Evaluation of Inventory Procedures for Hatchery Fish. III. Variation in Specific Gravities of Pacific Salmonids during Rearing

Abstract

Abstract Four salmonid species were examined for variations in specific gravity over time and between species. Possible causes of the variation were also examined. Monthly estimates of specific gravity ranged from 1.00 to 1.04. Specific gravity decreased over a 1-year rearing cycle for coho salmon (Oncorhynchus kisutch), spring chinook salmon (O. tshawytscha), winter steelhead (O. mykiss), and rainbow trout (nonanadromous O. mykiss). Although specific gravity decreased for all species, the patterns were different. Fish length did not appear to affect specific gravity. Lipid content had a slight effect on specific gravity, but only in fish fed at high feeding rates (only coho salmon and rainbow trout were tested). Deflation of the swim bladder significantly increased specific gravity in the species tested. Thus, the swim bladder and factors that affect it seem to be the main causes of variations in specific gravity. Fish inventories are often made by water displacement estimates based on a fish specific gr...