370
S.G. Robertson . A.K. Morison
technique is analogous to the counting of tree rings. More recently, radiometrie
methods (Bennett et al. 1982) and trends in radiocarbon levels (Kalish 1993) have
provided independent estimates of age. However, these methods are even more
expensive and time consuming and are mainly used to validate (show the accuracy
of) ages estimated from increment counts. The counting of growth increments on
otoliths is still the most commonly employed method of age estimation.
The process of age estimation by counting growth increments is not
straightforward and requires considerable experience to be done reliably. Age
estimates may be made as part of a validation study, for descriptions of age
composition and life his tory parameters, or as part of ongoing or "production"
ageing for commercially exploited species (Morison et al. 1998a). This ongoing
process providing estimates of the age composition of the catch from commercial
fisheries over many years also requires special attention to the issue of
maintaining accuracy across years, and often across readers. The importance of
the ageing data for fisheries assessments, and the recognition of problems of
consistency in age estimates, have led to considerable attention being given to the
procedures for the prevention, identification and quantification of biases and
errors in age estimates (Beamish and Fournier 1981; Chang 1982; Kimura and
Lyons 1991; Richards et al. 1992; Campana et al. 1995; Morison et al. 1998a;
Gröger 1999; Campana 2001). The problem of validation of a method of
estimating age is a important issue in itself that is often poorly done. This was first
highlighted by Beamish and McFarlane (1983) and has been recently revisited by
Campana (2001).
Differences between laboratories and between readers are common, and are not
helped by differences in methods of sampie preparation. The species and the
structure largely determine the preferred preparation techniques, but there is also
strong conservative element as particular laboratories will favour particular
preparation methods. A conservative approach is due partly to the available
equipment and the skills of staff, partly to the need to maintain consistency over
years, but also to the familiarity of staff with interpreting a particular form of
preparation. Staff become used to viewing specimens in a partieular orientation
and form of illumination (using either transmitted or reflected light) to the extent
that they can find it difficult to interpret sampies presented differently. This is
similar to the difficultly people have recognising faces presented upside down or
as a negative image, and points to an important aspect of the process: it is as much
one of pattern recognition as of counting increments. However, as a pattern
recognition problem it is probably closer to the character recognition in hand
writing than face recognition as there are a limited number of groups (age classes)
to be recognised. There may be much individual variation but each age class
shares common features.
S.G. Robertson . A.K. Morison
technique is analogous to the counting of tree rings. More recently, radiometrie
methods (Bennett et al. 1982) and trends in radiocarbon levels (Kalish 1993) have
provided independent estimates of age. However, these methods are even more
expensive and time consuming and are mainly used to validate (show the accuracy
of) ages estimated from increment counts. The counting of growth increments on
otoliths is still the most commonly employed method of age estimation.
The process of age estimation by counting growth increments is not
straightforward and requires considerable experience to be done reliably. Age
estimates may be made as part of a validation study, for descriptions of age
composition and life his tory parameters, or as part of ongoing or "production"
ageing for commercially exploited species (Morison et al. 1998a). This ongoing
process providing estimates of the age composition of the catch from commercial
fisheries over many years also requires special attention to the issue of
maintaining accuracy across years, and often across readers. The importance of
the ageing data for fisheries assessments, and the recognition of problems of
consistency in age estimates, have led to considerable attention being given to the
procedures for the prevention, identification and quantification of biases and
errors in age estimates (Beamish and Fournier 1981; Chang 1982; Kimura and
Lyons 1991; Richards et al. 1992; Campana et al. 1995; Morison et al. 1998a;
Gröger 1999; Campana 2001). The problem of validation of a method of
estimating age is a important issue in itself that is often poorly done. This was first
highlighted by Beamish and McFarlane (1983) and has been recently revisited by
Campana (2001).
Differences between laboratories and between readers are common, and are not
helped by differences in methods of sampie preparation. The species and the
structure largely determine the preferred preparation techniques, but there is also
strong conservative element as particular laboratories will favour particular
preparation methods. A conservative approach is due partly to the available
equipment and the skills of staff, partly to the need to maintain consistency over
years, but also to the familiarity of staff with interpreting a particular form of
preparation. Staff become used to viewing specimens in a partieular orientation
and form of illumination (using either transmitted or reflected light) to the extent
that they can find it difficult to interpret sampies presented differently. This is
similar to the difficultly people have recognising faces presented upside down or
as a negative image, and points to an important aspect of the process: it is as much
one of pattern recognition as of counting increments. However, as a pattern
recognition problem it is probably closer to the character recognition in hand
writing than face recognition as there are a limited number of groups (age classes)
to be recognised. There may be much individual variation but each age class
shares common features.