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Age and Growth Program

Thin Sectioning Procedure

Thin sectioning is used to supplement the break and burn method in the age determination of rockfish species and other difficult to age species. The AFSC thin section method is a modified version of a similar thin section technique developed in Australia (Smith et al, 1995). The modified thin section was used to develop the ageing criteria for shortraker rockfish and Pacific cod and is currently the primary method used to age shortraker rockfish. Below is a brief description of the procedure along with accompanying photographs.

Equipment: polyester resin with catalyst (Artificial WaterT), paper cups, glass and plastic pipettes, molds, slides, cover slips, saw, gloves, calipers.

Step 1: Mix 2 ounces of artificial water to 1 ml of catalyst. The mixture starts to harden in 5 minutes. This is enough resin to cover the bottom of 4 blocks on one mold. You can vary the amount of resin depending on the number of blocks needed. You can also vary the amount of catalyst mixed into the resin to adjust the curing time.

Step 2: Add enough resin to cover the entire bottom of the block. Let the bottom layer cure for 40 minutes. Score the resin to create two rows. Place the otoliths on the scored mark so that the otolith cores line up. The resin will be hard enough to prevent the otoliths from sinking into the resin. Add a label to identify to otoliths. Pour enough resin over the otoliths to completely cover them. Do not add too much resin. The idea is to keep the block thin because thinner blocks decrease cutting time.

Step 3: Use a high speed table saw such as a TyslideT with Pro-slicer T 0.004 inch diamond blade to cut the block into thin sections. Three thin sections are cut to insure that one thin section contains the first year of every specimen within that row. The Tyslide can make reliable cuts to a thickness of 0.4 mm.

Step 4: Loctite 349T is an adhesive resin used to mount the thin section onto slides. The adhesive is cured under UV light for 20 minutes. Labels are attached to the slides to identify specimens.

Step 5: The HillquistT grinding wheel is used to grind down the mounted thin section to any desired thickness. For example, the thin section thickness for shortraker rockfish is 0.2 mm.

Finished product:

Arrowtooth flounder thin section viewed with reflected light.

Shortraker rockfish thin section viewed with transmitted light.


    Smith, D.C., Fenton, G.E., Roberston, S.G., and Short, S.A. 1995. Age determination and growth of orange roughy, Hoplostethus atlanticus: a comparison of annulus counts with radiometric ageing. Can. J. Fish. Aquat. Sci. vol. 52 pp.391-401.


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