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Subject: Re: measures of volume/weight of food items in gut contents of 2-20 cm fishes
From: Chris Harrod <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Thu, 17 Feb 2011 09:45:54 +0000
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Gianluca
Have you seen this paper?
Hellawell, J. M. & Abel, R. (1971) A rapid volumetric method for the analysis of the food of fishes. Journal of Fish Biology, 3, 29-37.

All the best
Chris


-----Original Message-----
From: Scientific forum on fish and fisheries [mailto:[log in to unmask]] On Behalf Of Justin C. Johnston
Sent: 16 February 2011 20:39
To: [log in to unmask]
Subject: Re: measures of volume/weight of food items in gut contents of 2-20 cm fishes

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Gianluca,

Is there a lot of variability between like species of the "intermediate" sized items?  If not, what you might try is to subsample and measure many individuals of the same oddly shaped and similarly digested species (species/digested/size category) at once using a similar graduated cylinder method (perhaps try different graduated cylinders to find one that gives the best resolution) and divide the total volume of many critters by the number of organisms to obtain an average volume for the individuals in a subsample.  You could then take replicate subsamples of many of the like sized critters (maybe you could use 50-100 intermediate critters per subsample) to get an understanding of the variation in each species/digested/size category.  After that you could sort gut contents by species/digested/size category and multiply to get volume and error.  I would trust that method, but a statistician probably has some ideas to improve it.

Cheers,
Justin



-----Original Message-----
From: Scientific forum on fish and fisheries [mailto:[log in to unmask]] On Behalf Of Gianluca Polgar
Sent: Wednesday, February 16, 2011 2:21 PM
To: [log in to unmask]
Subject: measures of volume/weight of food items in gut contents of 2-20 cm fishes

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Dear all,

I am working on the gut contents of small fish species (approxim. 2-20 cm TL), and I would like to measure the VOLUME of the food items as a measure of bulk.

1) I may estimate the volume of the meiofauna, which usually includes small and transparent organisms (e.g. diatoms) by using the fine focus knob of an optical microscope to measure the thickness, and digital imaging to measure the area.

2) Much larger items (e.g. > 4 mL), could be measured by water displacement in 10 mL graduated cylinders (usually having an accuracy of  0.2 mL, that is about 5% of the minimum measured volume) ... IF the object (e.g. a large cheliped) fits inside the cylinder.

3) But what about intermediate objects (e.g. larger than about 1-2 mm)? 
These ones do not fit in the visual field of the microscope, and are usually not transparent. Large errors can be made with the fine focus knob. I was thinking about water displacement ... that would mean an instrument to measure the volume of small solids of irregular shape, in a range of 4 microL (+/- 0.2 microL) to 4 mL (+/- 0.05 mL).

What would you suggest? I thought using high performance syringes and calculate the volume with or without the object inside (e.g. 1 x 5 microL, 1x 10 microL, 1 x 100 microL, 1 x of 1 mL, 1 x 2.5 mL and 1 x 5 mL). Yet the internal diameter of the syringes might be too small to fit in the food items ...

I may also wish to measure the WEIGHT of the food items:

1) For smaller ones I could infer it from linear or volume measurements plus density estimates, either available in the literature (e.g. 
Eletheriou & McIntyre, 2005), or empirically determined (e.g. by extraction with Ludox sol for items such as diatoms or forams).

2) Larger items could be measured on a precision balance ... Yet would the former estimates be comparable with these direct measurements?
For example, since the dry weight (oven-drying them to constant weight) would not be comparable to the weight inferred from volume and density estimates (would it?), I should just "blot" or air-dry larger items for a few minutes... yet won't this determine a considerable difference of the percentage of evaporated water between different items?

I almost could not find anything in the literature, and this is my first experience with these methods ...
Could anyone share with me his opinions, comments, suggestions, criticisms, etc.? :-)

Thank you!

All the best,

Gianluca

--
Gianluca Polgar Ph.D.
Senior lecturer in Ichthyology
Institute of Biological Sciences
Institute of Ocean and Earth Sciences
Faculty of Science, University of Malaya
50603 Kuala Lumpur, Malaysia
Tel.: 017-6223549
Fax (ISB): 03-79674178
e-mail: [log in to unmask]
www.themudskipper.org

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