FISHERY BULLETIN: VOL. 84, NO. 4 



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Artemis TRIALS 



Artemia AND LARVAL TRIALS 



mean predator 

 error 



(during trials 

 with larvae) 



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 ® 2min 



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Artemia TRIALS 



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Artemia AND LARVAL TRIALS 



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TRIAL NUMBER 



Figure 1.— Variation in feeding performance of northern anchovy predators fed live 

 Artemia as a function of trial number (equivalent to elapsed time of experiment); 

 shaded area indicates trials in which northern anchovy were fed only Artemia; un- 

 shaded areas, Artemia trials alternated with larval trials. A, percent predator error 

 in capturing adult Artemia (percentage of attacks in which a northern anchovy missed 

 the prey); dashed line indicates mean. B, mean time required for predator group 

 to capture 3 adult Artemia; bars are 2 x SE of the mean. (N = 21.) 



Success of Avoidance Movements 



Larval vulnerability depended not only on the 

 responsiveness but also on the success of avoidance 

 movements. The proportion of larvae escaping 

 northern anchovy predators increased from 8% for 

 6 mm larvae to 92% for 33 mm larvae with an esti- 

 mated 50% of the 17 mm larvae escaping. The 

 percentage of larvae escaping the attacks of chub 

 mackerel was lower than for adult northern an- 

 chovy, but the curves given in Figures 2 and 3 had 

 a similar form. Weibull curves were fit to the data 

 to provide trend lines (equations and parameters 

 given in Figure legends). The fraction of larvae that 



escaped increased from 6% of 6.7 mm larvae to an 

 estimated 50% of the 30 mm larvae. Of the 50 mm 

 juvenile northern anchovy used as prey only 64% 

 escaped the attacks of the chub mackerel. 



The ability to successfully avoid predator attacks 

 was strongly affected by species-specific differences 

 in predator behavior since the fraction of larvae 

 escaping the attacks of northern anchovy increased 

 much more rapidly with larval length than did the 

 fraction escaping the attacks of chub mackerel. In 

 contrast, the fraction of smaller larvae (SL <20 mm) 

 responding to the attacks of these two predators 

 was similar (Fig. 4). This indicates that probability 

 of a larva responding to an attack is less affected 



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