Reactions of Salmonid Larvae to Light

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Abstract
I. The larvae of the brown trout, rainbow trout, and salmon were periodically active and negatively phototactic when illuminated. All three species exhibited a positive photo-orthokinesis, the duration of their activity being directly proportional to the log of the light intensity, over a limited range. The light threshold for the kinesis was about 0.005 m.c. but tended to be higher in many young larvae and lower in larvae of a later stage. An upper limit, although more variable, was demonstrated in some individuals, and lay between about 10 and 100 mc. in the older larvae. 2. It is suggested that a photo-orthokinesis, coupled with a negative phototaxis, berves well to maintain the larvae within the limits of their normal habitat at this stage of development. 3. No adaptation of the kinesis occurred after periods of up to 90 min. illumination. But larvae did exhibit a rhythm of activity, reaching peaks at intervals of about 15 min. 4. An increase in the general level of activity, at all light intensities, occurred as the larvae grew, until they were M y active throughout an observation period. At this point the behaviour of the young fish changed, in particular the negative phototaxis was lost and the fish soon began to feed. 5. The increase in the level of activity may be regarded as an index of development in trout and was shown to be related to the general growth of the embryo. 6. Rainbow trout ova hatch about a week earlier than brown trout at l0° C. This does not necessarily indicate a more rapid development in the rainbow trout, Since the latter takes a week longer to achieve full activity, both species reaching this stage at about sixty days after fertilization. The behaviour of the salmon larva does not seem to be strictly comparable with that of the trout during the later sbges of developement

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