FOLKVORD and HUNTER: VULNERABILITY OF NORTHERN ANCHOVY LARVAE 



PREDATOR - Engraulis mordax 



Escaping attack 



Taken in 5 min. 



i i i i 



B 



/ Responding to attack 



i i i i i i i 



2 6 10 14 18 22 26 30 34 38 42 46 50 

 LARVAL STANDARD LENGTH (mm) 



Figure 2.— Vulnerability of northern anchovy larvae to adult northern anchovy 

 predators as a function of larval length. A, Percentage of northern anchovy 

 larvae escaping attack; bars are 2 x SE; line is Weibull curve fit to six points 

 using Marquardt's least squares method (Pielou 1981); equation is N = K(\ - 

 exp (1 - (Lib) -A)) where K = 0.93, 6 = 17.85, A = 2.85, N is the percentage 

 of larvae and L = larval length; and predation rate of the northern anchovy 

 predators (percentage eaten in 5 min) where dashed line is data when ex- 

 periments with biased predator feeding motivation are omitted. B, Percentage 

 of northern anchovy larvae that responded to the attack of an adult northern 

 anchovy; bars are 2 x SE; and Weibull parameters for curve are K = 1.00, 

 b = 13.58, and A = 1.94. 



by differences in predator behavior than is its suc- 

 cess in avoiding the attack. 



The success of avoidance movements can be 

 separated from larval responsiveness by calculating 

 the avoidance success of responding larvae (numbers 

 escaping/numbers responding). Webb (1981) found 

 no change in this fraction over the larval size range 

 he examined (3-12 mm SL), indicating that changes 

 in responsiveness alone were responsible for the 

 decline in the vulnerability of northern anchovy lar- 



vae to Amphiprion with increasing larval length. In 

 the present study, no significant trend existed in the 

 success of avoidance movements over the size range 

 of larvae studied by Webb (1981) but success of 

 avoidance movements greatly increased in larger 

 larvae (Fig. 5). The figure also indicates that north- 

 ern anchovy larvae were much more successful in 

 avoiding Amphiprion than in avoiding adult north- 

 ern anchovy and that the larvae had the least suc- 

 cess in avoiding chub mackerel. 



863 



