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Subject: measures of volume/weight of food items in gut contents of 2-20 cm fishes
From: Gianluca Polgar <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Thu, 17 Feb 2011 03:21:04 +0800
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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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