The effect of age and environmental factors on the vertical migration and distribution of Chaoborus flavicans (Meigen) larvae

The effect of age and some environmental factors, especially light, on the vertical migration and distribution of Chaoborus flavicans larvae were studied both in the field and in the laboratory at Corbett Lake, British Columbia during the summer of 1963. Distribution and migration of Chaoborus larvae were studied largely by frequent horizontal Clarke-Bumpus plankton tows made at 1 metre intervals from the surface almost to the maximum depth of the lake. Marked differences were noted in daytime vertical distribution and diel migration of 5 size (or age) classes of larvae. These size classes probably corresponded approximately to larval instars. Class 0 and 1 larvae inhabited the epilimnion in the daytime throughout the summer, while class 4 larvae were largely confined to the hypolimnion during the day. Class 2 and 3 larvae occupied the epi-, meta-, and hypolimnion in the daytime during June and July, but were found chiefly in the hypolimnion during August and September. Only the older larvae (class 2, 3 and 4) underwent marked diel vertical migration which consisted of 4 phases: 1) daydepth, 2) ascent from day-depth to the surface, 3) gradual descent from surface, 4) rapid descent during dawn. The ascent occurred when subsurface light was rapidly diminishing at dusk, while the descent took place during darkness and was most marked when light started to penetrate the subsurface layers during dawn. Seasonal changes in timing of ascent and descent appeared to be correlated to seasonal changes in time of disappearance of subsurface light intensity during dusk. The rates of ascent and descent calculated from the analysis of echo traces were 13.6 and 1.1 m/hr respectively. Further analysis of the echo traces revealed that the Chaoborus scattering layer was in contact with the lake basin during daytime and descent, but not during ascent. Results from observations of larval migration in experimental tubes housed in a dark room corroborated those of the field. Class 2 larvae having similar daytime vertical distribution (surface and 5 m) as class 0 and 1 larvae underwent virtually no diel vertical migration in the tubes, while class 2 and 3 larvae taken from the deeper layers (10-14 m) of the lake did. The diel migration consisted of the same 4 phases observed in the field, as well as a "dawn rise" phase which was particularly evident for class 3 larvae. Complete migration cycles were induced by artificially changing the natural light intensity over an experimental tube during the period of relatively constant light (0900-1900 hours); the larvae responded most markedly to changes in light intensity at the 0-1000 lux range. Experiments indicated that the diel vertical migration of Chaoborus larvae is an exogenous rhythm controlled by light.Science, Faculty ofZoology, Department ofGraduat