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Thanks to all of you who contributed to the discussion on "fish thermal
basking" either to the list or directly to me.
In short, here are my conclusions:
The body temperature of a fish in a stream is primarily a function of the
ambient water column temperature. Other factors could include short-wave
and long-wave solar radiation, but only under very specific conditions,
depending on water depth, water clarity, water speed, fish "reflectivity"
and fish orientation. Most correspondents felt that the effects of solar
radiation were probably minimal, but that there were conditions where it
might be a factor. I'm interested one of these "specific conditions" where
fish spend considerable time in very shallow, slow-moving (virtually still),
clear water. Despite the risks associated with predation in these shallow
habitats (as mentioned by several respondents), some fish are willing to
expose themselves in these habitats, presumably for foraging and
thermoregulatory benefits.
For those who inquired about species-specific patterns, I've observed
shallow stream-margin use by juvenile cyprinids ("warm water guild")
juvenile catostomids ("coolwater guild") and juvenile and adult salmonids
("coldwater guild"). The cyprinids and catostomids utilized warm shallow
backwater habitats that are often warmer than the main river. The salmonids
used shallow stream-margin habitats that receive cold inflowing groundwater
and are in summer colder than the main river temperature. The salmonids were
presumable seeking thermal refuge from warm ambient stream temperatures. It
is in my efforts to quantify the "thermal experience" of these salmonids
that I encountered the possibility that the temperatures recorded by my
devices may not match the temperatures actually experienced by the fish.
As suggested by several people, I have experimented with shielded and
unshielded thermal probes. My thermistors were placed in clear moving
water, and half were shaded by a perforated PVC pipe. I found that at depths
less than 20cm, unshielded thermistors recorded temperatures that exceeded
ambient water temperatures.
I think, as suggested by several helpful correspondents, that this question
would be most conclusively answered by experimentation using either model or
live fish implanted with thermal tags or probes. The technology apparently
exists for this to be done. If anyone tries this, I would love to hear how
it goes!
Thanks again for all the great feedback,
-Joe Ebersole
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