A large number of different techniques are used for ageing eels, as illustrated by the preceeding sections. There are four main types of ageing techniques in use today, these are methods using ground otoliths or otolith slices, burnt and cracked otoliths, whole otoliths after clearing, and methods using advanced technologies. Each of these main techniques will be discussed separately. But first it has to be decided which criteria to use when accepting one technique over another. The first criterion must be that the method is validated; i.e. the accuracy of the method must be confirmed. Here a clear distinction must be made between the accuracy and the precision of the age determination. Precision relates to the reproduceability of age estimates and does not imply accuracy or validity (Beamish and McFarlane, 1983).
Numerous methods have been used in an attempt to validate age assessments of fish. The most powerful direct evidence for a method's validity is obtained by examining structures from fish of (partly) known age, e.g. stocked fish that have lived in the natural environment or have been reared in captivity under natural or seminatural conditions. Other ways of obtaining direct evidence of a method's validity is by mark-recapture experiments, preferably accompanied by use of internal markers such as tetracycline. Other methods for validating age assessments are analysis of length-frequency modes, monitoring of strong year classes, examination of the edge of a structure (i.e. otoliths) during the season, comparison of backcalculated growth with observed lengths of the corresponding age group, or comparison among different ageing techniques (Beamish and McFarlane, 1983).
Often it is not possible to validate or even use different structures to verify age assessment. In such cases, several readings should be made to increase the precision of the estimate. Ideally this replication should be done by several different interpreters (Casselman, 1983).
Grinding is the method in use at most institutes. This method has been modified so extensively that at present almost all institutes have their own versions. These small variations over the central theme probably are of minor importance. The use of otolith slices is similar to the use of ground otoliths.
Using otolith slices (as recommended at the 1979 meeting of the ICES/EIFAC Working Party on eels) and transmitted light Berg (1985) checked his interpretation of annual rings in the otolith parts deposited between tagging and recapture of eels in Lake Constance. In this way Berg was very close to a true validation of his otolith annuli. A modification of the grinding technique, the acetate peel technique, has been validated as giving correct age assessment of American eels of known age from ponds (Liew, 1974).
The burning and cracking method has been useful for ageing slowgrowing species from temperate regions. This is mainly because when growth is very slow, the dorso-ventral growth of the otoliths is greatly reduced and subsequent otolith growth is mainly lateral (Christensen, 1968). Thus, some form of cross section of the otolith may be necessary to estimate correct age (Skurdal et al., 1985). For eels it is unlikely that growth is so much depressed with age that dorso-ventral otolith growth stops. Using burning and cracking the different zones appear very clear due to the contrast between the burnt organic matrix and the unburnt calcium carbonate. Moriarty (1983) has also shown that the accuracy of the method is good by comparing growth rates from tagged and recaptured eels. Burning and cracking was also the best method in the studies of Moriarty and Steinmetz (1979) and Vøllestad and Næsje (1988). By monitoring the eel stock in the River Shannon Moriarty (1982) observed a decline in the fisheries occurring ca. 20 years after the inhibition of the elver ascent. This agrees closely with the observation that more than half of a sample of silver eels were aged over 19 years. Further, Aprahamian (1987) could recognize year-classes of stocked elvers in the upper reaches of the River Severn. The burning and cracking method is comparatively rapid and the prepared otoliths can conveniently be stored on microscope slides (20 – 25 otoliths on each slide). No expensive equipement is necessary.
The method using whole otoliths cleared in alcohol is rapid and inexpensive. After the first clearing the otoliths may be read repeatedly without further clearing. Vøllestad (1986) found that only one opaque and one translucent zone was deposited in the otoliths of brackish water eels from Norway. The reproduceability of the method is as good as for the burning and cracking method (Vøllestad, 1985), and Vøllestad and Jonsson (1988) give indirect evidence for the method's validity in the River Imsa, Norway. Otoliths from large and old eels, with thick otoliths, may be difficult to age using this method. The results of Vøllestad and Næsje (1988), however, indicate that clearing is unsuitable for ageing very slow growing eels.
The last methods, needing expensive equipement, will mainly be useful for pure research work. The scanning densitometer method (Deelder, 1976) and the electron microprobe X-ray analysis (Casselman, 1982) are too expensive and laborious for routine ageing of large numbers of otoliths. The difficulty in interpretation is transferred from the stage of looking at the otolith to the stage of looking at the graphs. Therefore for these method to be an improvement some objective measure should be developed to distinguish between the opaque and translucent zones. In the future automated image analysis may also be a method of interest.