FISH Scales & fin rays
The scales of most fish are composed of hydroxy apatite over a collagen protein layer. Since they grow in size by adding material to the outer margin, these annuli bands can be used for aging and can also preserve a record of the fish’s environmental conditions and diet. Scale sampling has the advantage of being a non-lethal monitoring method; however, unlike otoliths, mineral material from scales can be resorbed and altered by physiological processes to be utilized by the animal so the record is rarely complete or permanent.
Sampling fish fin rays (the base of the leading pectoral fin ray is usually clipped and used for analysis) is also a non-lethal monitoring technique, but they are much more analogous to otoliths in that the growth and environmental record they preserve is permanent and correlates well to water chemistry. Fin rays are composed of hydroxy apatite similar to scales.