What to Do When Dogfish Lie about Their Age?

Historical methods for ageing spiny dogfish (Squalus suckleyi) result in low precision of age estimates, particularly for older fish exhibiting fin spine erosion, prompting a search for improved methods. Spiny dogfish were aged by historical spine methods and by a new method involving vertebral thin sections obtained from the same specimens. We estimated inter-reader precision and variance associated with each structure. Both structures yielded similar ages for younger animals but not for older animals. The spine method was validated previously by both oxytetracycline and bomb radiocarbon dating, but between-reader agreement is poor. Moreover, worn or broken spines require another step, where lost annuli are estimated through regression, which introduces an additional source of error. In comparison, thin sections substantially improved between-reader agreement while not requiring regression, but processing of vertebrae is time consuming, thin section quality impacts age estimates, and age validation for larger animals has not yet been realized. In summary, the vertebrae thin section method is promising, but more work is required to examine individual variability in thin sections (i.e. quality) and ages need to be compared among the two methods from a larger sample size of age-validated large, old fish.