I'm using 800; 1000; 1200; 2500 grinding paper (water all the time), and polishing suspension at the end. The problem is that glass eel stage is seen ok, but not the annual rings. view is blurred, or the rings are not visible at all (something like this: http://www.diadfish.org/doc/document_2009/CD_otolithe_et_alose/CDotholite/images/planche/poncage1.jpg). Sometimes it looks like the otolith surface is very scratched. Any suggestions or ideas?
The scratches you see in the image suggest you are not polishing enough. In viewing polished sections, I usually place a drop of mineral oil on the otolith and add a cover slip, or submerge the slide in water. This greatly enhances the image. You may also try using reflected light and place the image on a black background. Electrical tape on the bottom of a petri dish works well. Good luck.
Alternatively you can use a thermoplastic resin like CrstalbondTM to attach the section to your slide using a heat plate and then put a coverslipe over the top of that such that the resin fills the scratches and makes the section easier to see. Heating the section on the heat plate also "burns" the protein within the matrix and sometimes essentuates the annuli. otherwise you can try to acid etch the section as well, but I found that by heating and using the resin, the slides were easier to see. If the eels are caught near the equator, you might find the differentiation in the annuli harder to see compared to others caught closer to temperate waters. Also try transmitted light, but use light sources off to the side (45 degrees onto the slide) rather than 90 degrees to the slide.
..as already recommended a little application of heat and etching with acid or EDTA will increase the visibility of the rings. Also try using the finer grade of carborundum/aluminium powder to make your otolith surface appear smoother.
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