Moriarty and Steinmetz (1979) compared five different techniques for ageing eels. They used otoliths from eels of known age from ponds. The number of otoliths was low but some interesting information was obtained. No method was equally good in ageing young and old eels, even though the eels were from the same pond. There were considerable differences, from three to ten years, between the ageing results using the different methods. Grinding or polishing gave the best results on young specimens, whereas burning and cracking gave the best result with the older eels. The eels used in this study were all slow growing. Eels from the same pond were some years later aged by burning and cracking, and clearing in ethanol. The mean difference between the methods was 4.5 years (Vøllestad and Næsje, 1988). Vøllestad (1985) compared the results using two methods (burning and cracking; clearing in alcohol) on otoliths from brackish water in Norway. Burning and cracking yielded an age estimate 1.25 years above that obtained by clearing in alcohol, and only 8 otoliths (3.7 %) yielded the higher estimate when cleared in alcohol. Studies by N. Barak (pers. comm.) and A. Richards (pers. comm.) further show close agreement between the two methods. Jarmatz et al. (1986) compared the result using burning and cracking, with sliceing and reading in stereomicroscopes. No systematic differences were found, and they concluded that the burning and cracking method was to be preferred because it was less time consuming. Lecomte-Finiger et al. (in prep.) compared 1800 otoliths from five different lagoons of the Gulf of Lion. First, whole otoliths, stored dry, were read using reflected light against a dark background in camomil essence. Observation and counting of the zones from four of the lagoons was easy. But between 26 and 47 % of the otoliths were unreadable in the fifth lagoon. The otoliths were then embedden in methyl metacrylate and treated according to the method of the french “Groupe National Anguille”. Grinding increased the number of readable otoliths in the fifth lagoon.
