We examined the digestion of hard remains of between one and four different size ranges of nine key North Sea prey taxa fed to seven captive harbour seals (Phoca vitulina). Percentage length reduction (mean 27.5%) and recovery rates (mean 42%) of experimental otoliths varied between species and were positively correlated to fish size and otolith robustness (mass/length). Mean length reduction of egested otoliths increased systematically with increasing size of ingested whiting and sandeel otoliths (p < 0.001), indicating that the size of larger fish may be underestimated. Intraspecific variation in otolith digestion was high (CV = 0.48-1.30), and to control for the artificial conditions of a captive study, external morphological features of otoliths were used to grade the degree of digestion and provide grade-specific correction factors. Bootstrap simulations were used to estimate 95% confidence intervals around correction factors and when partitioned indicated that calculation errors were in general less important than resampling errors. The application of species-, size-, and grade-specific correction factors progressively improved reconstructed estimates of prey biomass fed. As a consequence, estimates of prey size and diet composition require otoliths from faeces to be graded and more complex correction factors applied.