HUNTEK and NICHOLL: NORTHERN ANCHOVV SCHOOl.INC THRESHOLD 



Table 3. — Upper and lower bound and geometric mean for the 

 visual threshold for schooling of adult northern anchovy, 

 Engraulis mordax, in the various energy units. 



Energy units 



Schooling' 



No 

 schooling^ 



Geometric 

 mean^ 



7.785 X 10 " 6.051 x 10 



Radiometric 



(W cm ') 4.777 x 10 



Anchovy effective' 



(W cm 'anch. eff.) 3.079 x 10 '° 5.018 x 10 '^ 3.900 x 10 " 



Photometric* (mc) 2.048 x 10' 3.337 x 10' 2.594 x 10' 



'Lowest irradiance level at which schooling occurred. 



'Highest irradiance level at which anchovy failed to school. 



'Geometric mean of the irradiance at the upper and lower 

 bounds of the threshold. 



'Weighted by ERG action spectra for Engraulis encrasicholus 

 dark adapted retina (413-612 nm) (Protasov 1964). 



^Weighted by the 1964 human photopic response (413-600 nm). 



than that for jack mackerel (3.5 x lO"'' mc, Hunter 

 1968), a species associated with anchovy in the 

 California Current. Visual thresholds for schooling in 

 fishes range from about 1 x 10"-^tol x 10"' mc 

 with about 90% (14/16) of the literature values being 

 higher than anchovy (Blaxter 1970). We do not at- 

 tach much importance to such specific differences 

 because criteria for schooling differ widely and radio- 

 metric procedures in the older studies were primitive 

 by today's standard. We suspect the threshold for 

 jack mackerel may have been lower than the north- 

 ern anchovy because of use of a uniform and highly 

 reflective background in the apparatus and the use of 

 photometric brightness as a unit of measurement. In 

 our work the brightness to the sides and below was 

 much lower than the downwelling irradiation 

 whereas this was not the case in the jack mackerel 

 experiment. 



ACKNOWLEDGMENTS 



We thank Mike Sokol (Southampton College, NY) 

 for constructing the apparatus and for conducting 

 some of the experiments and Sandor Kaupp (Univer- 

 sity of California at San Diego) who provided advice 

 and assistance throughout the study. We also thank 

 Tilman Pommeranz and Geoffrey Moser for permit- 

 ting us to use their unpublished data on vertical 

 distribution of anchovy eggs, and Paul Smith, Tilman 

 Pommeranz, Roger Hewitt, and J. H. S. Blaxter for 

 reviewing the manuscript. 



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241 



