HOBSON ET AL.: CREPUSCULAR AND NOCTURNAL ACTIVITIES OF CALIFORNIA FISHES 



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PRESUNSET-30min,Z at 3m 



POSTSUNSET- lOmin 



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500 600 



WAVELENGTH (nm) 



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Figure 3. — Changes in down welling submarine spectral irradiance through sunset near the surface at Santa Catalina Island under 

 typical conditions. The shift toward the shorter wavelengths typical of surface waters at sunset (McFarland and Munz 1975c) is 

 apparent in the upper and center graphs, where those wavelengths that divide the number of visible photons into equal quarters are 

 identified by P25, P50, and P75. The bottom graph represents the underwater spectrum when a thin layer of clouds moved over the sky at 

 about 300 m altitude and demonstrates some of the variability that can stem from meteorological events. 



changes in APso- Thus, during our observations 

 under typical conditions, A.P5Q shifted from a pre- 

 sunset value of 527 to 509 nm (Figure 3), and 

 during bloom conditions from 540 to 528 nm (Fig- 

 ure 4). Presumably the APgQ would have been 

 closer to 500 nm had we made comparable mea- 

 surements during twilight when the water was 

 exceptionally clear. 



Submarine Nightlight 



Moonlight, starlight, and bioluminescence are 

 the major forms of submarine nightlight meaning- 

 ful to fishes. We did not measure these during the 



present study, but data available in the literature 

 permit a comparison of the spectra each would be 

 expected to produce in water like that typical of 

 Santa Catalina (Figure 5). The influence of moon- 

 light and starlight on activities in fishes and other 

 aquatic predators has been discussed (e.g., Hobson 

 1965, 1966). This report considers the role of 

 bioluminescent emissions of epipelagic plankton 

 in predator- prey relationships. 



Epipelagic bioluminescent plankton, especially 

 dinoflagellates, are widespread in most neritic and 

 oceanic seas (Tett and Kelley 1973). Sailing narra- 

 tives and logbooks of open water mariners are 

 replete with descriptions of the "phosphorescent 



