Are you looking at frequency of dace visit or time spent on stones as a
function of prey density? If you are examining either of those, why not
regression? If you are looking at how distribution of dace relates to that
of prey density, I don't think the size of stone matters.

At 04:38 PM 4/24/98 EDT, you wrote:
>Dear fish folks,
>
>     I have been studying foraging behavior by longnose dace, Rhinichthys
>cataractae, in the Coweeta Creek Drainage (southern North Carolina).  One of
>the questions I am addressing is whether longnose dace are responding to
>variability in macroinvertebrate density on individual stones by foraging
>with greater intensity on stones with high prey density.  In order to test
>this question, I observed longnose dace forage upon 100 stones, counted the
>number of bites taken from each stone, and collected macroinvertebrates from
>each stone.  There were great differences in macroinvertebrate density on
>individual stones, so I felt there was good reason for longnose dace to forage
>with higher intensity on stones with high prey density.  My question is:
>
>How can I say (statistically) that great differences existed in
>macroinvertebrate density per stone?
>
>Normally, one would take a variance/mean ratio or compare counts data with
>a poisson distribution in order to state that prey were clumped or uniformly
>distributed among individual stones.  However, 1) my data was collected from
>stones which varied in size (i.e. quadrant size differed among samples), and
>2) I am using macroinvertebrate biomass instead of number for my units. As I
>mentioned above, there were great differences in the biomass density of
>macroinvertebrates on stones (there was a 250 fold difference in biomass
>density between the lowest and highest density stones) and I just need a
>method to express this quantitatively.
>
>I realize this is a pretty specialized problem, but I would appreciate any
>feedback.
>
>Thank you very much,
>Andrew Thompson
>University of Georgia
>graduate student
>
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Ralf Riedel
Department of Ecology, Ethology, and Evolution
The University of Illinois, and
The Center for Aquatic Ecology - INHS
607 East Peabody Dr.
Champaign, IL 61820
217-244-5511

Home address:
Box 2731-A
Champaign - IL 61825
USA
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