WEBB: RESPONSES OF NORTHERN ANCHOVY LARVAE TO PREDATION 



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MEAN ESCAPE SPEED = 4.53 L + 2 73 



{r2=0.675i N=113) 



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TOTAL LENGTH (cm) 



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Figure 4. — The relationship (dots) between mean escape speed 

 and total length of northern anchovy larvae attacked by clown 

 fish. Vertical bars are ±2 SE. Circles show mean escape speeds 

 during chases. The line for mean burst speeds was taken from 

 Webb and Corolla 11981). 



the cumulative percentage of observations was 

 plotted on a probit scale against these values. A 

 logarithmic transformation of ALT values 

 linearized the relationship to the greatest extent 

 (Figure 5) implying ALT's were log-normally dis- 

 tributed in the population. Some curvature re- 

 mained after this transformation, probably due to 

 decision errors (Treisman 1975) and individual 

 variability in response latencies. Thus ALT values 

 were assumed to be log-normally distributed in 

 the larval populations sampled, and maximum 

 likelihood means and variances were calculated 

 for a type I singly censored sample (Table 2) as 

 described by Cohen (1961). Resulting mean ALT's 

 are shown as a function of total length in Figure 6. 

 The most important result from this analysis 

 was that ALT's decreased rapidly with increasing 

 size, and hence development, tending towards a 

 plateau for larger larvae and presumably for 

 adults. The relationship was best described by a 

 power function with a negative exponent (see Fig- 

 ure 6) but such a description should be treated 



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O 1 I66cm 

 A 0626 cm 

 D 399cm 



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LOOMING THRESHOLD (rod sec"') 



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Figure 5. — Representative relationships between cumulative 

 number of startle responses, expressed as percentages of total 

 attacks (probit scalei and the logarithm of ALT's for northern 

 anchovy larvae of three different total lengths. 





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TOTAL LENGTH (cm) 



Figure 6. — The relationship between the maximum likelihood 

 mean ALT and total length of northern anchovy larvae attacked 

 by clown fish. The relationship is shown for logarithmic coordi- 

 nates in (A) and for arithmetic coordinates in the inset (B). 

 Circles are for prefeeding yolk-sac larvae. 



with caution. This is because the ALT distribu- 

 tions were severely censored, with a resulting large 

 variance about each mean. In addition, the re- 

 sidual curvature in the transformed data could 

 influence mean ALT values, but this would not 

 alter the general shape of the inverse relationship 

 between ALT's and larval size. 



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