proximate responses to habitat changes, i.e. the cues which say it is time to move up
or down; and (ii) the ultimate or adaptive value of moving, the reason that selection
has repeatedly installed this behavior in many populations. The proximate response is
generally upward or downward swimming as illumination changes in the upper water
column. The response to light can be modified by other variables, for example
temperature structure, food availability, and the presence or absence of visual
predators. Many studies demonstrate the response to light, and some of the nicest are
studies of the movements of the “deep-scattering layers” of animals that reflect
downward-directed sound back up to a listening ship, as in the ADCP study
considered above. Kampa and Boden (1954) used a 12 kHz echosounder, which
means that most of the sound returned from midwater was reflected by swim bladders
of small fish. They identified a sound-scattering layer well before sunset, and lowered
an upward-looking irradiance meter to its top surface. Then, as the sun set, they raised
the meter so as to keep the irradiance constant at the meter, and recorded its depth
continuously. Thus, they obtained a time-series of the depth of the rising isolume,
which eventually passed out through the sea surface. The depth of the scattering layer,
recorded independently after the first data point, tracked the isolume upward exactly
(Fig. 8.17).
Fig. 8.17 Comparison of acoustic-scattering-layer movement from an echo-gram
(continuous line) with the vertical position of a single isolume (6.6 × 10−4 foot-
candles, dots) determined by irradiance metering.
(^) (After Kampa & Boden 1954.)
(^) Clearly, the reflecting fish moved to maintain the illumination around them at a
preferred level of irradiance. Moreover, when this same irradiance re-entered the
water column at dawn, the scattering layer followed it back down. Observations of