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Fishery Bulletin 106(4) 
Log I 
Figure 2 
Electroretinogram (ERG) responses to increasing light intensities (I, in log can- 
dela/m 2 ) in black rockfish (Sebastes melanops ) and Pacific halibut (Hippoglossus 
stenolepis). To construct response curves, ERG responses to 200-ms duration 
light flashes were recorded as light intensity from a white LED light source was 
increased in 0.2 log-unit steps from levels that produced no measurable responses, 
to those that produced maximal responses. Responses from individual trials 
were normalized by expressing them as a fraction of the maximal response. Data 
recorded at light levels beyond those that produced a maximal response have been 
omitted. Results obtained during the day and during the night were analyzed 
separately. Data points are means ±standard error; open circles show results 
obtained during the day and filled circles results obtained during the night. The 
Weibull four-parameter function contained within SigmaPlot for Windows vers. 
10 (Systat Software, San Jose, CA) was fitted to the mean V-log I response data. 
A sigmoidal curve was chosen because the ERG responses generally approached 
zero at low light intensities, became saturated (i.e., approached a maximum) at 
high light levels, and varied in between. Dotted lines show the predicted values 
based on data collected during the day and solid lines show predicted values 
based on data collected during the night. For illustrative purposes, vertical 
dotted and solid lines have been added showing the light intensities required 
to produce responses 50% of the maximal response during the day and night, 
respectively. 
fish were approximately 0.5 to 0.8 log units higher 
than the mean values for control fish (Table 1). In 
other words, the light levels required to reach the 50% 
response points in Pacific halibut exposed to simulated 
sunlight were three to seven times higher than the 
light levels required to reach the 50% response points 
in control fish. 
Flicker fusion frequency 
There were no day-night differences in the flicker fusion 
frequencies for either black rockfish or Pacific halibut. 
There was also no detectable influence of rapid decom- 
pression on flicker fusion frequency for black rockfish, 
nor an apparent effect of exposure to bright light on 
