation. 

 Only n7c of 8 mm sardine larvae (smallest size 

 tested) responded to attack by adult anchovy. 

 With increasing size more sardine larvae re- 

 sponded to attack. At 20 mm, the largest size 

 tested, 61% of the larvae responded to attack. The 

 response rate of Pacific sardine larvae was consis- 

 tently lower than that of northern anchovy larvae 

 of similar lengths (Fig. 1). Although this differ- 

 ence in responsiveness could be due to differences 

 in the observer, it may also be explained by the 

 difference in age of anchovy and sardine at the 

 same length. Sardine larvae are about 6.2 mm 

 when they begin feeding (age = 5 days from fertil- 

 ization at 17°C), whereas first-feeding anchovy 

 larvae are only about 4.3 mm (age = 5 days from 

 fertilization at 17°C) (Zweifel and Lasker 1976). 

 Sardine larvae also grow faster than anchovy lar- 



100 



o 80 1- 



z 



a. 



O 60 



(A 



U 

 OC 



40 



20 - 



ANCHOVY 



V.-- 



SARDINE 



8 12 16 



LENGTH (mm) 



24 



Figure 1. — Increase by size of the percentage of Pacific sardine 

 larvae, Sardinops sagax, and northern anchovy larvae, En- 

 graulis mordax, responding to attack by adult northern an- 

 chovy and 95% confidence intervals. Data on anchovy larvae 

 from Folkvord and Hunter (1986). 



vae at the same temperature (Butler and Rojas de 

 Mendiola 1985). Thus, sardine larvae are younger 

 at a given size than anchovy larvae. 



Since the latency of response to attack must be 

 related to the development of the central nervous 

 system (Webb 1981; Webb and Corolla 1981), it 

 may be more appropriate to compare sardine lar- 

 vae with anchovy larvae of the same age. For that 

 reason lengths of the larvae of both species were 

 converted to age using growth rates measured in 

 the field (Methot and Kramer 1979; Butler 1987). 

 Comparison of the percentage of larvae respond- 

 ing to attack at a given age (Fig. 2) reveals no 

 significant difference in the rate of development 

 of response to attack. Thus, the escape response 

 develops at the same rate in Pacific sardine and 

 northern anchovy, and the difference in propor- 

 tion of larvae responding at a given size (Fig. 1) 

 is due to the difference of size at hatching and 

 the difference in growth rates of the two 

 species. 



100 



o 



z 



a. 

 < 

 O 

 If) 



LU 



o 



OC 

 UJ 

 Q. 



80 - 



60 - 



40 



20 



10 



20 

 AGE (days) 



30 



40 



Figure 2. — Increase by age of the percentage of Pacific sardine 

 larvae, Sardinops sagax, and northern anchovy larvae, En- 

 graulis mordax, responding to attack by adult northern an- 

 chovy and 95% confidence intervals. Size categories of reared 

 larvae have been converted to ages using growth rates esti- 

 mated from the field. Data on anchovy larvae from Folkvord 

 and Hunter (1986). 



Probability of Escaping Attack 



The ability to successfully avoid attack in- 

 creased with size of Pacific sardine as well as 

 northern anchovy. Few small larvae of either spe- 

 cie escaped attack by adult northern anchovy. 

 Only 3% of 8 mm sardine larvae escaped attack 

 and the percentage of larvae escaping increased 

 to only 11% for 17 mm larvae and 13% for 20 mm 



165 



