Department of Fisheries Resource Management, Tokyo University of Fisheries, Minato, Tokyo 108-8477, Japan
Effects of the degree of statolith grinding on counting the number of daily increments in Japanese common squid Todarodes pacificus were studied. We used left statolith from 15 squids of 236-292mm in mantle length. Statolith that was ground to half its thickness was defined as completely ground and a 0.04mm thicker stage before complete grinding as incompletely ground. The numbers of increment counted by two readers were compared. Although the readers were unable to count increments in several incompletely ground statolith, all became countable after complete grinding. The average number of increments in incompletely ground statolith by one reader was greater than in complete ones by 24 (N=11) and the other reader counted 26 increments less than complete ones (N=12). The average difference of the increment number of incompletely ground statolith between readers was 43 (N=10). In completely ground statolith, the average number of increments was 188 and 192 (N=15) with the difference of only 4 increments between readers. The difference of increment number between incompletely ground statolith seems to be caused by counting increments in the peripheral zone.