164
THE BIOLOGY OF EUPHAUSTTDS
pronounced thermocline, however, was associated with faster speeds
of ascent, more time spent in the surface layers, and slower speeds of
descent a t dawn. Meganyctiphanes norvegica and Thgsanoessa raschii
migrate through the thermocline when it is present in the Clyde but the
investigations have not been detailed enough to show any effects it may
have on the pattern of the migration.
The most detailed work on the physical factors of the environment
controlling the vertical migration of euphausiids is that of Lewis (1954).
He uses the following terms in describing the results of his work.
50% level. The depth above which half the individuals present
in the water column between the surface and 400 fathoms depth
occurs.
Day level of maximum abundance. This is the mode of the
vertical distribution.
Upper deviation level. The depth above which one half of the
individuals present in the water column between the surface and
day level of maximum abundance occurs.
Lower deviation level. The depth above which one half of the
individuals present in the water column between the day level of
maximum abundance and 400 fathoms occurs.
Spread. The distance between the day level of maximum abundance and the upper or lower deviation levels.
This is a measure of diffuseness away from the surface.
Lewis examined the effects of light and temperature on the extent of
the vertical migration by correlating the depths of the low8 isolume at
noon and the 15°C isotherm with the 50% level for all species combined.
He found a significant correlation between light and depth of occurrence, but no correlation with the isotherm. He, therefore, concludes
that the vertical diffuseness of the population is influenced by light
but not by temperature. He examined these correlations for species
in the two genera Euphausia and Stylocheiron, and found a significant
correlation between the depth of the 50% level and the depth of the
isolume for Euphausia but only a positive, but not significant,
correlation between these parameters for Xtylocheiron species. He further
analysed the relationships between light and temperature on the one
hand and the day level of maximum abundance on the other for species
in these two genera (Table VI). The day level of maximum abundance
of species of Euphausia is directly related to the
isolume and
therefore suggests that there is an optimal light intensity which species
of this genus seek by moving vertically in response to altering light
conditions. No such significant correlation existed between the
THE BIOLOGY OF EUPHAUSTTDS
pronounced thermocline, however, was associated with faster speeds
of ascent, more time spent in the surface layers, and slower speeds of
descent a t dawn. Meganyctiphanes norvegica and Thgsanoessa raschii
migrate through the thermocline when it is present in the Clyde but the
investigations have not been detailed enough to show any effects it may
have on the pattern of the migration.
The most detailed work on the physical factors of the environment
controlling the vertical migration of euphausiids is that of Lewis (1954).
He uses the following terms in describing the results of his work.
50% level. The depth above which half the individuals present
in the water column between the surface and 400 fathoms depth
occurs.
Day level of maximum abundance. This is the mode of the
vertical distribution.
Upper deviation level. The depth above which one half of the
individuals present in the water column between the surface and
day level of maximum abundance occurs.
Lower deviation level. The depth above which one half of the
individuals present in the water column between the day level of
maximum abundance and 400 fathoms occurs.
Spread. The distance between the day level of maximum abundance and the upper or lower deviation levels.
This is a measure of diffuseness away from the surface.
Lewis examined the effects of light and temperature on the extent of
the vertical migration by correlating the depths of the low8 isolume at
noon and the 15°C isotherm with the 50% level for all species combined.
He found a significant correlation between light and depth of occurrence, but no correlation with the isotherm. He, therefore, concludes
that the vertical diffuseness of the population is influenced by light
but not by temperature. He examined these correlations for species
in the two genera Euphausia and Stylocheiron, and found a significant
correlation between the depth of the 50% level and the depth of the
isolume for Euphausia but only a positive, but not significant,
correlation between these parameters for Xtylocheiron species. He further
analysed the relationships between light and temperature on the one
hand and the day level of maximum abundance on the other for species
in these two genera (Table VI). The day level of maximum abundance
of species of Euphausia is directly related to the
isolume and
therefore suggests that there is an optimal light intensity which species
of this genus seek by moving vertically in response to altering light
conditions. No such significant correlation existed between the