Changes in Midorbital to Hypural Length and Morphology in Maturing Sockeye Salmon

Abstract

Abstract The morphology of sockeye salmon Oncorhynchus nerka changes dramatically as the fish mature. The extent of those changes and whether or not midorbital to hypural length (MOHP) also undergoes change have not been examined. We measured MOHP, body depth, girth, and snout length of migrating sockeye salmon as they entered their natal lake, tagged them, and measured them again after recapture on the spawning grounds. All measured dimensions changed; MOHP decreased 5% in males, while body depth increased 18% and snout length increased 25%. Females changed less than males. Midorbital to hypural length did not change in a way that was explained by allocation of those resources into basal metabolism, secondary sexual development, or gonadal development. In order to use MOHP of mature adults to establish harvest rates on a small population within a fishing district, the change in MOHP should be established, as well as a conversion from the length measurement used at the time of fishing.