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Subject: Response to fish feeding selectivity question
From: Douglas Megargle <[log in to unmask]>
Reply-To:Academic forum on fisheries ecology and related topics <[log in to unmask]>
Date:Wed, 2 Apr 1997 12:40:00 -0700

text/plain (218 lines)

Fish Folks,

A while back I posted a question regrading the biological significance
of different relative diet ration composition (biomass and frequency),
similar diet, and different food item electivity values of shovelnose
sturgeon from the Missouri River from different locations (Montana and
South Dakota). I put forth the question as to whether a fish species can
exhibit "selective" foraging strategies and "neutral" feeding strategies
in riverine habitat.  If a fish species feeds selectively in one habitat
(river location), and posses a different diet (ration composition)
downstream (~ 500 river miles downstream) with different food item
electiviy values, is it still a selective forager or an opportunistic
forager (adapted to different forage availability)?  Or both.  Generally
the responses indicated a caution regarding interpreting electivity
data. For those who requested a public posting of the responses, I have
included some of the responses.  I wish to thank all those who reponded
to my question.

Dear Douglas,

The concept of selective/opportunistic feeding has been developped for
species, which find their prey visually. So each time the fish sees a
potential prey it can decide, whether it will try to feed on it or not.
I think most benthic fish do not feed predominantely visually
orientated. In most cases the fish is just taking up substrate,
processing it and keeping whatever it is able to keep. So the choice the
fish can make is mainly where and when it starts to dig up substrate. It
may be quite selective in this respect, e.g. prefer certain substrates
over other ect. On the other hand it may be screen the environment and
feed on the most profitable patches of food. How much information do you
have on the macrozoobenthos samples? Your electivity values may look
quite differently if you calculate them seperatly for different
substrate qualities. On the other hand, if your fish are e.g. patch
selective you could simply have a tendency of the individual fish to
"specialize", that means it has mainly one type of food in the gut. By
the way: many people use Jacobs electivity instead of Ivlevs (although I
dont think it makes any essential difference). I'm also working with
benthic fishes and would be very interested on what you decide to do
with your data.

All the best

Irene Zweimueller            e-mail: [log in to unmask]
Inst. of Zoology            phone:  ++43 1 31 336 1337 or 1342
Dept. of Limnology          fax:    ++43 1 31 336 778
Althanstr. 14
A-1090 Wien

You have raised some interesting questions, that are difficult to
answer.  I do not view "selective" and "opportunistic" are opposing
predator characteristics. Let me use an example (fictitious) to
illustrate my point:  Imagine that a predator has 3 prey taxa available
(chironomidae, emphemeroptera,trichoptera) and it consumes them in
proportion to their abundance (non-selective).  A few days later, there
is a mass emergence of ephemeroptera,which makes the nymphs much easier
to locate.  As a result,100% of the predator's diet is emphemeroptera -
despite the fact that there still are substantial numbers of
Chironomidae and trichoptera available.  Selectivity indexes would
indicate that the predator has become much more selective during the
emergence, but is it correct to consider the predator less opportunistic
during the emergence?  The predator may be ignoring some resources
(chironomidae and trichoptera), but what if it is consuming more food
for the same foraging effort by feeding exclusively on emerging
mayflies?  In my view, the predator is demonstrating its opportunistic
nature during this emergence.  I would consider the predator less
opportunistic if it did not take advantage of the abundant emerging
mayflies.  Based on this, I find it difficult to place the "selective"
and "opportunistic" labels on opposite ends of a continuum.  I am more
comfortable treating them as independent characteristics.

I am not subscribed to FISH-ECOLOGY, but I would very much appreciate it
if you would forward any other responses you receive to me.

Jeremy D. Trexel
Department of Fisheries and Wildlife
University of Minnesota
St. Paul, MN 55108

[log in to unmask]

        In response to your question about different percent composition
in the diet, I would think that spatial heterogeneity in prey
distribution could account for different percent compositions in the
The species could be opportunistic but have different prey available to
it due to prey patchiness,  etc.
        Another complication is differential digestion rates which may
be tied to temperature and to prey type. If the study occurs over
differing time scales temperature may be a problem.  I am also skeptical
of gut content analysis of soft-bodied and easily digested organisms
which may be abundant in the environment and also significant components
of the diet but are under-represented in gut analysis due to rapid
digestion or incomplete preservation.
        I am working on a similar problem of spatial and temporal
variation in the diet of striped bass (Morone saxatalis). I am
interested in how you obtained electivity indices and also in how you
obtain fish samples and preserve gut contents.  Any information you
could provide me would be useful.  Thanks.

John F. Walter, III
Department of Fisheries Science
Virginia Institute of Marine Science
Gloucester Point, Virginia 23062
email: <[log in to unmask]>
home:  (804) 684-9995
work:  (804) 684-7316

Hi Doug,
  You've got an interesting problem there. I have used Ivlev's index,
but I was looking at selectivity of deep-sea fishes where the prey field
is largely uniform (little temporal variation in abundance). Your
situation is different because you've got a variable prey field for a
single species. My suggestion would be to treat the different sturgeon
(populations?) as "ecological subunits", standardized to location (since
the Ivlev index relates to prey abundance), i.e. each location with a
characteristic prey field should be considered separately. There is no
reason why a fish couldn't be selective in one prey field and
non-selective in another; there are many scenarios where this could be
energetically desirable. Overall, it sounds like the sturgeon are
opportunistic. If prey field is variable, then an opportunist would also
show a variable diet (usually in the form of percent diet differences).
If all of the sturgeon you looked at had the same basic diet in
locations with different resources, then that would suggest
selectivity.  Was any of this useful?

Tracey Sutton                                 [log in to unmask]
Oceanic Biology Laboratory                    (813) 893-9187
Department of Marine Sciences                 FAX     X-9189
University of South Florida
140 7th Avenue South
St. Petersburg, FL 33701 USA

Hi, Douglas -

It seems to me to be a fairly straightforward question:  if percent
composition in the gut is the same as in the environment, you have an
opportunistic feeder; if it's different, you have a selective one.
these proportions with a chi-square test - you may get some information
from the significance levels).  Can fish switch from being opportunistic
to being selective?  Werner and Hall (1974. Ecology 55:1042-1052)
proposed that the advantage of being (size) selective may disappear if
the abundance of preferred food items decreases.  You may also get some
ideas by thinking about the biology of your prey species and differences
in environmental conditions between the two areas.  (I'm assuming the
length composition of these animals is about the same between the two
locations, and so this is not a factor that would influence your results
- I modestly offer Gabriel and Pearcy (1981. Fish. Bull. 79:749-763) The
explanations may become complex, and it's difficult to generalize from
the small bit of information you present here.

Wendy L. Gabriel

Wendy L. Gabriel
[log in to unmask]
Northeast Fisheries Science Center              PH  (508) 495-2213
National Marine Fisheries Service               FAX (508) 495-2393
166 Water Street
Woods Hole, MA  02543

Dr. Megargle,

I'm not sure I understood your question, but on first reading, yes.  You
didn't say how availability of food differed between sites, but if they
differed, then one would expect differences among diets of opportunistic
(non-selective) feeders.

I am preparing to develop a population model for white sturgeon that
will include some simple feeding relationships. At the moment I'm pretty
ignorant on the subject and I would really appreciate it if you would be
so kind as to share your references on food habits of related sturgeon
species with me.

Thanks in advance,


Yetta Jager
Environmental Sciences Division
Oak Ridge National Laboratory
P.O. Box 2008, MS 6036
Oak Ridge, TN  37831-6036
OFFICE: 423/574-8143
FAX:    423/576-8543
[log in to unmask]

<:=< <:=< <:=< <:=< <:=< <:=< <:=< <:=< <:=< <:=<
<:=<      Douglas J. Megargle                <:=<
<:=<      1512 Yellowstone Avenue,  Unit #1  <:=<
<:=<      Billings, Montana  59102           <:=<
<:=<      (406) 254-9328                     <:=<
<:=<      Email:  [log in to unmask]           <:=<
<:=< <:=< <:=< <:=< <:=< <:=< <:=< <:=< <:=< <:=<

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