Our new otolith sectioning system

Because stock assessments are relying more and more heavily on age data, our group has had to provide more ages in the same amount of time as before without any appreciable increase in staff. It was, therefore, necessary for us to increase our efficiency. For otoloith sectioning, we obtained a new high speed saw system from Benetec Ltd. in the United Kingdom. Similar systems are being used in ageing laboratories in Europe, but this is the first of its kind in North America and only the eighth in the world.

Our traditional sectioning method had been to align one otolith on a cardboard tag and mount it in paraffin wax. This tag was then inserted into a custom-made chuck on an Isomet low-speed saw. Two closely spaced diamond-impregnated blades, separated by a thin spacer, were used to cut a thin section. This section was removed, placed in paper, and then returned to the envelope from which it came. More information on this method can be found in Pentilla and Dery (1988) or via the Ageing Manual link.

With the new otolith sectioning system, otoliths are embedded in polyester resin in aluminum molds. The sulcus of each otolith is aligned for sectioning through the use of a video monitor.  Four to eight otoliths are aligned in each row, with six rows per mold.

The blocks of resin are then cut using a high-speed saw with one thin metal-bonded diamond blade. Two cuts are made for each row of otoliths, yielding a thin strip of resin in which the sections are embedded. These six strips are then glued onto a Plexiglas slide.

Because this system allows up to 48 otoliths to be sectioned at once, it greatly increases efficiency. It takes less than 30 minutes to cut one block of otoliths. Also, mounting the sections on Plexiglas slides makes it much easier for the age reader, by eliminating the need to pull each individual section out of an envelope. It is hoped that this machine will also reduce breakage of otolith sections, and eliminate the need for fine-tuning the thickness of the cuts. Overall, we are finding that this new system reduces the amount of time needed to section otolith samples, allowing this laboratory to provide more age data to other groups.
 

Validation of the new technique

In order to ensure that ages obtained from otoliths sectioned with the new system are identical to the ages that would have been observed if the otoliths had been sectioned with traditional methods, a study was undertaken to compare the consistency between ages determined with the new system and prior sectioning methods.

For this comparison, both otoliths from 219 haddock were sectioned, one otolith with each technique. One person aged all the samples using standard procedures (Penttila & Dery 1988). Precision of ages obtained with the two sectioning methods was measured by the coefficient of variation (CV), which is the ratio of the standard deviation to the mean, averaged over all the fish (Chang 1982).

Of the 219 fish, 198 were successfully aged by both methods. Ages ranged from 0 to 9 years, with a high level of agreement (90.4%).

The mean CV for all samples was 2.15%. This is lower than most reported CV values for haddock, which range from 1.4 to 14.4% in the Scotian Shelf (Campana 1995, Campana et al. 1994). There may have been a very slight bias toward lower ages with the new system at ages 3 to 5, but the average ages differed minimally between the two sectioning methods.
 

The high agreement level and low CV indicate there is a high level of precision between the two sectioning methods. If the method generates accurate ages, as we assume it does, this means ages from the new system are also accurate. Therefore, future population assessments do not need to take into account which sectioning method was used to generate age data.
 

Summary

Our new otolith sectioning system is clearly a step in the right direction toward modernizing and streamlining the process of ageing fish. This system enables us to mount and section up to 48 otoliths per block, depending on the species and otolith size, rather than one otolith at a time. It yields ages nearly identical to those obtained with the previous system, which means no conversion will be necessary to incorporate ages obtained with the new system into the age database.

A great amount of time is saved by this new system, merely because so many otoliths can be sectioned at once. The actual time required to cut the sections is about 5 minutes per thin section, and less than half an hour for an entire block. Admittedly, three days are needed to complete the entire process, and many of the steps require two people. However, these steps only take up a portion of a given workday, with long periods of waiting for the resin to harden. It is possible to prepare, mount, and section 16 blocks in one week. This could account for as many as 750 fish, what would have taken a few weeks of dedicated work on the old system.

With the previous method, much work was required to ‘fine-tune’ the saw blades so that the proper thickness was attained for each species, and to avoid breaking sections while cutting. The new system will eliminate the need for fine-tuning, as all the otoliths are mounted uniformly and the sectioning machine has only one blade. Another advantage is the ease of ageing samples mounted on Plexiglas slides. Age readers previously had to remove each individual section from an envelope, but now can view many sections at once. This is both more efficient and ergonomic.

At this moment, we are still working out some bugs in the system. Currently, a portion of the otolith sections are lost or broken with the new sectioning machine. Even so, the otolith halves, embedded in the 16 mm sections, are still available and ageable. Further practice avoiding trapped air bubbles near the mounted otoliths, and obtaining a thinner blade, should reduce this problem. Also, it seems the new technique occasionally creates more hyaline sections than before, which could increase ageing error. This can be avoided by viewing both the sections and the otolith halves. Finally, we are seeking a better method of affixing the sections onto Plexiglas, so that both sides of the sections are readable.

As with all progress, it will take a while to adjust to the new methodology, but in a few years, we will wonder how we ever lived without it.
 
 

References

Campana, S.E. (1995) Expert age determination of 4VW and 4X haddock otoliths by national and international laboratories. Report No. Research Document 95-120, Department of Fisheries and Oceans Atlantic Fisheries.

Campana S.E., Annand M.C., and McMillan,  J.I. (1994) Graphical and statistical methods for determining the consistency of age determinations. Transactions of the American Fisheries Society 124:131-138.

Chang, W.Y.B. (1982) A statistical method for evaluating the reproducibility of age determination. Canadian Journal of Fisheries and Aquatic Sciences 39:1208-1210.

Penttila J. and Dery, L.M. (1988) Age determination methods for Northwest Atlantic species. Report No. NOAA-TR-NMFS-72, National Marine Fisheries Service, Woods Hole, MA (USA).
 
 
 
 

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