FISHERY BULLETIN: VOL. 79, NO. 1 



ent, and underwater visibility exceeded 20 m, but 

 at other times heavy phytoplankton blooms re- 

 duced visibility to <3 m. (We define underwater 

 visibility as the horizontal distance over which we 

 could see major environmental features in day- 

 light.) Most of the time, however, conditions were 

 intermediate between these extremes. Although 

 our irregular observations of visibility do not 

 permit a precise figure, we estimate that at least 

 70% of the time horizontal visibility in daylight 

 was between 8 and 12 m. The images that we saw 

 in these more typical conditions were relatively 

 sharp — more like images seen in clear water than 

 the relatively fuzzy images seen in turbid water. 

 So in characterizing the photic environment in 

 the surface waters, we recognize three sets of con- 

 ditions: 1) clear — when visibility exceeded about 

 15 m amid relatively small amounts of visible sus- 

 pended or dissolved materials, 2) bloom — when 

 visibility was less than about 5 m owing to dense 

 phytoplankton, and 3) typical — the more usual 

 condition, when circumstances were intermediate 

 to the above. Thus, when visibility was about 20 m 

 in July 1976, the spectral radiance showed the 

 essential blue-greenness of the water (Figure 2: 

 clear curve), whereas when visibility was about 3 

 m during a phytoplankton bloom in May 1974, the 

 spectra showed a shift toward the yellow-green 



Figure 2. — Horizontal spectral radiance in warm-temperate 

 coastal waters (Santa Catalina Island) under typical, bloom, and 

 clear conditions, and in a tropical sea (Enewetak Atoll) under 

 clear conditions. Cosine detector was 3 m below surface oriented 

 horizontally (90° from the zenith). All values normalized, with 

 typical condition at Santa Catalina stippled for emphasis. 



wavelengths (with >50% of the photons in the 

 visible spectrum located between 500 and 600 nm; 

 Figure 2: bloom curve). Under bloom conditions, 

 therefore, the radiance was similar to that of lakes 

 rich in plants (McFarland and Munz 1975c). The 

 more usual intermediate condition, however, was 

 closer to the clear than to the bloom condition 

 (Figure 2: typical curve). Nevertheless, even under 

 the clearest conditions encountered at Santa 

 Catalina, the water was greener than it was under 

 similar circumstances in a tropical lagoon (Figure 

 2: tropical-sea curve). 



The spectra depicted in Figure 2 represent a 

 horizontal view, which effectively measures the 

 background spacelight (or horizontal backlight- 

 ing) against which the fishes studied here see most 

 objects. At the same time we also measured 

 downwelling and upwelling spectra, but they add 

 nothing to the topics considered in this paper that 

 is not illustrated by the horizontal readings. So 

 they will be included in our later paper on photic 

 conditions during midday. 



Twilight Spectra 



The broad spectrum of downwelling light in 

 near-surface waters at Santa Catalina shifted to- 

 ward the blue during twilight (Figures 3,4), even 

 though skylight acquires relatively more red 

 photons at this time. Fading daylight characteris- 

 tically loses photons between 550 and 700 nm 

 more rapidly than it loses photons below 550 nm, 

 so that as twilight progresses the proportion of 

 photons at the shorter wavelengths steadily in- 

 creases (McFarland and Munz 1975c; McFarland 

 et al. 1979). The pattern varies in response to 

 changing local conditions, however. For example, 

 on 21 November 1974, events at day's end pro- 

 ceeded typically under a clear sky until shortly 

 after sunset (Figure 3, top and middle panels). At 

 this time, the sky suddenly was covered by a layer 

 of cirrostratus clouds and immediately acquired a 

 red-orange hue (through refraction of the sun's 

 rays). Although we did not record atmospheric 

 spectra at this time, an underwater spectrum re- 

 corded 10 min after sunset (Figure 3, bottom 

 panel) was essentially flattened across the visible 

 wavelengths. Other variations in the twilight 

 shift to shorter wavelengths occur under differing 

 water conditions, as exemplified by a weakening of 

 the phenomenon during phytoplankton blooms 

 (Figure 4). The extent of the blue shift during 

 underwater twilight can be measured by the 



