Walleye pollock Theragra chalcogramma during transformation from the larval to juvenile stage: otolith and osteological development

Abstract

Walleye pollock [Theragra chalcogramma (Pallas, 1814)] were collected from the Gulf of Alaska from 1991 to 1998 to examine the relationship between otolith and osteological development during transformation from the larval to juvenile life stage. Five stages of otolith development were determined for walleye pollock in conjunction with landmarks in osteological development during juvenile transformation. The first stage of otolith development (12–19 mm SL) is identified by the first accessory growth center (AGC) forming on sagittal otoliths. At this stage, the notochord is undergoing flexion as the teeth, jaws, branchiostegal rays, neural spines, and abdominal vertebral centra are ossifying. The second and third stages of otolith development (19–21 and 21–24 mm SL) are defined by the formation of the anterior, posterior, and dorsal AGCs on the otolith. This occurs during postflexion in conjunction with ossification of the principal caudal-fin rays, haemal spines, and caudal vertebral centra. The fourth stage of otolith development (24–37 mm SL) is defined by the AGCs on the ventral edge of the otolith. By this stage of development, the pelvic-, pectoral-, dorsal-, and anal-fin ray elements have started ossification and the fish has entered the transformation stage. The fifth stage of otolith development (36–48 mm SL) is identified by a completely formed sagittal otolith, which has AGCs that have grown together so that the otolith shape resembles an adult otolith, the ossification of all vertebrae and fin rays is complete, and the fish is a juvenile. By estimating what length a fish was, based solely on otolith development, would prove helpful for those who wish to reconstruct length frequencies of walleye pollock from otoliths of fish that have been damaged due to gear and/or poor handling during the preservation process.