ewest, I'm intrigued by your "not fully reliable for exactness" comment.. Can you elaborate a bit on what you mean there?
Its easy to destroy/miss information in samples especially that of the early years particularly if/when the growth was small. One must view in cross section and so this requires intersecting the 0-year.
There are number of methods for aging fish (e.g. scales, spines, and otoliths). They all work by the same principle, seasonal growth causes the growth darker margins where growth is subdued. Provided the subdued growth is annual, then counting the rings is an indication of age. I am not aware of any real world conditions where fish are developing more than one ring per year but it is in principle possible. Generally, rings indicate a minimum age that can be taken as true but it is possible that age is underestimated.
I've read that spine collection can miss early growth and that usually field samplers try take spine as far below the outer surface as practical without unduly harming the fish. Spines are often preferred due to the ease of sectioning (the collected sample is sectioned for example). Like otoliths, growth rings are preserved over time. Scale growth can destroy information of early life.
