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Subject: quantification of "patchiness"
From: andrew thompson <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Fri, 24 Apr 1998 16:38:49 EDT

TEXT/PLAIN (38 lines)

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

Thank you very much,
Andrew Thompson
University of Georgia
graduate student

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