OCCURRENCE OF A CHECK IN OTOLITHS OF REARED AND SEA-CAUGHT LARVAL WALLEYE POLLOCK THERAGRA-CHALCOGRAMMA (PALLAS) AND ITS RELATIONSHIP TO EVENTS IN EARLY-LIFE HISTORY

In frontal sections of postlarval otoliths of walleye pollock Theragra chalcogramma (Pallas), some increments were observed to be fused, forming a band structure (check) on the outer distal side. Backward counts of otolith daily growth increments showed the check was formed at different dates for different individuals, beginning from early to late in March and terminating from late in March to mid-April. A comparison between samples from various areas around Hokkaido revealed that the check was formed at almost the same body length (8-11 mm), independent of their age in days and growth patterns. Otoliths of larvae reared under constant environmental conditions also showed the band structure, appearing at the same body length as for wild fish. Comparing these observations with those published in the literature, morphological or physiological changes seem to be responsible for the check formation that the fish experience at 8-11 mm body length.