Marine fish larvae typically are visual feeders and require a minimal light 
intensity above 10 "-10 lux to feed optimally (14, 16). Light levels reported 
by aquaculturists for successful culture of marine fish larvae have ranged from 
250-10,000 lux (9, 42). A 500-3000 lux range has been used most often. A 
minimum light intensity is necessary for initial detection of prey, visual 
recognition and prey selection. At light intensities close to the threshold level 
there is a gradual reduction in larval activity and in feeding performance (16). 
Blaxter (19) recently has summarized information on the anatomy of eyes in 
fish larvae and also has discussed the development of vision. A pure cone 
retina, which requires relatively high light intensities to be effective, appears to 
be typical of larval stages of fish and such retinas have been identified in the 
Atlantic herring (22), the plaice (12) and other species (19, 23). In general, for 
the few species of larvae that have been studied, the light intensity range in 
which feeding activity decreases is approximately 10 1 -10" 1 lux (11, 12, 13). 
This is near the dusk-dawn light intensity range of 10"-10’^ lux (16). 
Laurence (60) estimated the number of hours required daily by winter 
flounder larvae at 8°C to consume a ration that exceeds the maintenance 
ration. For a prey concentration of 3.4 cal/1 (= 500 copepod nauplii/1), the 
minimum suitable for survival and growth, first feeding winter flounder larvae 
would require about 20 hours per day to consume the maintenance ration. The 
diurnal light period, when light intensities are above 10" lux, is most important 
at low prey concentrations. For prey concentrations exceeding 6.8 cal/1 (= 
1000 nauplii/1) winter flounder larvae could meet their daily food 
requirements in less than 10 hours and at prey levels above 13.6 cal/1 (= 2000 
nauplii/1) established feeders could meet requirements in only 5 hours. It is 
apparent that the seasonal variation in day length and light intensity are 
important elements in any model predicting survival of fish larvae, particularly 
in high latitudes where seasonal variation is greatest. 
Possible harmful effects of natural sunlight on larvae are poorly known. 
Ultraviolet light near the sea surface may be deleterious to pelagic eggs (63). 
Some information on effects of ultraviolet light on pelagic embryos may apply 
to larval stages. Pommeranz (75) tested effects of ultraviolet light on plaice 
embryos, using an artificial UV intensity of 0.05 ly/min (1 ly n 1 cal/cm") at 
the water surface of 350 ml incubators with a 200 ml/min water exchange. 
Although results were not conclusive, lower percentages of embryos survived in 
12 hours and 24 hour-exposed incubators than in control incubators which 
were in the dark. Pommeranz (75) also exposed plaice embryos to natural 
daylight, natural daylight with UV wavelengths filtered out, and natural 
daylight with long wave infrared above 1400 nm filtered out. Average light 
intensities in two experiments were 257 ly/day and 462 ly/day. Only the high 
intensity experiment caused high mortality of embryos (35 percent). 
Ultraviolet light was considered to be the lethal agent because corresponding 
mortalities were not observed in incubators where ultraviolet was filtered out. 
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