Slijtage van otolieten in de maag van een Aalscholver Phalacrocorax carbo

Fish otoliths are often used in diet studies of piscivorous predators. One of the problems associated with this method is otolith wear. Particularly clupeids have fragile otoliths, that easily wear down or get lost during digestion, which will lead to an underestimation of numbers, size and mass of these fish. A possible solution is to select uneroded otoliths, or to correct for wear in eroded specimens. For the latter, otoliths need to be classified (e.g still present in the fish skull and hence uneroded, loose hut apparently uneroded, slightly eroded or heavily eroded). It is thus of interest whether classification is feasible. We compared herring and sprat otoliths from the same meal in a drowned Great Cormorant. Loose hut apparently uneroded otoliths were found in the stomach, while others were taken from intact fish skulls, still present in the proventriculus. The sample size for sprat was too small, but adequate for herring. Eventhough the herring otoliths in the stomachs showed no sign of wear, their lengths were on average 6.5% smaller, leading to an underestimation of mean fish length by 5.6% and offish mass by 12.9% as compared to the otoliths still present in the fish skulls. A correction for wear thus appears to be in order, even in apparently uneroded clupeid otoliths in birds' stomachs.